Laser-Induced Breakdown Spectroscopy

Preface

The acronym LIBS has a history almost parallel to the more popular acronym LASER with the difference that the former is about twenty years younger and its first letter stands for laser. Although the production of sparks in air by a focused beam from a pulsed ruby laser was observed in 1963, the use of spark emission for elemental analysis became a reality only around 1983, due to the pioneering spectroscopic investigations of D. A. Cremer and L. J. Radziemski at Los Alamos National Laboratory in U.S.A. They also coined the name Laser-Induced Breakdown Spectroscopy (LIBS) for this technique in which spectra of laser-produced plasmas were used for qualitative as well as quantitative spectrochemical analysis of condensed and gaseous samples without any elaborate preparation. A thorough description of the physics of laser breakdown processes and laser-produced plasma is given in Laser Induced Plasma & Applications co-edited in 1989 by the two pioneers of LIBS. The authors not only summarized the work carried out in this field during the previous 25 years but also pointed out its advantages and disadvantages. In view of the rapid developments in laser and detection technologies, they predicted its widespread use in the future. During the past decade and a half, technology has produced more reliable lasers, charge coupled detectors, and miniature spectrographs with its capabilities of recording spectra over a wide range of wavelengths. The combination of these technologies has produced unprecedented enhancements in the signal-to-noise ratio. LIBS has rapidly developed into a major analytical technology with the capability of detecting all chemical elements in a sample without any preparation, of real-time response, and of close-contact or stand-off analysis of targets. The present book includes the latest developments in the experimental techniques and applications of LIBS. It should be useful to analytical chemists and spectroscopists as an important source of information and also to graduate students and researchers engaged in the fields of combustion, environmental science and planetary and space exploration. Understanding the major components in a LIBS experiment and the physics of laser-target interactions are essential to appreciate the new vision of LIBS performance capabilities. These basic ingredients are discussed in Part I (Basic Physics and Instrumentation) of the book, comprising the first five chapters. The first chapter contains the effects of laser beam characteristics on its focusing behavior and on the production of laser sparks in gaseous samples and of plasma plumes from solid samples. The principle of charge-coupled detectors (CCD) and their incorporation in compact spectrographs for broad-band detection are also briefly described. In Chapter 2, a brief account of the electronic structure of atoms and their quantum states is given. The allowed and forbidden transitions are discussed in the electric-dipole approximation, and the origins of continuum as well as line emission from atoms are explained. The broadening of spectral lines is related to the physical conditions around the radiating atoms and the effects of electric fields in a plasma environment are discussed in detail. Applications

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Preface

of atomic emission spectroscopy in determining electron density, electron temperature and qualitative as well as quantitative spectrochemical analysis of the source are briefly discussed. Ablation forms the subject matter of Chapter 3. The term ‘ablation’ describes the explosive vaporization of material irradiated by a laser beam. In general, the ablation rates depend on the material, the laser wavelength, the ambient atmosphere and the geometry of the laser beam. Two different types of ablation mechanisms can be distinguished: the photochemical and the photothermal. Regardless of the mechanism, the dominating effect of every kind of ablation is an extreme short-term temperature increase of the irradiated material surface which is the starting point of different physical and chemical reactions. The characteristics of a radiating plasma produced by a laser is strongly dependent on its pulse duration and irradiance. The influence of laser ablation on LIBS is discussed in this chapter. The physics of LIBS involves many processes of which ablation and plasma formation are of great significance, but the process of optical emission from the plasma is the crucial one for obtaining spectroscopic information about the constituent atomic species. A significant fraction of the incident laser pulse energy is absorbed in the expanding plasma plume, causing the atoms and ions to reach different states of excitation and subsequent optical emission. The physics of these optical processes involving absorption of laser radiation and emission from the plasma plume forms the subject matter of Chapter 4. The origins of continuum and line emission from laser-produced plasmas is discussed with a view to emphasize the importance of spatial as well as temporal resolution of the optical emission. Designs of experimental setups for obtaining maximum sensitivity of measurement are described in great detail. The contents of Chapter 5 deal with LIBS instrumentation. It has three major components: the laser, the ablation chamber, and the detection system for optical emission. LIBS is, however, a versatile technique for detection and identification of elements in a variety of environments. Each one of these situations requires some kind of modification of the standard LIBS instrumentation. Some of these unusual experimental arrangements are discussed with emphasis on remote detection systems and portable detection systems. Part II of the book comprises of Chapters 6 to 8, dealing with New LIBS Techniques. The technique of dual laser pulse LIBS (described in Chapter 6) has proved fruitful in improving the signal-to-background ratio (S/B) and signal-to-noise ratio (S/N) relative to conventional single-pulse technique. Experimental configurations with collinear and orthogonal impact of two laser pulses separated by a few microseconds have been used resulting in a ten-fold or more enhancement in the LIBS signal. Although the mechanism of increase in signal is not very well understood at present, this new LIBS technique has been very widely used in the recent past to improve the reproducibility and limit of detection. The use of femtosecond lasers in LIBS is discussed in Chapter 7. The interaction of such ultra-fast laser pulses with materials is very different from that of nanosecond laser pulses commonly employed in LIBS. The fundamental physical processes involved during laser ablation and applications of this technique are presented in this chapter. Chapter 8 contains the results of the new technique of micro-LIBS, employing laser pulses with energy in the range of micro-joules to less than a milli-joule. Such low energy laser pulses permit two-dimensional microanalysis of material surfaces with spatial resolutions approaching a micron. This technique has great potential in the development of portable LIBS systems. The variety of LIBS applications are covered in Part III of the book in Chapters 9 through 18. LIBS can be utilized in the detection of trace metals in the off-gases from

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industrial plants, traffic, volcanoes, wild fire and combustion processes. The results presented in Chapter 9 demonstrate that LIBS can be used as continuous emission monitor and also for the metallic species in the exhaust of rocket engines. Analysis of liquid samples is of great importance in the context of environmental studies and of molten metals. The use of LIBS for determining the chemical composition of such samples and many others, and also methods of enhancing the precision of such measurements are described in Chapter 10. One of the major problems, in glass, aluminum, and steel industries is the need for real-time measurement of constituents of the melt. LIBS can provide rapid, in-situ melt composition measurements. It also allows chemical additions to be made to the melt so that an acceptable product composition is achieved prior to draining a furnace. In Chapter 11 experimental arrangements are described, based on fiber optic LIBS sensor to measure in-situ elemental composition of solid and molten samples. Chapter 12 deals with the elemental analysis of powder samples using LIBS. Powder materials, both granular as well as fine powders, represent the most common form of raw material in industries, like chemical, pharmaceutical, glass and ceramics, food and others. It has been shown with examples, that LIBS can be used for on-line monitoring of the elemental composition of the powder material before it is fed into a process. The detection of chemical and biological agents that pose threat to human life, form the subject matter of chapter 13. Such agents are complex molecules and intricate living structures and it is not readily clear as to how an elemental analysis technique should be useful in their analysis. The application of broadband spectrometers to LIBS in recent years has led to very accurate data on elemental ratios making it possible to determine stoichiometry of a broad range of compounds. The use of LIBS in the analysis of chemical and biological agents in air, water and particulate matter has been discussed in detail. Life science applications of LIBS discussed in chapter 14 deals with the analysis of elemental composition of biological samples. The capability of LIBS for estimating trace elements in a single cell has potential medical applications. The relative concentrations of major as well as trace elements in normal and malignant cells have been determined. Physical parameters during laser ablation of teeth have also been studied. Chapter 15 is concerned with the determination of total carbon content of soil using LIBS. This has great significance in view of suggestions that soils and vegetation could be managed to increase their uptake and storage of CO2 and thus become ‘land carbon sink’ to reduce anthropogenic emissions of carbon dioxide. LIBS for space exploration, one of the most exotic and exciting applications is described in Chapter 16. This application is based on the stand-off capability of LIBS and results of measurements on atmospheric conditions simulating Mars are discussed. Chapter 17 is devoted to the detection and analysis of chemical composition of aerosol particles – a complex mixture of nitrates, sulphates, chlorides, water etc- originating from both natural and anthropogenic sources. Quantitative aerosol analysis is presented in terms of the aerosol-sampling problem followed by direct and indirect aerosol measurements. It is always a difficult task to predict the future course of developments in the field of science and technology but one can point out the existing shortcomings and possible directions for future researches in LIBS. This has been done in chapter 18 of this book. Many existing applications have not been put to practical use due to insufficient accuracy and precision of LIBS. The extension of echelle spectrometer to VUV range will permit the detection of non-metals (S, P, Cl, Br) which are very important in process analysis. There is no suitable theoretical model at present to explain the laser ablation and plasma

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formation in a LIBS experiment. Developments along these and some other directions are expected to make LIBS a very important field for science and technology in the future. We gratefully acknowledge the imaginative contributions by the authors who spared time from their busy schedule of research and teaching for this book. We are also grateful to the members of Laser and Spectroscopy Laboratory at the Banaras Hindu University and of the Institute for Clean Energy Technology at the Mississippi State University for their enthusiastic help during the preparation of the manuscript. Special thanks are due to Mr. Sushil K. Singh, Dr. Vineeta Singh, Dr. Rajamohan R. Kalluru and Dr. S. B. Rai for their valuable editorial suggestions and help. We wish to record our gratitude to our wives Mrs. Shila Singh and Mrs. Vaidehi Thakur for their exemplary cooperation, patience and understanding.

Contributors

S. Michael Angel Department of Chemistry and Biochemistry University of South Carolina Columbia, SC 29208, USA Steve Buckley Department of Mechanical and Aerospace Engineering University of California, San Diego La Jolla, CA 92093, USA David A. Cremers 4300 San Mateo Boulevard Applied Research Associates, Inc. Albuquerque, NM 87110, USA I. V. Cravetchi Department of Electrical and Computer Engineering University of Alberta Edminton, Alberta, T6G2V4, Canada R. Fedosejevs Department of Electrical and Computer Engineering University of Alberta Edminton, Alberta T6G2V4, Canada C. T. Garten Jr. Environmental Sciences Division Oak Ridge National Laboratory Oak Ridge, TN 37831, USA J. J. Gonzalez 1 Cyclotron Road Lawrence Berkeley National Laboratory Berkeley, CA 94720, USA

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David Hahn Department of Mechanical Engineering and Aerospace Engineering University of Florida Gainesville, FL 32611, USA Akashya Kumar Department of Physics Tuskegee University Tuskegee, AL 36088, USA Bansi Lal Center for Laser Technology Indian Institute of Technology Kanpur 208016, India X. L. Mao 1 Cyclotron Road Lawrence Berkeley National Laboratory Berkeley, CA 94720, USA M. Martin Environmental Sciences Division Oak Ridge National Laboratory Oak Ridge, TN 37831, USA A. V. Palumbo Environmental Sciences Division Oak Ridge National Laboratory Oak Ridge, TN 37831, USA Ulrich Panne Department of Chemistry Humboldt-Universitaet zu Berlin Richard-Willstaetter-Str. 11 12489 Berlin, Germany A. K. Rai Department of Physics Allahabad University Allahabad 211002, India V. N. Rai Laser Plasma Division Centre for Advanced Technology Indore 452 013, India

Contributors

Contributors

D. K. Rai Department of Physics Banaras Hindu University Varanasi 221005, India R. E. Russo 1 Cyclotron Road Lawrence Berkeley National Laboratory Berkeley, CA 94720, USA Mohamad Sabsabi National Research Council Canada Boucherville, Québec, J4B 6Y4 Canada Louis St-Onge National Research Council Canada Boucherville, Québec, J4B 6Y4 Canada J. Scaffidi Department of Chemistry and Biochemistry University of South Carolina Columbia, SC 29208, USA Jagdish P. Singh Institute for Clean Energy Technology (ICET) Mississippi State University Starkville, MS 39759, USA M. T. Taschuk Department of Electrical and Computer Engineering University of Alberta Edminton, Alberta, T6G2V4, Canada Surya N. Thakur Department of Physics Banaras Hindu University Varanasi 221005, India Y. Y. Tsui Department of Electrical and Computer Engineering University of Alberta Edminton, Alberta, T6G2V4, Canada S. D. Wullschleger Environmental Sciences Division Oak Ridge National Laboratory Oak Ridge, TN 37831, USA

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J. Yoo 1 Cyclotron Road Lawrence Berkeley National Laboratory Berkeley, CA 94720, USA F. Y. Yueh Institute for Clean Energy Technology (ICET) Mississippi State University Starkville, MS 39759, USA

Contributors

Acronyms

Å AAID AAS AES API APXS ArF BBO BKG CBE CCD CE CEM CFFF CIR CM CM CPA CPM CRM CW DCP-AES DIAL DM DMA DOE DU EP EPA Er: YAG ESAWIN FDA FO FRAS FRC fs

Angstrom Advanced Analytical Instrumentation Demonstration Atomic Absorption Spectrometry Atomic Emission Spectroscopy Active Pharmaceutical Ingredient Alpha Proton X-Ray Spectrometer Argon Fluoride Barium Borate Background Conduction Band Electron Charge Coupled Device Coronal Equilibrium Continuous Emission Monitoring Coal Fired Flow Facility Cumulative Intensity Ratio Corona Model Chlorpheniramine Maleate Chirped Pulse Amplification Colliding Pulse Mode (Locked laser) Collisional-radiative Model Continuous Wave Direct Current Plasma Atomic Emission Spectrometry Diagnostic Instrumentation and Analysis Laboratory Dichroic Mirror Differential Mobility Analyzer Department of Energy Depleted Uranium European Pharmacopeia Environmental Protection Agency Erbium Yttrium Aluminum Garnet Echelle Spectra Analyzer software for WINdows Food and Drug Administration Fiber Optic Facility for Remote Analysis of Small Bodies Field Research Center femtosecond

xxii

FTIR FWHM GRIN HEPA HMX HPLC IB ICCD ICP-AES ICPES ICP-MS ID IDAD IPCF IR JPL KrF KTP LA LASER LASIK LBR LDRD LEAF LEAFS LIBS LIDAR LIF LIP LIPS LMA LNR LOD LPF LSC LSD LSR LTE LTSD MACT MALDI MALIS MER MHD MIP-AES

Acronyms

Fourier Transform Infrared Full Width at Half Maximum Gradient Index High Efficiency Particulate Air (filter) High Melting eXplosive (octogen and cyclotetramethylene tetranitramine) High Performance Liquid Chromatography Inverse Bremsstrahlung Intensified Charge Coupled Device Inductively-Coupled Plasma Atomic Emission Spectrometry Inductively Coupled Plasma Emission Spectroscopy Inductively Coupled Plasma Mass Spectrometry Inner Diameter Intensified Diode Array Detector Instituto per I Processi CHimio Fisici Infrared Jet Propulsion Laboratory Krypton Fluoride Potassium Titanium Oxide Phosphate (KTiOPO4) Laser Ablation Light Amplification by the Stimulated Emission of Radiation Laser Assisted in situ Keratomileusis Line-to-Background Ratio Laboratory Directed Research and Development Laser Enhanced Atomic Fluorescence Laser Excited Atomic Fluorescence Spectrometry Laser Induced Breakdown Spectroscopy Light Detection and Ranging Light Induced Fluorescence Laser Induced Plasma Laser Induced Plasma Spectroscopy Large Mode Area Line-to-Noise Ratio Limit of Detection Laser Photofragmentation Laser Supported Combustion Laser Supported Detonation (waves) Laser Supported Radiation Local Thermodynamic Equilibrium Lens-to-Surface Distance Maximum Achievable Control Technology Matrix Assisted Laser Desorption/Ionization Mars elemental Analysis by Laser Induced Breakdown Spectroscopy Mars Exploration Rover Magnetohydrodynamics Microwave Induced Plasma Atomic Emission Spectrometry

Acronyms

MIS MIT mJ MPI MS MSE MSL MSU MW NA NABIR NASA Nd: YAG Nd:YLF NETL ng NIR NIST nm NMR NRC ns OD ORNL P/B PAT PC PCA PDA PETN PLD PM PMT ppb ppm ps PVA PVC RA RBC RCRA RDX R-FIBS RKIS RLIBS RM RMS

xxiii

Metal Insulated Semiconductor Massachusetts Institute of Technology milli-Joule Multiphoton Ionization Mass Spectrometry Mountain States Energy Mars Science Laboratory Mississippi State University Megawatt Numerical Aperture Natural and Accelerated Bioremediation Research National Aeronautics and Space Administration Neodymium Yttrium Aluminum Garnet Neodymium: Yttrium Lithium Fluoride National Energy Technology Laboratory nanogram Near Infrared National Institute of Standards and Technology nanometer Nuclear Magnetic Resonance National Research Council nanosecond Outer Diameter Oak Ridge National Laboratory Peak-to-Base Process Analytical Technology Personnel Computer Principal Component Analysis Photodiode Array Pentaerythritol Tetranitrate Pulsed Laser Deposition Particulate Matter Photomultiplier Tube part per billion part per million picosecond Polyvinyl Alcohol Polyvinyl Chloride Relative Accuracy Red Blood Cell Resource Conservation and Recovery Act Royal Demolition eXplosive (1,3,5-trinitro-1,3,5-triazine) Remote Filament Induced Breakdown Spectroscopy Rotary Kiln Incinerator Simulator Resonance Laser Induced Breakdown Spectroscopy Reference Method Root Mean Square

xxiv

rpm RSD RSP S/B S/N SAIC SEM SEM-EDX SESAM SHL SMA SMR SOM SSC SSTB STP TE TEM TFP TNT TOF TTL TW U.S. USN USP UV UV-VIS VUV XeCl XRF J s

Acronyms

revolutions per minute Relative Standard Deviation Repetitive Spark Pair Signal-to-Background Ratio Signal-to-Noise Ratio Science Applications International Corporation Scanning Electron Microscopy Scanning Electron Microscopy – Energy Dispersive X-ray Semiconductor Saturable Absorber Mirror Superheated Liquid SubMiniature version A (fiber connector) Surface Map-ping Rate Soil Organic Matter Stennis Space Center Slip Stream Test Bed Standard Temperature and Pressure Thermodynamic Equilibrium Transverse Electric Mode Thin Film Polarizer Tri-nitro Toluene Time-of-Flight Transistor-Transistor Logic Tera-Watt United States Ultrasonic Nebulizer United States Pharmacopeia Ultraviolet Ultraviolet Visible Vacuum Ultraviolet Xenon Chloride X-ray Fluorescence Micro-Joule microsecond

Chapter 1

Fundamentals of Laser Induced Breakdown Spectroscopy S. N. Thakura and J. P. Singhb a

Laser and Spectroscopy Laboratory, Department of Physics Banaras Hindu University, Varanasi-221005, INDIA b Institute for Clean Energy Technology, Mississippi State University Mississippi State, U.S.A

1. INTRODUCTION The devastating power of the laser was demonstrated soon after its invention when a focused laser beam produced a bright flash in air similar to the spark produced by lightening discharge between two clouds [1]. Another spectacular effect involved the production of luminous clouds of vaporized material blasted from a metallic surface and often accompanied by a shower of sparks when the laser was focused on a metal surface [2,3]. These laser effects have found many technological applications in the fields of metalworking, plasma production, and semiconductors. When a pulsed laser beam of high intensity is focused, it generates plasma from the material. This phenomenon has opened up applications in many fields of science from thin film deposition to elemental analysis of samples. The possibility of using a high-power, short-duration laser pulse to produce a high temperature, high-density plasma was pointed out by Basov and Krokhin [4] as a means of filling a fusion device by vaporizing a small amount of material. Laser ablation of solids into background gases is now a proven method of cluster-assembly [5,6]. In this method, a solid target is vaporized by a powerful laser pulse to form partially ionized plasma that contains atoms and small molecules. Not much is known about the formation and transport of particles in laser ablation plumes. In recent years there has been notable interest both in an increased understanding of laser induced plasmas (LIP) and in the development of their applications. Emission spectroscopy is used for elemental analysis of targets from which the luminous plasma is generated and it can also be applied to determine the temperature, electron density and atom density in the LIP [7]. The history of laser spark spectroscopy runs parallel to the development of highpower lasers, starting with the early use of a ruby laser for producing sparks in gases [8]. In subsequent years the spectral analysis of LIP became an area of study that has significantly matured at present. The current developments of this technique for chemical analysis can be traced to the work of Radziemski and Cremers [9] and their co-workers Laser-Induced Breakdown Spectroscopy Jagdish P Singh, Surya N Thakur (Editors) © 2007 Elsevier B.V. All rights reserved.

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S. N. Thakur and J. P. Singh

at Los Alamos National Laboratory in the 1980s. It was this research group that first coined the acronym LIBS for laser induced breakdown spectroscopy. During the last two decades, LIBS has undergone a dramatic transformation in terms of hardware, software and application areas. It has become a powerful sensor technology for both laboratory and field use. In order to obtain a reliable quantitative elemental analysis of a sample using LIBS, one needs to control several parameters that can strongly affect the measurements. Some of these parameters are the laser wavelength, its irradiance, the morphology of the sample surface, the amount of ablated and vaporized sample, and the ability of the resulting plasma to absorb the optical energy. If these and related parameters are properly optimized, the spectral line intensities will be proportional to the elemental concentration. In the following sections we briefly describe the basic components and the underlying physical processes that are essential to appreciate the range of applications and power of LIBS.

2. LASERS FOR LIBS The main properties of laser light which distinguish it from conventional light sources are the intensity, directionality, monochromaticity, and coherence. In addition the laser may operate to emit radiation continuously or it may generate radiation in short pulses. Some lasers can generate radiation with the above mentioned properties and that is tunable over a wide range of wavelengths. Generally pulsed lasers are used in the production of plasmas and also in laser induced breakdown spectroscopy (LIBS). We consider only those properties of lasers relevant to plasma production in gaseous, liquid and solid samples so that the role of various types of laser systems used in LIBS experiments is clearly understood in the later chapters of this book. It is possible to generate shortduration laser pulses with wavelengths ranging from the infrared to the ultraviolet, with powers of the order of millions of watts. Several billions to trillions of watts and more have been obtained in a pulse from more sophisticated lasers. Such high-power pulses of laser radiation can vaporize metallic and refractory surfaces in a fraction of a second. It is to be noted that not only the peak power of the laser, but also the ability to deliver the energy to a specific location is of great importance. For LIBS, the power per unit area that can be delivered to the target is more important than the absolute value of the laser power. The power per unit area in the laser beam is termed “irradiance” and is also called “flux” or “flux density.” Conventional light sources with kilowatts powers cannot be focused as well as laser radiation and therefore are not capable of producing effects that lasers can. The next property of laser radiation that is of interest is the directionality of the beam. Laser radiation is confined to a narrow cone of angles which is of the order of a few tenths of a milliradian for gas lasers to a few milliradians for solid state lasers. Because of the narrow divergence angle of laser radiation, it is easy to collect all the radiation with a simple lens. The narrow beam angle also allows focusing of the laser light to a small spot. Therefore the directionality of the beam is an important factor in the ability of lasers to deliver high irradiance to a target. Coherence of the laser is also related to the narrowness of the beam divergence angle and it is indirectly related to the ability of the laser to produce high irradiance. However, coherence is not of primary concern in LIBS. Provided that a certain number of watts per square centimeter are delivered to a

Fundamentals of LIBS

5

surface, the effect will be much the same whether the radiation is coherent or not. The monochromaticity of the laser as such plays very little role as far as plasma production is concerned because it is the power per unit area on the target that matters irrespective of the fact whether the radiation is monochromatic or covers a broad band. In special cases, one may require highly monochromatic laser radiation to probe the plasma using resonance excitation of atomic species. The frequency spread of gas lasers is of the order of one part in 1010 or even better and for solid lasers, it is of the order of several megahertz. In specifying the frequency spread, we have taken the width of a single cavity mode of the laser, although most lasers operate in more than one cavity mode so that the total frequency spread may cover the entire line width of the laser transition. The frequency spread of each of the cavity modes is much narrower than the line width of the laser transition and the former is used to characterize the frequency stability of the laser.

2.1. Mode Properties of Lasers The optical cavity of a laser is determined by the configuration of the two end mirrors. The stationary patterns of the electromagnetic waves formed in the cavity are called modes. For a cavity formed by two confocal spherical mirrors separated by a distance L, the frequency  of a mode (mnq) is given by mnq = c/2Lq + 1/2m + n + 1

(1)

where c is the velocity of light, q is a large integer, and m and n are small integers. The axial modes correspond to m = n = 0 and involve a standing wave pattern with an integral number q of half wavelengths with q /2 = L, between the two mirrors with a node at each mirror. The separation between frequencies of two consecutive axial modes (c/2L) is of the order of gigahertz for typical solid state lasers. The transverse modes of the laser are designated by TEMmn . They affect the focusing properties of the laser beam. The smallest focal spots and highest irradiance is obtained with beams containing the lowest transverse modes with the smallest pair of values m, n. The higher transverse modes have radial intensity distributions which are less and less concentrated along the resonator axis with increasing values of m or n. These modes are also known as off-axis modes and their diffraction losses are much higher than that of the fundamental modes TEM00q . Some of the patterns for transverse modes are shown in Fig. 1. The presence of higher transverse modes (large m, n) increases the divergence angle and affects the focusing of the laser beam. If there is no control over the mode properties, the modes present in the laser pulse can change from one shot to the next and different pulses from a high power laser would be focused differently. High brightness is essential for delivering high irradiance. The brightness of a source is the power emitted per unit

TEM00

TEM10

TEM20

TEM11

Fig. 1. Transverse modes showing off-axis intensity distribution in selected higher modes.

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S. N. Thakur and J. P. Singh

area per unit solid angle. As laser power increases, the number of transverse modes increases with little increase in brightness. The technology to produce high irradiance in a laser beam thus involves decreasing beam divergence as much as increasing power.

2.2. Spatial Intensity Distribution and Focusing of Laser Beam In order to determine the irradiance produced by a laser, it is necessary to know the spot size to which the beam can be focused. It is impossible to focus the beam to a geometrical point and the minimum spot size is dependent on diffraction. Since optical systems are not perfect, the actual spot size is larger than the limit set by diffraction. Maximum irradiance is obtained with minimum focal area of the laser spot. The spatial distribution of the output of a continuous gas laser follows the mode patterns shown in Fig. 1 and for the lowest transverse mode, the intensity distribution is given by I00 r = exp−2r 2 /w2 

(2)

where w is called the Gaussian radius of the TEM00 mode. The output of a high-power solid state laser has a complicated spatial intensity distribution and does not exhibit the recognizable mode patterns shown in Fig. 1. The output is a superposition of many modes along with distortions caused by inhomogeneities of the crystal. This irregular spatial distribution leads to problems in focusing the laser beam to the minimum size. A schematic spatial profile of a solid laser is shown in Fig. 2. The spatial profile of the laser beam can change during the course of the laser pulse [10]. Many methods have been employed for improvement of the mode properties of high power solid state lasers. One is shown in Fig. 3 where an aperture was introduced at the focus of a lens system contained within the laser cavity. This arrangement can

Fig. 2. Schematic contour of irradiance in unfocused high power ruby laser. Mirror

Ruby rod

Aperture

Lens

Output mirror

Lens

Fig. 3. Schematic diagram of laser cavity with an aperture to remove higher order transverse modes.

Fundamentals of LIBS

7

reduce the off-axis mode content significantly, because the high order modes have large diffraction losses at the aperture. The output from a ruby laser can be made more spatially uniform than that shown in Fig. 2 and can have a divergence angle close to the diffraction limit [11]. The number of axial modes in a laser output can be reduced with an optical cavity in which one mirror is made up of a number of uncoated interferometric flats. Laser oscillation occurs at those wavelengths which are simultaneously modes of the total cavity and of the individual interferometers formed by each pair of flat parallel surfaces. Since the gain of the laser is nonlinear, the output power is funneled into a single or a few axial modes. In an optical cavity, introduction of a dye mode-selector has been used to produce a single TEM00 axial mode output from a ruby laser [12]. The design and fabrication of a single mode laser oscillator followed by amplifiers has led to diffraction-limited lasers of high brightness [14]. An important concept in the context of lasers is the distinction between near and far field spatial patterns. In the near field, the intensity pattern is the same as at the output mirror of the laser and it follows the mode patterns shown in Fig. 1. If ‘a’ is the aperture diameter of the output mirror and the laser beam is approximated by a Gaussian beam, then the near field pattern persists for a distance of the order of a2 / where  is the wavelength of laser light. However for larger distances from the output mirror, the well defined mode pattern in the near field would be washed out by diffraction effects; the spreading angle is of the order of /a. Gaussian beams have the same phase across the entire wave front, and they are capable of being focused to the minimum possible size [13]. Ruby lasers with an ordinary optical cavity can be focused with a simple lens to produce spots with diameters of the order of 300 microns whereas those with an apertured optical cavity have focal spot sizes of the order of a few microns. A focal area of 10−3 cm2 is typical for a ruby laser focused with a simple lens and the following peak power and irradiance can be obtained by different types of ruby lasers: Laser Type

Peak Power

Irradiance

Normal Pulse Laser Q-Switched Laser Picosecond Pulse Laser

105 W 108 W 1011 W

108 W cm−2 1011 W cm−2 1014 W cm−2

2.3. Time Behavior of Laser Pulses Solid state lasers, such as ruby lasers, Nd: glass and Nd: YAG lasers that produce high powers are generally pulsed with widely different pulse durations and with different methods of pulsing. If the laser is pumped by a flashlamp, pulse widths in the range of 100 to1000 microseconds are typical. In many cases, the laser emission is not uniform, but consists of many microsecond duration spikes called relaxation oscillations whose amplitudes and spacing are not uniform. The presence of these spikes in the laser pulse causes heating and cooling of the target surface and is not suitable for producing a uniform plasma plume. Laser pulse durations in the range of 10 to 1000 nanoseconds can be produced by Q-switching techniques, where laser operation is suppressed and population inversion

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S. N. Thakur and J. P. Singh

in the solid rod increases greatly over the normal threshold condition [14]. If the Q-switching component in the laser cavity is changed to a transparent condition, the laser rod, now in a highly inverted state, gets coupled to the two mirrors of the cavity and the stored energy is emitted in a pulse of much higher power and much shorter duration than without Q-switching. It is possible to produce laser pulses of picoseconds duration by the phenomenon of mode locking. If there are N resonant axial modes of the cavity simultaneously present in the linewidth of the lasing transition, then these can be coupled by using a Q-switching dye with nonlinear transparency. This coupling of modes leads to a locking of the phases of different axial modes. In the time domain, a single ultrashort pulse circulates in the cavity with a time period equal to the round trip transit time (2L/c). The laser output is in the form of identical pulses whose spacing is equal to 2L/c. The width  of each pulse is approximately the inverse of the frequency spread of the laser output. Thus, for N axial modes in the lasing transition linewidth, the pulse duration is given by  = 1/Nc/2L = 2L/Nc

(3)

Femtosecond laser pulses are produced by the technique of colliding pulse mode locking (CPM) which utilizes the collision of two counter-propagating pulse trains in a thin saturable dye jet. The interaction of the counter-propagating pulses creates a transient grating of the population of dye molecules, which synchronizes, stabilizes and shortens the pulse. The operation of femtosecond pulses is very sensitive to mirror coatings. The short duration and high electric field intensities encountered in amplifying femtosecond pulses introduce new problems in amplifier design. Improvement in amplification techniques has permitted generation of femtosecond laser pulses of gigawatt intensities [15].

2.4. Measurement of Laser Power and Energy In order to study the physics of laser induced plasmas, reliable measurements of laser beam power, beam energy, beam divergence angle, and spatial intensity distribution of the beam cross-section are needed. The most common detectors used in the measurement of laser power are referred to as square law detectors because they respond to the square of the electric field. Photomultiplier tubes (PMTs) and single stage vacuum photoemissive detectors are sensitive in the ultraviolet (UV), visible, and near infrared, whereas photoconductive detectors are used for lasers emitting at wavelengths longer than one micron. For pulsed lasers, the phototube output is displayed on a fast oscilloscope to determine the pulse shape. The response speed of the photodetector must be fast and circuitry must be carefully designed to preserve the pulse shape. Intense laser output tends to saturate the output of the detectors, so absorbing filters are used to keep the detector in the linear portion of their operating range and to make them blind to background radiation. Another widely used detector is the semiconductor photodiode which is a photovoltaic device. Laser radiation incident on the detector produces a voltage across the p-n junction even in the absence of an external bias. When light falls on a back-biased diode, the reverse current increases sharply. Room temperature devices are used in the visible region and up to 3.6 microns whereas liquid nitrogen-cooled devices operate up to 5.7 microns.

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9

The total energy in a laser pulse is measured by calorimetric methods using blackbody absorbers of low thermal mass in contact with thermocouples or other temperature measuring devices. In one common form, the absorber is a small hollow cone of carbon such that radiation entering the base of the cone cannot be reflected out of the cone. Thermistor beads, forming an element of a balanced bridge circuit, are placed intimately in contact with the cone. As the cone is heated by a pulse of energy, the resistance of the thermistors changes, resulting in an imbalance of the bridge and a voltage pulse which decays as the cone cools to ambient temperature. The magnitude of the voltage pulse gives a measure of the energy in the laser pulse.

2.5. Varieties of Lasers A laser is not one single device, but there are a wide variety of different lasers with many different characteristics. Each type has its own properties of wavelength and operating parameters. Even within one type, there are many varieties of construction. Now several thousands laser lines are known which span a whole spectral range from extreme ultraviolet to the far-infrared region. Developments in LIBS have taken place by using the laser wavelengths provided by existing technology. In 1962 a ruby laser at 694 nm was used by Breach and Cross [16], but its pulse-to-pulse stability was very poor and LIBS was not considered to be a very reliable technique for spectrochemical analysis. The next phase of LIBS development was marked by the sophisticated pulsed-laser technology of the 1980s which led to very reliable Nd: YAG lasers in the near-IR, visible, and UV regions and to excimer lasers in the UV region. At present many more laser wavelengths have become available to study their effects on LIBS measurements [17–19]. Lasers commonly employed in LIBS are listed in Table 1 along with properties associated with these lasers. These are representative values, not necessarily the highest or the best ever achieved. Table 1. Characteristic properties of some lasers for LIBS[20] Laser Type

Wavelength

Pulse Duration

Energy/Pulse

CO2 Repetitive CO2 Q-switched Er:YAG Q-switched Nd:YAG Ruby Normal Pulse Ruby Q-switched Ruby Picosecond Pulse Nd:YAG Second Harmonic Nd:YAG Third Harmonic N2 Laser XeCl Excimer Nd:YAG Fourth Harmonic KrF Excimer ArF Excimer

10 6 m 10 6 m 2 94 m 1 06 m 694.3 nm 694.3 nm 694.3 nm 532.0 nm 354.7 nm 337.1 nm 308 nm 266 nm 248 nm 193 nm

10–100 s 200 ns 170 ns 5–10 ns 0.2–10 ms 5–30 ns 10 ps 4–8 ns 4–8 ns 3–6 ns 20–30 ns 3–5 ns 25–35 ns 8–15 ns

0.1–5 J 0.1 J 25 mJ 1–3 J 1–500 J 1–50 J 0.01–0.5 J 0.5–2 J 0.2–0.7 J 0.1–0.6 mJ 0.5–1 J 0.1–0.3 J 0.5–1 J 8–15 mJ

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S. N. Thakur and J. P. Singh

3. LASER INDUCED PLASMAS To produce a spark in air or a gas requires laser intensities of the order of 1011 W cm−2 . Sparks are caused by the breakdown of the gas due to the electric field associated with the light wave. Breakdown thresholds are of the order of 106 to 107 V cm−1 . The spark is accompanied by production of charged particles, absorption of laser light, and re-radiation of light from the spark. If the temperature of the plasma at the position of the gas breakdown becomes high enough, X-ray emission is also observed, in addition of visible and UV radiation. This phenomenon was termed laser induced breakdown in analogy with the electrical breakdown of gases [21]. The breakdown results from strong ionization and absorption by gases that are usually transparent to light. The breakdown is marked by a threshold irradiance below which virtually no effects are observed. The onset of the breakdown is a sudden, dramatic phenomenon occurring at an easily determined threshold. Its spatial as well as temporal profiles make interesting study [22]. The breakdown in the focal volume of the lens in which the peak laser irradiance occurs can be understood as occurring in two steps. First, the production of the initial ionization and the subsequent cascade by which the ionization grows resulting in the breakdown. Multiphoton ionization, where simultaneous absorption of many quanta by an atom produces an ion-electron pair, is considered to be a plausible mechanism for the initial ionization. An alternative possibility is multiphoton excitation of an atom to an excited state with many other excited states between it and the free electron continuum. Single photon absorption processes may rapidly ionize the atom from this excited level. A free electron in the focal volume absorbs photons and gains enough energy to ionize additional atoms by collisions. In each such ionization process, the colliding electron is replaced by two electrons with lower energy in the free electron continuum. These in turn absorb photons so that an avalanche or cascade of ionization will occur. The absorption of a photon by an electron may be visualized in two equivalent ways. (1) It can be considered as an inverse Bremsstrahlung process in which a single light quantum is absorbed by an electron in the field of an atom or ion. (2) Secondly it can be considered as analogous to microwave-generated breakdown, in which the electron oscillates in the electric field of the incident radiation.

3.1. Laser Induced Breakdown in Gases One of the most striking observations is the extinction of laser light by a plasma plume produced by breakdown in gases. When the laser irradiance is less than the threshold, no significant attenuation is observed, but with laser irradiance exceeding the threshold, the absorption is so strong that it is often used as a critical test of whether breakdown has actually occurred. Fig. 4 shows the shape of the original laser pulse and the pulse transmitted through the plasma when breakdown occurs. It is evident that early in the transmitted profile, there is little attenuation, but at later times, after breakdown occurs, the plasma becomes very opaque. The abrupt shutoff of the transmitted light occurs simultaneously with initiation of the spark. When light transmission is studied for a series of laser pulses with increasing energy, breakdown occurs earlier in the pulse as laser irradiance increases. The time to breakdown as a function of intensity depends

Fundamentals of LIBS

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Original laser profile

Laser profile transmitted through plasma

10

20

30

40

50

ns

Fig. 4. Schematic temporal profile of laser pulse in the absence and in presence of gas breakdown showing attenuation of laser beam.

on the focal area [23]. For a small focal volume, higher laser intensity is required to produce breakdown within the same time. Although the laser induced blue-white spark appears spatially uniform to the naked eye, it is indeed elongated along the direction of the incoming laser beam. For laser powers of the order of 100 MW, the spark may be 1 cm long and a few millimeters in diameter. A schematic shape of the spark is shown in Fig. 5 where its expansion back toward the laser essentially fills the converging cone of laser radiation. The growth of the spark in the direction opposite to the light flux has led to the model of a radiationsupported detonation wave. A detonation wave is a shock wave which is fed by release of energy behind the shock wavefront. In this case the energy is supplied by the absorption of the incoming laser beam. This is analogous to the detonation of reacting gases, with the reaction energy of the gases replaced by the absorbed laser energy. A shock wave propagates from the focal region into the undisturbed gas and absorption of energy from the laser beam drives the shock wave, causing it to spread. The motion of the luminous front has been measured as a function of time and two time regions have been identified. The plasma front has been found to move faster before the end of the laser pulse but its expansion is slowed down after the end of the pulse. This is schematically shown in Fig. 6.

Fig. 5. Schematic shape of laser-produced spark in air. The intense core is indicated by the white contour. Arrows indicate the propagation of the focused laser beam.

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S. N. Thakur and J. P. Singh 10

Distance 5 3

Laser off

1

1

2

4

6 8 10

20

40

Time (ns)

Fig. 6. Relative displacement of expanding luminous front as a function of time.

Spectroscopic investigations of laser induced spark in air show that its emission consists of spectral lines of N and O atoms as well as a strong continuum [24]. It has been found that in the early part of the development of the spark, the continuum is the dominant component of emission in addition to broad lines of ions and neutral atoms. When the spark has expanded and cooled, less broadened lines from neutral atoms are observed.

3.2. Plasma Production from Solid Targets When a high-power laser beam strikes a solid surface, it produces a plasma plume due to rapid melting and/or vaporization of the sample surface. The vaporization of a tungsten surface by a Nd:glass laser pulse was found to be accompanied by a shower of sparks characteristic of molten material expelled along with vaporization, whereas a plume of glowing material was emitted by a pulse from a ruby laser beam on a carbon target [25]. The plasma is produced by vaporization of the opaque target surface and subsequent absorption of laser light in this vaporized material. The phenomena observed in this interaction are in many ways similar to the phenomena accompanying gas breakdown, but the initial density of the material is much lower in the latter case. Plasma production studies are carried out at laser irradiances of the order of 109 W/cm2 or greater which produce a denser, more absorbing blow-off material. There is a great difference in the behavior of surfaces struck by laser pulses with millisecond duration as compared to those with pulse durations in the nanosecond region. The short pulses of very high power do not produce much vaporization, but instead remove only a small amount of material from the surface, whereas longer, low-power pulses produce deep, narrow holes in the target. For laser pulses of picosecond (ps) and femtosecond (fs) duration there is no reheating of the plasma due to absorption of laser radiation as in the case of nanosecond (ns) laser pulses (see Fig. 4). Thus the volume of plasma produced in the cases of ps and fs laser pulses is much smaller than in the case of ns laser pulses. The plasma plume produced by ns laser pulses gets elongated towards the incident laser beam as a result of reheating as shown in Fig. 5.

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The interaction of the laser with a target surface is considerably modified by the presence of material emitted from the surface by ns pulsed high power laser irradiation [26]. It exerts a high pressure on the surface and changes the vaporization characteristics of the surface. Since the laser flux density is very high, the ejected material can be heated further by absorption of incoming laser radiation. It becomes thermally ionized and opaque to the incident radiation. The absorbing plasma prevents light from reaching the target surface, which is effectively cut off from the incoming radiation for a large fraction of the laser pulse. At the end of the laser pulse, the blow off material becomes so hot that it begins to radiate thermally and some of this radiation may reach the surface, causing further vaporization. The temporal evolution of the depth vaporized by the high-power laser pulse is schematically shown in Fig. 7. The processes involved in vaporization by a ns laser can be understood in terms of a simple model [27]. It takes into account the pressure produced by a small amount of the blow-off material early in the laser pulse. This recoil pressure raises the boiling point of the target above its usual vaporization temperature. If the increase in vaporization temperature is sufficiently high, the surface will be prevented from vaporizing further and the material will continue to heat to a high temperature (above the normal vaporization temperature) as more and more laser light from the pulse is absorbed by the target surface. Eventually, the target surface will reach the critical point and at that point vaporization can occur. This model has been used to estimate the maximum depth at which the critical temperature is exceeded. At depths greater than this, removal of the material which is heated above the critical point will continue to exert a sufficiently high pressure so that no vaporization will occur. The heat will eventually be conducted into the interior of the target. This model does not take into account the shielding of the target surface from the incoming laser light as the blow-off material becomes hot, ionized and opaque. The absorption of laser radiation at a surface can produce large pressure waves in the target material. One mechanism is evaporation of material from the surface, with recoil of the heated material against the surface leading to motion of the target as a whole.

Laser pulse

Arbitrary units

Depth vaporized

10

20

30

Time (ns)

Fig. 7. Schematic representation of depth vaporized in a metal target as function of time showing effect of shielding by the blow-off material.

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There is another mechanism which does not necessarily involve removal of any material from the surface. In this case, as laser radiation is absorbed in a thin layer near the surface, the internal energy of that layer increases and it will expand by thermal expansion. The thermal energy is deposited very rapidly by a short pulse laser and the expanding layer of material exerts pressure on the adjacent layer, thus sending a compressive shock wave into the target.

3.3. Radiation from Laser Induced Plasmas The plasmas produced from solid targets also exhibit strong anisotropy in their expansion. The flow of the plasma has maximum velocity perpendicular to the surface, and it is independent of the angle at which the laser beam is incident on the surface. Photographic measurements determine the motion of the plasma which emits light by recombination or de-excitation of atoms [9]. X+ + e− → X + h X

+∗

+

→ X + h

X∗ → X + h

recombination

(4)

(de-excitation of ions)

(5)

(de-excitation of atoms)

(6)

A schematic diagram of expanding plasma is shown in Fig. 8 where the radius of its outer luminous edge is plotted as a function of time. The results of one the earliest studies of a plasma production by a Q-switched ruby laser from a carbon target indicated that a bright plume of emission began somewhat after the peak of the 45-ns laser pulse, reaching its maximum intensity about 120-ns after the start of the laser pulse [28]. Optical spectroscopic studies of laser-produced plasmas reveal both continuum and line radiation. The continuum radiation originates near the target surface and covers the spectral range from about 2 nm to 600 nm. The line spectrum shows the presence of highly ionized atoms as well as neutral atoms. The most highly ionized species are present near the plasma center, while lines of lower ionization and neutral species are observed near

Radius in arbitrary unit

Luminous edge

Laser pulse

0

50

100

Time (ns)

Fig. 8. Size of the luminous edge of expanding plasma produced by a short-pulse Q-switched laser as a function of time.

Fundamentals of LIBS

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the outer regions of the plasma plume. The spectra of neutral atoms are found to originate in a larger spatial region, indicating that neutral atom emission dominates after the plasma has expanded and cooled. The time variation of spectral line intensities indicates that the highest ionized states are present fairly early and lower ionized states appear later.

4. PROGRESS IN DETECTION OF LIBS The early measurements of spectral emission from laser induced plasmas employed photographic detection using prism or grating spectrographs. The system was far from satisfactory due to the fact that emission spectra consist of lines as well as continuum. Photometric detection with provision for time resolving the emission signals was not widely available and spectrally resolved light could be detected using a photomultiplier tube (PMT) only since the early 1980s [29]. The availability of a gated integrator made it possible to integrate the PMT current only during a time period selected by a gate pulse. The gate pulse is synchronized in time to the arrival of the laser pulse at the target. To suppress the detection of continuum from laser induced plasmas (LIP) present in the early part of the pulsed emission, the high voltage to the dynodes is gated so that full gain of the PMT is not realized until several microseconds after plasma formation. The disadvantage of gated PMT detection is that its gain does not remain constant. The other detector for spectrally resolved emission is a photodiode array (PDA) consisting of a series of photosensitive silicon detector elements known as pixels, lined up in a row. Time resolution in this case is achieved by time-gating the voltage applied to the microchannel-plate image-intensifier in front of the PDA. Time resolution of a few nanoseconds could be obtained with a PDA and its gain could be controlled by a factor of 106 . Due to the nature of the photodiode detectors, the PDA can only be cooled to temperatures reached by thermoelectric devices. At such temperatures, the dark current is on the order of 500 counts/pixel/sec, which is relatively high so that PDAs are best suited for medium and high intensity signals. The selection of the spectral range of plasma emission to be recorded in an experiment could be made by using (i) a narrow bandpass filter, (ii) a monochromator, or (iii) a spectrograph between the detector and the plasma plume. If only a single emission line is to be recorded at a time, the filter-detector combination or the monochromatordetector combination would be used depending on the presence of isolated or closely spaced emission lines. The simultaneous recording of several lines with high resolution would require a spectrograph-detector combination. The PMT can be used in all of these cases, and it is positioned behind the filter or the exit slit of the monochromator. In the latter case, the wavelength of the monochromator can be scanned to record several spectral lines in emission from the LIP provided the nature of plasma plume does not change from one pulse to another during the period of the scan. In the case of a sample containing many elements, either PMTs or PDAs can be used with the spectrograph to record several spectral lines simultaneously. A slit-PMT combination located in the focal plane of the spectrograph has to be used for each spectral line and the number of lines to be recorded is limited by the length of the focal plane. Another disadvantage of PMT detection is that continuous wavelength coverage of the spectrum is not possible. PDA detection is more versatile because it has continuous wavelength coverage over the

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S. N. Thakur and J. P. Singh Laser spark Spectrograph

PDA

Display

Pulsed laser

Time control

Fig. 9. Schematic diagram for spectral analysis of plasma plume with time-gated PDA.

array length and it can record a spectrum from a single laser shot. A typical detection system is shown in Fig. 9. There has been a tremendous growth in the range and sophistication of photo-detectors since 1990 due to progressive research and improvements in optical technology. The advent of high quality solid-state detectors has led to a quantum leap in applications of LIBS [30–34].

4.1. CCD and ICCD Detectors A charge-coupled device (CCD) is a micro-electronic device that is used in memory, signal processing and imaging applications. CCDs were initially conceived as an electronic analogue of the magnetic bubble device. To function as memory, there must be a physical quantity that represents a bit of information, a means of recognizing the presence or absence of the bit, and a means of creating and destroying the information. In the CCD, a bit of information is represented by a packet of electrons. These charges are stored in the depletion region of a metal insulator semiconductor (MIS) capacitor and moved about in the CCD circuit by placing the MIS capacitors so as to allow the charge to spill from one capacitor to the next and hence the name charge-coupled device. The CCD must perform four tasks in generating an image, viz. charge generation, charge collection, charge transfer, and charge detection. The first step occurs when free electrons are liberated due to incident photons. In the second step, the photoelectrons are collected in the nearest collecting site, referred to as pixels. Pixels are defined by electrodes called gates formed on the surface of the CCD. The third operation is accomplished by manipulating the voltage on the gates in a systematic way so that signal electrons move down vertically from one pixel to the next. At the end of the columns is a horizontal register of pixels. This register collects a line at a time and then transports the charge packets in a serial fashion to an output amplifier. The final operating step is performed by the CCD when the charge packet from the horizontal register is converted to an

Fundamentals of LIBS

17

output voltage by the on-chip amplifier. This voltage is amplified, processed and digitally encoded off chip and stored in a computer to reconstruct image on a television monitor. CCDs provide the multichannel advantage of array detectors and since it is a twodimensional array, it can record multiple spectra simultaneously. The large format, two-dimensional nature of CCDs is ideal for high-resolution or echelle spectroscopy. High-resolution spectra with overlapping orders are produced by a grating; each order contains information in successive spectral regions. The different order spectra are separated in the orthogonal direction by a cross-dispersing element. The resulting twodimensional spectrum is imaged onto the CCD. In this way, it is possible to obtain spectra covering the UV to the near IR range with 0.01-nm resolution. The CCD is the most sensitive multichannel detector. It can be cooled with liquid nitrogen to 140 K where the dark current is less than 1 electron/pixel/hour. At this temperature the detector can be exposed to a signal for hours without any significant contribution from the dark current. CCDs have a large dynamic range which is defined as the ratio of the smallest distinguishable measurable charge to the largest before saturation. A 16-bit converter used with the CCD will allow the measurement of signals that are 1/65536th of the full scale signal. CCDs also offer variable gain which is important in the measurement of weak signals. By increasing the gain to measure signal levels which are very close to the noise, the signal-to-noise ratio (SNR) can be improved while maintaining the same integration time. In other words one can achieve the same signal-to-noise ratio in less time. Experiments involving rapid kinetic measurements require an intensified CCD. The ICCD is a CCD with a multichannel plate intensifier attached. Light hits the photocathode on the front of the multichannel plate and is converted to electrons which are multiplied and hit a phosphor to produce photons which are detected by the CCD. Since the intensifier adds noise to the signal, causes blurring of the image and has a non-uniform photocathode response, ICCDs are used for time-domain measurements. The intensifier is gated and the time between the pulsing of the laser and opening of the multichannel plate can be set to within better than 5 ns accuracy.

4.2. The Spectrograph-Detector Combination LIBS makes use of the atomic emission from plasma plumes generated by a laser from solid, liquid or gaseous samples to identify the constituent elements present in the sample. An ideal experimental system should be capable of simultaneous multielemental monitoring of both high- and low-Z elements. In many applications, rapid, near real-time standoff detection capabilities are required. Typically a lens or a fiber optic collects the radiation from the plasma and couples it to a spectrograph. Emission from different atomic species may occur at different times during the pulsed laser spark and timeresolved detection is necessary to obtain a spectral fingerprint of the atomic species that are present in the sample. The wavelengths of the atomic emission lines most commonly analyzed with LIBS range from 190 to 850 nm. Detection below 190 nm is limited by atmospheric absorption but some elements with nonmetallic character have their strongest lines in the near vacuum ultraviolet (110–190 nm). Special efforts are required to minimize attenuation due to ambient air in the VUV region [35–38].

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S. N. Thakur and J. P. Singh

An ideal spectrograph-detector combination to detect all possible elements in a sample should have the following features: 1. Wide wavelength coverage (130–950 nm) to record simultaneously several elements. 2. High resolution (0.003–0.01 nm) to resolve closely spaced spectral lines and to avoid interferences. 3. A large dynamic range (6–7 orders of magnitude) for the detector to provide the optimum SNR for a large range of elemental concentrations. 4. High quantum efficiency of the detector particularly in the near IR and UV. 5. Short readout and data-acquisition time (less than the time lapse between laser pulses) for rapid analysis. The ICCD array detector coupled to a grating spectrograph or integrated into a compact high-resolution Czerny-Turner spectrometer has been widely used as the detector platform for a great variety of LIBS applications. In some applications ensembleaveraged spectra are used to smooth pulse-to-pulse variations frequently seen in LIBS [39,40]. In applications that require rapid sorting or emission from single particles, single-shot spectral measurements have to be made [41,42]. The use of non-ICCD arrays in LIBS is not common although correlation analysis in the identification of stainless-steel standards has been carried out using this detector [43,44]. The much lower costs of non-ICCD detectors are an important factor in their increasing use in research laboratories. The performance and sensitivity of a non-ICCD array and an ICCD array detector system have been compared in a recent publication [45]. Many applications of LIBS require remote and rapid multichannel analysis in hostile environments which implies large spectral coverage with high resolution [46]. Conventional Czerney-Turner spectrometers provide high resolution only in a limited spectral range and it takes many laser shots to make sequential measurements for the analysis of many elements. In contrast an echelle spectrometer coupled with an ICCD detector can cover a large spectral range. Bauer and coworkers [47] were the first to couple an ICCD camera to an echelle spectrometer, but they had to use a mobile mirror to obtain large spectral coverage. The efforts of several workers have shown that a large ICCD camera is necessary for wide spectral coverage without any moving parts [48–51].

5. APPLICATIONS OF LIBS The technological developments leading to the emergence of broadband high-resolution spectrometers has led LIBS into the 21st century with unprecedented capabilities to extract spectral information from microplasmas. It is now possible to detect almost all chemical elements in the periodic table by analyzing the UV, visible and IR emission prevalent in laser-generated sparks. Broadband high-resolution detection enables simultaneous analysis of multiple component elements of targeted samples. For the first time in the history of LIBS [52], there is a hope to obtain qualitative as well as quantitative information on complex biological molecules in a sample. LIBS-based technologies are developing rapidly. It is not inconceivable that it would be possible to develop LIBSsensors capable of the detection and identification of almost all forms of matter. In such

Fundamentals of LIBS

19

a case, it is difficult to make future predictions about the course of LIBS applications. In the following paragraphs, we will attempt to briefly summarize some novel features of this rapidly expanding field. The use of femtosecond laser pulses in LIBS experiments has led to better precision and better reproducibility in emission measurement as compared to nanosecond pulses. This improvement is attributed to high peak powers in the range of 1014 W/cm2 . Femtosecond lasers consistently create well-defined craters and lead to better ablative reproducibility than nanosecond lasers [53]. Extremely short fs-laser pulses account for some remarkable features as atomizers. In contrast to ns-lasers, the impact of the fs-laser energy on the sample has ceased before the plasma is formed. There is no shielding by the plasma and hence no dissipation of laser energy by it. The ablation threshold is lower than for ns-lasers and the energy is more localized in the sample leading to better spatial resolution [54]. Femtosecond-LIBS is being used for enhancement of signal and measurement of atom density distributions in the laser induced plasma [55,56]. The analysis of single microscopic particles, aerosols and cells has received great interest in recent years. A novel feature of LIBS for single particle analysis is its ability to provide elemental mass composition and size data for individual particles [57]. The presence of aerosols in ambient air has been cause of great concern because of their hazardous effects on human health, visibility, and climate change [58]. LIBS has found increasing application in studies on aerosols including effluent waste and real-time monitoring [59,60]. Bioaerosols which include pollen, fungi, bacteria, and viruses are found nearly everywhere; although their concentration is not high, they can cause disease or allergic reaction when inhaled even in very minute amounts [61–63]. The use of LIBS technology in field-portable instruments has given rise to a spurt of research activity in order to deal with social problems arising from criminal and terrorist activities [64–67]. LIBS is the preferred detection and identification technique because of its many characteristic features, including flexibility of point detection or operation in a stand-off mode, and fast, real-time response. The performance of LIBS can be enhanced with the use of an array of Geiger photodiodes as the detector in echelle spectrometers. Single photon detection in room temperature conditions is possible without complex gating-timing circuitry [68]. A compact design and high sensitivity would make this instrument very handy for standoff detection when low levels of plasma emission are to be collected. This development is very attractive in view of earlier work which shows that LIBS would provide an extremely useful tool for space and planetary exploration [69].

REFERENCES [1] P. D. Maker, R. W. Terhune and C. M. Savage, Quantum Electronics, Eds. P. Grivet and N. Bloembergen, Columbia Univ. Press, New York (1964) p. 1559 [2] S. Namba and P. H. Kim, Jap. J. Appl. Phys. 3 (1964) 536 [3] R. H. Fairbanks and C. M. Adams, Welding J. 43 (1964) 97s [4] N.G. Basov and O.N. Krokhin, Sov. Phys. JETP 19 (1964) 123 [5] H. W. Kroto, J. R. Heath, S. C. O’Brien, R. F. Curl and R. E. Smalley, Nature 318 (1985) 162 [6] F. Kokai, K. Takahashi, K. Shimizu, M. Yudakasa and S. Iijima, Appl. Phys. A69 (1999) S691 [7] D. A. Rusak, B. C. Castle, B.W. Smith and J. D. Winefordner, Crit. Rev. Anal. Chem. 27 (1997) 257

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[8] R. G. Meyerand and A. F. Haught, Phys. Rev. Lett. 9 (1963) 401 [9] L. J. Radziemski and D. A. Cremers. Eds., Laser-Induced Plasma and Applications, Marcel Dekker, New York (1989) [10] E. S. Dayhoff and B. Kessler, Appl. Opt. 1 (1962) 339 [11] J. A. Baker and C. W. Peters, Appl. Opt. 1 (1962) 674 [12] F. J. McClung and D. Weiner, IEEE J. Quantum Electron. QE-1 (1965) 94 [13] A. L. Bloom, Gas Lasers, Chapter 4, Wiley, New York (1968) [14] F. J. McClung and R. W. Hellwarth, Proc. IEEE 51 (1963) 46 [15] C. V. Shank, R. L. Fork and R. T. Yen, Picosecond Phenomena III, Eds. K. B. Eisenthal, R. M. Hochstrasser, W. Kaiser and A. Laubereau, Springer Verlag, New York (1982) p. 1 [16] F. Brech and L. Cross, Appl. Spectrosc. 16 (1962) 59 [17] W. Sdorra, J. Brust and K. Niemax, Mikrochim. Acta 108 (1992) 1 [18] C. W. Ng, W. F. Ho and N. H. Cheung, Appl. Spectrosc. 51 (1997) 976 [19] M. Martin and M. D. Chang, Appl. Spectrosc. 54 (2000) 1279 [20] Table of Laser Lines in Gases and Vapors, 3rd Revised and Enlarged Edition by R. Beck, W. Englisch, and K Gürs (Springer-Verlag, Berlin, 1980) [21] E. Damon and R. Thomlinson, Appl. Opt. 2 (1963) 546 [22] Y-L. Chen, J. W. L. Lewis and C. Parigger, J. Quant. Spectrosc. & Rad. Transfer. 67 (2000) 91 [23] R. W. Waynant and J. H. Ramsey, J. Opt. Soc. Amer. 55 (1965) 602 [24] S. L. Manel’shtam, Sov. Phys. JETP 20 (1965) 1344 [25] J. F. Ready, Effects of High-Power Laser Radiation, Academic Press, New York (1971) p. 95 [26] G. A. Askar’yan and E. M. Moroz, Sov. Phys. JETP 16 (1963) 1638 [27] J. F. Ready, J. Appl. Phys. 36 (1965) 462 [28] J. F. Ready, Appl. Phys. Lett. 3 (1963) [29] Y. Talmi, Ed, Multichannel Image Detectors, ACS Symp., Series No. 102, ACS, Washington, D.C. (1983) [30] M. J. Pilon, M. B. Denton, R. G. Schleicher, P. M. Moran and S. B. Smith, Appl. Spectrosc. 44 (1990) 1613 [31] S. Vasile, P. Gothoskar, R. Farrell and D. Sdrulla, IEEE Trans. Nucl. Sc. 45 (1997) 720 [32] Q. S. Hanely, C. W. Earle, F. M. Pennebaker, S. P. Madden and M. B. Denton Anal. Chem. 68 (1996) 661A [33] J. M. Harnly and R. E. Fields, Appl. Spectrosc. 51 (1997) 334A [34] F. M. Pennebaker, D. A. Jones, C. A. Gresham, R. W. Williams, R. E. Simon, M. F. Schappert and M. B. Denton, J. Anal. At. Spectrom. 13 (1998) 821 [35] C. J. Lorenzen, C. Carlhoff, U. Hahn and M. Jogwich, J. Anal. At. Spectrom. 7 (1992) 1029 [36] V. Strum, L. Peter and R. Noll, Appl. Spectrosc. 54 (2000) 1275 [37] M. A. Khater, J. T. Costello and E.T. Kennedy, Appl. Spectrosc. 56 (2002) 970 [38] S. Kaski, H. Hakkanen and J. Korppi-Tommola, Appl. Opt. 42 (2003) 6036 [39] F. Colao, R. Fantoni, V. Lazic and V. Spizzichino, Spectrochimica Acta B57 (2002) 1219 [40] A. K. Rai, F-Y. Yueh J. P. Singh and H. Zhang, Rev. Sci. Instrum. 73 (2002) 3599 [41] C. Aragon, V. Madurga and A. J. Aguilera, Appl. Surface Sci. 197–198 (2002) 217 [42] J. E. Carranza, B. T. Fisher, G. D. Yoder and D.W. Hahn, Spectrochimica Acta B56 (2001) 851 [43] I. B. Gornushkin, B.W. Smith, H. J. Nasajpour and J.W. Winefordner, Anal. Chem. 71 (1999) 5157 [44] S. I. Gornushkin, I. B. Gornushkin, J. M. Anzano, B. W. Smith and J. D. Winefordner, Appl. Spectrosc. 56 (2002) 433 [45] J.E. Carranza, E. Gibb, B. W. Smith, D. W. Hahn and J. D. Winefordner, Appl. Opt. 42 (2003) 6016 [46] P. Fichet, P. Mauchien and C. Moulin, Appl. Spectrosc. 53 (1999) 1111

Fundamentals of LIBS [47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57] [58] [59] [60] [61] [62] [63] [64] [65] [66] [67] [68] [69]

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H. E. Bauer, F. Leis and K. Niemax, Spectrochimica Acta B53 (1998) 1815 P. Lindblom, Anal. Chim. Acta 380 (1998) 353 S. R. Goode, S. L. Morgan, R. Hoskins and A. Oxsher, J. Anal. At. Spectrom. 15 (2000) 1133 S. Florek, C. Haisch, M. Okruss and H. Becker-Ross, Spectrochimica Acta B56 (2001) 1027 P. Fichet, D. Menut, R. Brennetot, E. Vors and A. Rivoallan, Appl. Opt. 42 (2003) 6029 L. Radziemski, Spectrochimica Acta B57 (2002) 1109 K. L. Eland, D. N. Stratis, D. M. Gold, S. R. Goode and S. M. Angel, Appl. Spectrosc. 55 (2001) 286 V. Margetic, T. Ban, F. Leis, K. Niemax and R. Hergenroder, Spectrochimic. Acta B58 (2003) 415 J. Scaffidi, J. Pender, W. Pearman, S. C. Goode, B. W. Colson, Jr., J. C. Carter and S. M. Angel, Appl. Opt. 42 (2003) 6099. O. Samek, F. Leis, V. Margetic, R. Malina, K. Niemax and R. Hergenroder, Appl. Opt. 42 (2003) 6001 D. W. Hahn and M. M. Lunden, Aerosol Sci. Technol. 33 (2000) 30 J. H. Seinfeld and S.N. Pandis, Atmospheric Chemistry and Physics: From Pollution to Climate Change, Wiley, New York (1998) J. P. Singh, F-Y. Yueh, H. Zhang and R. L. Cook, Process Control Qual. 10 (1997) 247 J. E. Carranza, B. T. Fisher, G. D. Yoder and D. W. Hahn, Spectrochimica Acta B56 (2001) 851 W. C. Hinds, Aerosol Technology: Properties, Behavior and Measurement of Airborne Particles, 2nd Ed., Wiley, New York (1999) A. R. Boyain-Goitia, D. C. S. Beddows, B. C. Griffiths and H. H. Telle, Appl. Opt. 42 (2003) 6119 A. C. Samuels, F. C. DeLucia, Jr., K. L. McNesby and A. W. Miziolek, Appl. Opt. 42 (2003) 6205 J. M. Anzano, I. B. Gornushkin, B. W. Smith and J. D. Winefordner, Polym. Eng. Sci. 40 (2000) 2423 R. T. Wainner, R. S. Harmon, A. W. Miziolek, K. L. McNesby and P. D. French, Spectrochimica Acta B56 (2001) 777 C. R. Dockery and S. R. Goode, Appl. Opt. 42 (2003) 6153 F. C. DeLucia, Jr., R. S. Harmon, K. L. McNesby, R. J. Winkel, Jr., and A. W. Miziolek, Appl. Opt. 42 (2003) 6148 R. A. Myers, A. M. Karger and D. W. Hahn, Appl. Opt. 42 (2003) 6072 A. K. Knight, N. L. Scherbarth, D. L. Cremers and M. J. Ferris, Appl. Spectrosc. 54 (2000) 331

Chapter 2

Atomic Emission Spectroscopy S. N. Thakur Laser and Spectroscopy Laboratory, Department of Physics Banaras Hindu University, Varanasi-221005, INDIA

1. INTRODUCTION The light emitted from a gaseous discharge when examined by a spectrometer to form a spectrum, is found to consist of discrete lines, bands and sometimes an overlying continuum. Discrete lines (and sometimes accompanying continuum) are characteristic features of emission from neutral atoms and ions in the discharge source. The spectral lines are characterized by three properties: wavelength, intensity and shape. These properties are dependent on the structure as well as the environment of the emitting atoms. Atomic emission spectroscopy can be used to determine the identity, the structure and the environment of atoms by analyzing the radiation emitted by them. From the measurement of wavelengths we may deduce the energy levels (or stationary states) of the atom and it provides experimental basis for the theories of atomic structure. If we know the characteristic lines emitted by an atom then their appearance in the spectrum establishes the presence of that element in the source. Measurement of intensities of spectral lines of different atoms in a given source provides information about their number densities. The physical parameters of the discharge source, such as temperature and pressure, affect the intensities and also the shape of spectral lines and these parameters can be determined by analyzing the shapes of the spectral lines. The Bohr theory of hydrogen in 1913 established the first link between the spectra and structure of atoms. The theoretical developments in quantum mechanics during 1920s have their roots in the accurate experimental measurements on the fine structure and the hyperfine structure of spectral lines. The experimental measurement of Lamb shift in the spectrum of hydrogen atom in 1947 added a new dimension to theoretical physics. In recent years, the availability of fast computers has established a high degree of cooperation between theory and atomic spectroscopy. The aim of this chapter is, however, not to discuss the details of atomic structure but to provide the basis for the other two applications of atomic spectroscopy, namely the identification of atoms together with their relative abundance and the determination of the physical conditions of plasma discharge in which these atoms are located. In the following sections we briefly discuss the measurement of spectral lines, the electronic structure of atoms and nomenclature of atomic states, the radiative transitions in atoms Laser-Induced Breakdown Spectroscopy Jagdish P Singh, Surya N Thakur (Editors) © 2007 Elsevier B.V. All rights reserved.

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and intensities of spectral lines. The environment of the atom affects its stationary states by the presence of electric and magnetic fields due to moving electrons and ions in addition to the electron-atom, ion-atom and atom-atom collisions which result in the broadening of spectral lines emitted by the atom. In the case of plasma, Stark broadening is the major cause of change of atomic lineshapes which depend on the electron density and temperature of the plasma. The last section contains a brief account of application of atomic emission spectroscopy of the light emitted from a plasma source.

2. MEASUREMENT OF SPECTRAL LINES The first step is the recording of the spectra using a prism or grating as the dispersing element to spread the light spatially according to its wavelength. A typical spectrometer is shown in Fig. 1. A narrow slit S allows the light from the source to be collimated by lens L1 so as to form a beam of parallel rays incident on the dispersing element. Parallel rays of light corresponding to different wavelengths come out at different angles from the dispersing device and are focused at different points in the focal plane of lens L2. The capacity of the spectrometer to separate two closely spaced wavelengths is known as the spectral resolving power and depends on the narrowness of the slit for a given dispersing device. A narrow exit slit can be placed in the focal plane of lens L2 with a photomultiplier tube behind it and the spectral lines can be recorded by rotating the dispersing device. Alternatively a photographic plate (or a CCD plate) can be placed in the focal plane of lens L2 to record many spectral lines simultaneously. Intensity measurements from photographic plates are cumbersome and in modern spectrometers photomultiplier tubes or CCD plates are used for more reliable intensity measurements. In case a concave grating is used as the dispersing element, lenses L1 and L2 are not required because the concave grating acts as its own collimator and focusing lens. The wavelengths of spectral lines have to be measured accurately. The primary standard is the red line of the isotope of Krypton Kr 86  whose vacuum wavelength is 605780210 × 10−10 meter. A number of spectral lines measured interferometrically against the primary standard have been accepted as secondary standards. These are mostly lines of neon, argon, iron, thorium etc and are updated from time to time by a commission of the International Astronomical Union [1]. There are also many tertiary standards and wavelengths of many atoms [2] quite accurate enough to calibrate the

Dispersing element L1

S

L2

Focal plane

λ2 λ1

Fig. 1. Schematic diagram of a prism or grating spectrometer.

Atomic Emission Spectroscopy

25

spectra recorded on any spectrometer. The unit of wavelength is nanometer (nm) but many spectroscopists prefer Angstrom (A)

3. ELECTRONIC STRUCTURE OF ATOMS An atom consists of positively charged nucleus and a number of negatively charged electrons. In a neutral atom, the total negative charge of all the electrons is equal to the total positive charge of the nucleus. The forces holding the atom together are predominantly electrostatic, consisting of attractions between each electron and the nucleus and repulsions among all the atomic electrons. A simple theory of electrostatics, however, fails to account for the stability of atoms and for the characteristics of their spectra. The early attempts of Bohr and de Broglie to give a plausible theory of atomic structure led to the much more sophisticated, probabilistic quantum mechanics of Heisenberg, Schrödinger and others. The object of the theory of atomic structure is the study of stationary states of isolated atoms. At first sight, such a study seems to be unrealistic since an atom is never totally isolated and in order to be observed it must interact with photons. However, in conditions of low pressure and low photon density, the effects of atom-atom and photon-atom interaction are weak enough to be neglected. Even in the case of an isolated atom, its energy depends not only on the charge of the nucleus but on its volume and the charge distribution inside this volume. Therefore the problem is very complicated and cannot be solved without approximations. The first approximation is to consider the nucleus as a point charge with infinite mass. The relative velocities of electrons in atoms are small enough to be neglected and the interactions can be described as electrostatic Coulomb. To reproduce important experimental features of atomic spectra, it is necessary to introduce a magnetic term: the spin-orbit interaction. As regards the remaining effects, they are small enough to be introduced by means of perturbation theory. Although we do not want to get involved with the mathematical details of quantum mechanics, it is necessary to use its language and its results to describe atomic structure. The atom is described by the fundamental differential equation of quantum mechanics called Schrödinger equation H = E

(1)

In Eq. (1),  is a function of the coordinates of the system called the atomic wavefunction and characterizes the state (electronic configuration) of the atom. The H is a differential operator called the Hamiltonian operator of the atom and is composed of the operators for kinetic energy of electrons and the nucleus, the attractions between electrons and the nucleus and the inter-electronic repulsions. Except in the case of hydrogen atom Eq. (1) does not have an exact solution and the average energy of atom in the state  is given by  ∗  Hd E = − (2)  ∗ d − In Eq.(2) ∗ represents the complex conjugate of the wavefunction  and d represents the volume element in three dimensional space. The Schrödinger equation is insoluble for all but hydrogenic atoms and the energy E is based on the approximate solutions for

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S. N. Thakur

atoms with two or more electrons. It is within the framework of these approximations that we will describe the structure of atoms with many electrons.

3.1. Hydrogenic Atoms The atomic wavefunctions nm for hydrogenic atoms are expressed in terms of the coordinates of the system and depend on three parameters called quantum numbers n,  and m which result from the exact solution of the Schrödinger equation. The principal quantum number n determines the average energy and radius of the hydrogenic atom in the corresponding stationary state, whereas  and m determine the average value of angular momentum and its component along a fixed direction respectively. For a given value of n the value of  for the wavefunction can be: 0 1 2  n − 1. Similarly the values of quantum number m for a given  are: − − + 1 − + 2  0 1 2  − 1 . Thus, for any value of angular momentum quantum number , there are 2 + 1 possible values of magnetic quantum number m and for a given value of n there are n different values of . The wavefunction nm is said to represent an atomic orbital. Atomic orbitals belonging to a given value of n define a shell and those associated with a given value of  constitute a subshell. Hydrogenic orbitals belonging to the same shell have the same energy and are said to be degenerate. Atomic orbitals corresponding to  = 0 1 2 3 4  5 are called s, p, d, f, g, h .orbitals respectively. Thus the atomic orbitals associated with n = 3 are, 3s, 3p, 3d and those for n = 5 are, 5s, 5p, 5d, 5f and 5g. Dirac included the effects of special relativity in the solution of Schrödinger equation and was able to derive not only the quantum numbers n, , and m, but also a fourth quantum number s, called the spin quantum number. The spin quantum number is related to the magnetic moment of the electron and it can only have two values +1/2 and −1/2. Thus an electron occupying a hydrogenic orbital nm can have either s = +1/2 or s = −1/2. The four quantum numbers n, , m and s play a fundamental role in the electronic configurations of atoms with many electrons. If we neglect the fine structure, the stationary states of hydrogen atom are as shown in Fig. 2. The zero of energy is defined for the electron and proton at rest at infinite separation. The energy corresponding to the principal quantum number n is −RH /n2 where RH is the Rydberg constant for hydrogen. The ground state energy of hydrogen atom is −RH corresponding to n = 1. The separation between consecutive energy states decreases as n increases till the ionization limit corresponding to the complete removal of the electron. The states of positive energy correspond to proton + electron + kinetic energy, the energy is no longer quantized and there exists a continuum of states. The transition of atom between a pair of discrete states is possible under certain conditions resulting in a spectral line. Transitions between the continuum and a discrete state and transitions within the continuum give rise to continuum and are known as free- bound and free-free transitions respectively.

3.2. Many Electron Atoms The model for the ground state of a neutral atom of nuclear charge Z is constructed by assigning Z electrons to the hydrogenic orbitals in such a way that electronic

Atomic Emission Spectroscopy

27 Continuum

Ionization limit

3

2

Emission

Absorption

n=1

Fig. 2. Energy level diagram of hydrogen atom.

configuration of lowest potential energy is obtained. In a many- electron atom, there are repulsions between each pair of electrons but the hydrogenic orbitals do not account for this. The degeneracies of occupied orbitals of the same n but different  may be removed in many-electron atoms. However, the degeneracies of orbitals having the same values of n and  but different values of m are not removed in the absence of external magnetic fields. The orbital capacities and order of assigning electrons to atomic orbitals are governed, respectively, by the Pauli’s exclusion principle and the Hund’s rule of maximum multiplicity. In its simplest form, the exclusion principle states that no two electrons in the same atom can have four identical quantum numbers. Thus an orbital specified by n, , m can accommodate a maximum of two electrons one with s = +1/2 and the other with s = −1/2. The Hund’s rule has its basis in Coulomb’s law and states that in the case of degenerate orbitals, the configuration of minimum potential energy would be obtained by allowing the electrons occupying these orbitals to stay as far apart as possible. Thus in filling degenerate orbitals, each orbital accepts one electron before double occupancy (pairing) occurs, because the separate orbitals occupy different regions of space whereas, two electrons, paired (opposite spins) in one orbital are close together, resulting in greater repulsive potential energy. The application of Pauli’s principle together with Hund’s rule of maximum multiplicity has been quite successful in predicting the ground state electronic configurations of lighter atoms. This is not so, however, in the assignment of ground state electronic configurations of heavier atoms. Thus iron (Fe) with Z = 26 would be assigned a configuration 1s2 2s2 2p6 3s2 3p6 3d8 with two unpaired electrons in d orbitals. This configuration is not consistent with chemical and magnetic properties of iron which require four unpaired electrons in 3d orbitals with the ground state configuration 1s2 2s2 2p6 3s2 3p6 3d6 4s2 . This discrepancy is due to repulsions produced on the n = 3 electrons by the inner shell

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S. N. Thakur

Table 1. The Periodic Table 113 114………118 7p Fr Ra Ac 7s Tl Pb Bi Po At Rn 6p Cs Ba La 6s In Rb Sr 5s Ga K Ca 4s Al Na Mg 3s B Li Be 2s H He 1s

Rf Ha Sg Ns Hs Mz………...112 6d Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lw 5f Hf Ta W Re Os Ir Pt Au Hg 5d Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 4f Sn Sb Te I Xe 5p Y Zr Nb Mo Tc Ru Rh Pd Ag Cd 4d Ge As Se Br Kr 4p Sc Ti V Cr Mn Fe Co Ni Cu Zn 3d Si P S Cl Ar 3p C N O 2p

F

Ne

*Atomic weight increases as we read the table from left to right and then upward

electrons. The occupied d orbitals are repelled to energies greater than that of the unoccupied 4s orbital and as a result the 4s orbital becomes occupied by two electrons in preference to the 3d orbitals. A modern form of periodic table due to LonguetHiggins [3] is given in Table 1 where each group contains elements with similar electron configuration.

3.3. Classification of Electronic States Atomic electrons produce an orbital magnetic moment as a result of their orbital motion, which is located at the nucleus and is directed at right angles to the orbit plane, collinearly with the orbital angular momentum vector associated with the occupied orbital. The magnitude of magnetic moment due to a single electron is directly proportional to its orbital angular momentum . Similarly the magnetic moment due to the spin angular momentum is located at the position of the electron and is directed either up or down along the direction of the spin angular momentum vector of the electron depending on whether s = +1/2 or s = −1/2. The individual spin and orbital magnetic moments may be added to give a resultant magnetic moment for the atom. This is synonymous with the addition of spin and orbital angular moments of electrons in the atom. There are two ways in which individual electronic  and s values may be added vectorially to give the resultant atomic angular momentum quantum number J. In the first scheme the  and s of each electron may be added vectorially to give a resultant

Atomic Emission Spectroscopy

29

one-electron angular momentum quantum number j. The j values of all atomic electrons may then be added vectorially to give J. This is known as j-j coupling scheme. In the second scheme, the individual  values of each electron may be added vectorially to give a total orbital angular momentum quantum number L. The s values of of each electron may similarly be added vectorially to give a total spin angular momentum quantum number S. The values of L and S may then be added vectorially to give J for the atom. This is known as L-S (or Russell Saunders) coupling scheme. The two addition methods for electronic angular momenta correspond to different physical situations. In the L-S coupling, L, S as well as J can be used to describe electronic states but in the j-j coupling, L and S have no physical meaning and J is the only good angular momentum quantum number. The orbital angular momentum vector of an electron is located at the nucleus whereas the spin angular momentum vector is located, roughly, on the electronic orbit, the coupling of  and s is very weak unless the electron spends a considerable portion of its time near the nucleus. The probability density of electron is large near the nucleus in atoms with high nuclear charge (heavy atoms) which exhibit appreciable spin-orbit coupling and their atomic angular momentum is given by j-j coupling. On the other hand, interelectronic repulsion is strong if several electrons have high probability densities in the same region of space. If the atom has a nucleus of low nuclear charge, all the individual electronic orbital angular momenta, located at the nucleus, will couple strongly and similarly will the individual spin momenta, located on the electronic charge distribution. The coupling of  and s will be secondary to the coupling of ’s and s’s and hence L-S coupling will adequately describe the situation. For most atoms, even when appreciable spin-orbit interaction occurs, the L-S coupling is retained, with an appropriate perturbation treatment to account for the interaction. In the remainder of this discussion we will describe the stationary states of atoms in the framework of L-S coupling. The general nomenclature of an atomic state is based on the representation 2S+1 LJ , where the state is labeled as S, P, D, F, G corresponding to L = 0, 1, 2, 3, 4 respectively. Thus, with L = 2, and S = 1, the values of J will be 3, 2 and 1 and the resulting states will be represented as 3 D3  3 D2 and 3 D1 respectively. It is to be noted that the total angular momentum quantum number J and the corresponding magnetic quantum number M are always good quantum numbers, irrespective of the coupling scheme and the atomic wavefunction is represented as JM . The corresponding atomic state is 2J + 1 fold degenerate in the absence of external electric or magnetic field. For further information on atomic structure the reader is advised to see some of the excellent books [4–9].

4. RADIATION FROM ATOMS The intensity of a spectral line depends on the atomic population of the initial level and also on the intrinsic probability of transition to the final level. The transition probability is defined in terms of Einstein’s A and B coefficients shown in Fig. 3 where E1 and E2 are two discrete quantum levels of the atom with populations of N1 and N2 atoms/cm3 respectively. The frequency of the spectral line resulting from a transition between the two levels is given by h 12 = E2 − E1

30

S. N. Thakur N2 ________________________________________________________ E2

ρ B12N1

ρB21N2

A21N2

N1 ________________________________________________________ E1

Fig. 3. Emission and absorption processes between a pair of energy levels.

There are three kinds of radiative processes that transfer atoms between the energy levels E1 and E2 : (1) An atom with energy E2 may spontaneously make a transition to energy state E1 with emission of energy h 12 The probability of this transition per second is A21 and the number of such transitions per second per cm3 is A21 N2 (2) Under the influence of external radiation of density  12  an atom may make a transition from state E1 to E2 with absorption of energy h 12 . The probability of this transition per second is B12 and the number of transitions is B12 N1 sec−1 cm−3 . (3) An atom in state E2 may undergo a stimulated (or induced) transition to state E1 in the presence of external radiation of density  12 . The probability of this transition is B21 per second and the number of transitions is B21 N2 sec−1 cm−3 . The Einstein’s coefficients for spontaneous emission A21 , stimulated emission B21 and absorption B12 , are intrinsic properties of the atom and they can, in principle, be calculated if the wave functions of the two states are known. If the energy states E1 and E2 are degenerate with degeneracy parameters g1 and g2 respectively then the relations between Einstein’s coefficients are as follows: g1 B12 = g2 B21 A21 = 8 h 3 /c3 B21

(3a) (3b)

where = 12 = 21 is the frequency of the spectral line resulting from the transition between the two states.

4.1. Electric Dipole Selection Rules If we assume the external electromagnetic radiation as a time dependent perturbation to the atom, it is easy to visualize that the perturbation should be as effective for the stimulated emission E2 → E1 as for the absorption E2 ← E1 , so that B21 = B12 if the two states are non-degenerate. The B coefficients can be calculated from quantum mechanics

Atomic Emission Spectroscopy

31

by treating the oscillating electric and magnetic fields as a time dependent perturbation leading to the following expression: B12 = 2 2 /30 h2  Mx2 12 + My2 12 + Mz2 12 

(4a)

where 0 is the permittivity of free space and Mx, My and Mz are the components of the transition moment vector M12 given by M12 =



 ∗ J2M2 er J1M1 d

(4b)

In Eq. (4b) J2M2 and J1M1 are the wavefunctions of the upper and lower states respectively e is the electronic charge and r is the operator corresponding to the displacement of electronic charge in the atom as a result of the transition (er is the dipole moment vector) and the transition between the two states is said to be allowed if the integral is different from zero. The quantum numbers that define the wavefunctions of the two states satisfy certain relations for a non- zero transition moment and these relations are called selection rules. The selection rules for the total angular momentum quantum number J for dipole allowed transitions are given by J = J2 − J1 = 0 +1 −1

(5a)

If the spin-orbit coupling is so weak that the orbital motion is practically the same as in the absence of electron spin then the orbital quantum number L retains its significance and the corresponding selection rules are L = L2 − L1 = +1 −1

(5b)

M = M2 − M1 = 0 +1 −1

(5c)

The selection rules for M are

The selection rule M = 0 corresponds to emission of light from the atom with its electric vector oscillating along the z-axis. This radiation can not be seen along the z-direction and it gives rise to linearly polarized light when viewed along a direction in the x-y plane. The selection rules M = +1 and M = −1 correspond to right and left circular polarization when viewed in the z-direction and linearly polarized when viewed in a direction in the x-y plane. The selection rules for the quantum number M and the corresponding polarizations of the emitted radiation are physically meaningful if a fixed direction in space is defined. Thus the direction of the magnetic field in Zeeman effect or that of the electric field in Stark effect defines the polar axis for the atom. In the cases of scattering or fluorescence from the atom in the presence of a linearly polarized incident beam, the electric vector of the incident radiation can be used to define the polar axis.

32

S. N. Thakur

4.2. Parity Selection Rules The energy states of free atoms can be divided into two classes according to their parities. If the orbital angular momentum quantum number i of individual electrons is taken into consideration, the parity of the sum i is the parity of the resulting energy state. The energy state is said to have odd or even parity depending on the odd or even value of i . Odd parity of a state is indicated by a superscript ‘o’ such as 2 Po 3/2 . The parity property is well defined for an atom with any number of electrons with any kind of coupling of their orbital and spin angular momenta. The rigorous selection rule based on parity is known as Laporte rule: Only transitions between even and odd states are allowed for a dipole radiation. It can be shown that all the selection rules in Eq. (5) obey parity selection rule.

4.3. Forbidden Transitions Transitions which are forbidden in the dipole approximation may appear very weakly as quadrupole radiation or magnetic dipole radiation. The matrix elements for electric quadrupole radiation take the following form Q12 =



 ∗ J2M2 exy J1M1 d

(6a)

and it does not vanish when J2M2 and J1M1 have the same parity because the parity of the operator xy is clearly even. The selection rules for quadrupole allowed transitions are L = 0 +2 −2

J = 0 +1 −1 +2 −2

(6b)

The Laporte rule in this case states: Only transitions from even to even or from odd to odd terms (states) give rise to quadrupole radiation. Magnetic dipole transitions appear with the same strength (with intensity 10−6 times the intensity of electric dipole allowed transitions) as electric quadrupole transitions. They result from the fact that radiation field produced by a system of moving charges in the atom cannot be adequately described in terms of electric dipole, quadrupole, octopole radiation but also need terms containing magnetic dipole, quadrupole etc. The selection rules for magnetic dipole allowed transitions are: L = 0

J = +1 −1

(6c)

The forbidden transitions which are forbidden as dipole radiation only to a first approximation may become allowed under special conditions. Inter-combination lines resulting from transitions between states of different multiplicities fall in this category. The electric dipole selection rules hold strictly only for free atoms and external fields due to ions in a crystal or those in a discharge or even the fields resulting due to neutral atoms can give rise to forced electric dipole transitions of observable intensity.

Atomic Emission Spectroscopy

33

4.4. Line Strength If we adopt  the following notation for the x-component of the electric dipole transition moment:  ∗ 2 ex 1 d = ex12 , Eq.(4a) takes the following form B21 =2 2 e2 /30 h2  x12 2 + y12 2 + z12 2 

(7a)

The electric dipole line strength S of the transition is defined as S = S21 = S12 = e2 x12 2 + y12 2 + z12 2 

(7b)

From Eq.(3) and (7b) we get the following relations for non degenerate states B21 = 2 2 /30 h2 S12

(7c)

A21 = 16 3 3 /30 hc3 S12

(7d)

If the two states involved in the transition are degenerate and the degeneracies of E1 and E2 are g1 and g2 respectively then the line strength is given by S = S21 = S12 = e2

g1 g2 M1

M2

x12 2 + y12 2 + z12 2 

(8a)

The degeneracies will cause the atomic populations to be divided amongst g1 and g2 number of sublevels for E1 and E2 respectively and the number of atoms in E1 will reduce by a factor 1/g1 and those in E2 by a factor 1/g2 . The expressions for Einstein’s coefficients will take following form: B12 = 2 2 /30 h2 S/g1

(8b)

B21 = 2 2 /30 h2 S/g2

(8c)

A21 = 16 /30 hc S/g2

(8d)

3 3

3

4.5. Oscillator Strength The classical model of emission from an atom has the electron performing simple harmonic motion at a characteristic frequency v but with amplitude decreasing with time because of the energy radiated away. The emission of radiation acts as the damping agent for the electron’s motion and the damping constant is given by  = 2 e2 2 /30 mc3 

(9)

where m is the electron mass. The lifetime of a classical oscillator is the inverse of damping constant  = 1/ and it is of the order of 10−8 sec for emission in the visible region.

34

S. N. Thakur

When light passes through an absorbing medium, the energy absorbed by atoms is proportional, to the thickness of the medium and also to the incident flux of light. The transmitted light intensity at frequency across a thickness  of the absorbing medium is given by I = I0 exp −kv 

(10)

where k is in units of cm−1 and is called the absorption coefficient. Absorption is also expressed in terms of atomic absorption coefficient (or atomic cross section) 

= k /N1  where N1 is the population density of the lower energy state E1 .  has the dimension of area. It is to be noted that k is dependent on the profile of the spectral transition (line shape) and its relation with B12 is given by  (11a) k dv = h 0 /cN1 B12 where the integration is to be carried over the line profile and v0 is the frequency corresponding to the peak of the line profile. If the number density N2 of atoms in the upper energy state E2 is appreciable, there would be stimulated emission, putting photons back into the incident beam and the relation between k and B12 would become  (11b) k dv = h 0 /cN1 B12 1 − g1 N2 /g2 N1  The quantum mechanical and classical models of light emission are related through a quantity called oscillator strength. This is achieved by equating the light absorbed by N1 atoms in the transition E2 ← E1 to that absorbed at the same frequency by N classical oscillators so that N = f12 N1 . The oscillator strength ‘f’ is related to the Einstein’s coefficient B by f12 = 4mh0 /e2  12 B12

(12a)

If we replace 0 by 1/4 the relation between f and B (in c.g.s. units) becomes f12 = mh/ e2  12 B12

(12b)

The normal meaning of ‘f’ is the oscillator strength f12 but the emission ‘f’ value f21 is a negative quantity given by f21 = −g1 /g2 f12

(12c)

The oscillator strength ‘f’ is also interpreted as the effective number of electrons per atom for a particular transition. Thus f should be approximately 1 for one valence electron atoms (hydrogen and alkalis) and 2 for the alkaline earths. Since each electron can participate in several different transitions, the total oscillator strength is accordingly split between several spectral lines. Thus f represents the fraction of the available electrons participating in a particular transition and has values in the range 1 to 0.01 for strong spectral lines.

Atomic Emission Spectroscopy

35

Ec

Eu

Ei

El

Fig. 4. Transitions involved in f-sum rule.

The f-sum rule states that the sum of all transitions from a given state should be equal to the number of optical (or valence) electrons z. Thus  u fiu + l fil + fic dc = z (13a) c

where iu refers to transitions upwards from a particular state Ei , il refers to transitions downwards and ic refers to transitions to the continuum Ec as shown in Fig. 4. The downward il transitions represent stimulated emission and if we use oscillator strengths for absorption only, Eq (13a) takes the following form:  u>i fiu − gl /gi l 0 and hence C4 > 0. A spectral line broadened by quadratic Stark effect becomes asymmetric and its center shifts to longer wavelengths.

5.2. Theory of Stark Effect We present a qualitative explanation of Stark broadening in terms of interaction of the emitting atom with fast moving electrons and the slowly moving ions in plasma.

Atomic Emission Spectroscopy

39

5.2.1. Impact approximation The atom emitting radiation is assumed to collide with electrons and the duration of collision tc is taken to be extremely small in comparison to the time between two successive collisions. The collision is assumed to cause a phase change in the optical wave train emitted by the atom before and after the collision and not to stop it altogether by knocking out the atom from its excited state. The interaction between the electron and the emitting atom is thus regarded as an optical collision which results in a sudden phase change of the light wave emanating from the atom and there is no effect on the state of the atom. Let us assume that the colliding electron is moving with velocity u¯ along the x-axis and the impact parameter for collision with the atom is . Since there is a frequency change  r in the radiation emitted by the atom when it is located at a distance ‘r’ from the electron, the phase change of the light wave during a small interval of time dt would be 2  rdt and the total phase change during the collision period tc is   =



tc 0

2  r dt =



 −

2  r dt

(18a)

Where  r is given by Eq.(17b) and limits on the integral have no effect on the collision because  r is zero outside the interaction duration tc . From Fig. 5 we have = r cos , x = rsin = tan  dt = dx/¯u =  /¯usec2 d and from Eq.(17b) we have  r = Cn /r n = Cn cosn / n Hence from Eq.(18a) we get   = 2 Cn /¯u



/2 − /2

cosn−2 d = 2 Cn /¯u n−1 an

(18b)

Where an is a numerical factor of the order of one and depends on the value of n. In a well known theoretical treatment of collision induced phase change of light wave by Weisskopf [10], the coherence of the wave train is completely destroyed if the phase change   = 1 and the corresponding impact parameter 0 known as Weisskopf radius is obtained from Eq.(18b) 0 = 2 Cn /¯u1/n−1

(19a)

According to Weisskopf theory the line shape is Lorentzian with a FWHM given by w = 0 2 u¯ Ne = 2 Cn /¯u2/n−1 u¯ Ne where Ne is the number density of electrons in the plasma. x

ρ θ

u-

r

Fig. 5. Optical collision of electron with the radiating atom.

(19b)

40

S. N. Thakur

Experimentally it is found that atomic lines in plasma sources are generally red shifted with a Lorentzian profile whereas the Weisskopf theory leads to a broadened but unshifted line. This comes from the fact that the theory ignores small phase shifts originating from impact parameters > 0 . Lindholm [11] was the first to include the contributions of small as well as large phase shifts and later Foley [12] and Anderson [13] included the effects of inelastic collision to obtain a Lorentzian profile with phase shift  and width ‘w’. The spectral line shape in the light of these modifications is given by I  = I0 / − 0 − 2 + w/22   D Where  = 2 N¯u sin   d and

w = 4 N¯u



(20a) (20b)

0

D 0

1 − cos    d

(20c)

The upper limits of the above integrals D is called Debye shielding radius such that the emitting atom is shielded from the effects of all charged particles located at distances greater than or equal to D . This limit on is necessary to avoid the electron-atom interaction time  /¯u from becoming very very large. Thus the upper limit for the duration of electron-atom interaction is D /¯u and its reciprocal is the plasma frequency p . The Debye radius is given by D = 0 kT/2e2 Ne 1/2 ≈ 50T/Ne 1/2

(20d)

where T is in Kelvin (K), Ne is in m−3 and D is in m. For very large values of    is very small and sin  ≈  has the same sign as Cn for all electrons interacting with the atom, but for  in the neighborhood of (both below and above), sin  has both positive and negative values and its average contribution to  is very small (see Eq.(20b). Thus, for 0 ≤  ≤  sin  has a non- zero average and it accounts for a shift of line center towards red if Cn >0 and towards blue if Cn < 0. When the values of are very large cos  ≈ 1 and its contribution to line width w is extremely small (see Eq.(20c). For  →  cos  → −1 and contributes a large value to ‘w.’ The above description leads to the conclusion that electron collisions for which ≈ 0 are responsible for most of the line broadening (w) while collisions with >> 0 contribute greatly to the shift of the line center . We can group electron-atom collisions into two types: weak collisions (large , small ) produce line shift and the strong collisions (small , large ) produce most of the broadening.

5.2.2. Quasi-static approximation The interaction between slowly moving ions and radiating atoms can be approximated by a perturbation which remains nearly constant over the whole time that the atom is radiating. Following Holtsmark [14] the motions of ions are neglected and their perturbing action is included in an electric field F that produces static Stark effect. In the next step the statistical average of Stark effect over various values of the ion field-strength F, is taken. In the final step of the calculation, each element of the line

Atomic Emission Spectroscopy

41

is considered to be broadened and shifted by the electron impacts. The quadratic Stark effect produces asymmetrically broadened lines whereas the linear Stark effect gives rise to symmetrically broadened line shapes. A large amount of theoretical and experimental work has been carried out in the case of spectral line broadening by charged particles. With the help of existing theoretical models it is possible to calculate line profiles that fit the experimental ones [15–17].

6. APPLICATIONS In the preceding sections we have seen that the intensities, shapes and widths of atomic lines depend on the atomic structure as well as on the temperature, pressure and electron density of the discharge plasma. Analysis of the spectral lines can give information about the physical state of the the emitting gas without in any way interfering with the plasma The use of lasers based optical techniques in recent years have replaced spectroscopy as a diagnostic tool to some extent. Nevertheless spectroscopy still plays a major part in determining the physical processes going on in the plasma. It is the most reliable method of detecting and identifying trace elements in a source. In the following sections we present a brief account of these applications.

6.1. Determination of Electron Temperature When the temperature of a molecular vapour is increased, molecules tend to dissociate into atoms and atoms into ion plus electrons; some of the molecules, atoms and ions are excited to higher energy states; and the kinetic energy of all these particles and of the free electrons increases. The spectroscopic determination of electron temperature of a source of radiation is based on the assumption that local equilibrium conditions must exist in each small volume that contributes to emission. Complete thermodynamic equilibrium (TE) exists when all forms of energy distribution are described by the same temperature. In the following sections we first discuss the approximate conditions that may prevail in plasma and then describe methods of determining temperature of the source from the measurements of spectral lines emitted by it.

6.1.1. Temperature and Equilibrium Maxwell’s distribution for velocities of particles of mass ‘M’ gives the number of particles dN with velocity between v and v + dv in terms of their number density N and temperature T as follows: dN v = N M/2 kT3/2 exp−mv2 /2kT 4 v2 dv

(21a)

The Boltzmann distribution of particles Nj having excitation energy Ej is Nj = N gj /UT exp−Ej /kT where UT = gj exp−Ej /kT is called the state sum or partition function.

(21b)

42

S. N. Thakur

The condition of equilibrium for ions, electrons and neutral atoms is given by Saha’s equation Ne N i 22 mkT3/2 = N0 h3

Ui T exp −/kT U0 T

(21c)

where Ne is the number density of electrons and Ni and N0 that of the ions and neutral atoms respectively, regardless of the energy levels they occupy, m is the mass of electron and  is the ionization energy of atom. The factor of 2 represents the state sum for the two possible spin states. The equilibrium distributions for kinetic energy, excitation energy and ionization energy are represented by Maxwell, Boltzmann and Saha equations respectively and it may happen that there is equilibrium distribution of one of these forms of energy but not for the others. In that case the temperature parameters for Eq. (21a), (21b) and (21c) are all different. Complete thermodynamic equilibrium exists when all forms of energy distribution are described by the same temperature parameter. Thermodynamic arguments require that for equilibrium to hold, for every photon emitted by the system, a photon of the same energy be absorbed and for every excitation by electron collision there must be a de-excitation by electron collision. In practice, however, photons do leak out from the plasma, no matter how large or dense the plasma is, otherwise we would be unable to observe the plasma. Thus the condition of near thermodynamic equilibrium requires that such losses be small compared to the total energy. Many plasmas can be described by a state known as local thermodynamic equilibrium (LTE), in which it is possible to find a temperature parameter for every point in a region of space that fits the Boltzmann and Saha relations for the population density of excited and ionic states and the Maxwell distribution of velocities among the electrons. The criterion for LTE is that collisional processes must be much more important than radiative, so that the deficit of radiative energy is extremely small. In other words the probability of de-excitation by inelastic collision for an excited state must be very large compared to that of spontaneous emission. This is possible at very high electron densities in the plasma such that Ne >> A21 /v21 

(22a)

If S is the line strength, the excitation cross-section corresponding to electron velocity v at threshold is given by 12 v ∼ e/4 0 h2 S/v2 . Putting this value of 12 in Eq. (22a) makes the right hand side proportional to vA21 /S. Since A21 is proportional to S 3 and v is proportional to T1/2 , the value of Ne is proportional to 3 T1/2 and numerical relationship for LTE is given by Ne >> 16 × 1012 T1/2 E2 − E1 3 cm−3

(22b)

where T is electron temperature in K and E2 − E1 is the energy difference in electron volts between the two neighboring states with an allowed transition. It is possible to determine the relative population of atoms in various excited states even in the absence of LTE provided the collisional cross-sections and radiative transition probabilities are known. Another approximation is to assume that level E2 in Fig. 3 is populated entirely by electron collision and depopulated entirely by spontaneous

Atomic Emission Spectroscopy

43

radiation. This is called coronal equilibrium (CE) since it is applicable to sun’s corona where temperature is high 106 K, electron density is low 108 cm−3  and radiation density is also low. The populations of higher states are much lower in CE than in LTE. Coronal equilibrium can hold only if Ne is below a critical value and it holds good only for the lower excited states. It is quite possible that the atomic populations of higher states follow LTE while those of the lower states follow CE.

6.1.2. Temperature from Relative Intensities of Lines The method for determination of temperature in LTE plasma is based on the fact that the number densities in various excited states follow Boltzmann distribution. The temperature in terms of relative intensities of lines from the same element and same state of ionization is given by kT = E2 − E1 / loge I1 31 g2 f2 /I2 32 g1 f1 

(22c)

where I1 is the total intensity integrated over the line profile, 1 is the wavelength and f1 the oscillator strength of the spectral line with excitation energy E1 and I2  2 and f2 are the corresponding quantities for the line with excitation energy E2 , The statistical weights for energy states E1 and E2 are g1 and g2 respectively. Relative intensities can be measured with an accuracy of better than 10 per cent but errors in the oscillator strengths are more. Since kT is of the order of the largest energy separation E2 − E1 between excitation energies of non-resonance lines, the uncertainty of oscillator strengths is reflected in the errors associated with the temperature. The method described above based on the relative intensities of lines from the same atom and ionization stage generally leads to inaccurate temperature. The main reason being the small separation between E1 and E2 which is typically smaller than or equal to kT and it renders the line-intensity ratio somewhat insensitive to temperature variations. This shortcoming is removed if spectral lines from successive ionization stages of the same atom are compared. The effective energy difference is enhanced by the ionization energy leading to increased sensitivity to temperature changes. In LTE the relation between relative intensities and the source temperature is given by I /I = f g 3 /fg 4 3/2 a03 Ne −1 kT/EH 3/2 exp E + E − E − E /kT 3

(22d)

where primed quantities correspond to the spectral line from the higher ionization stage and E is the reduction of the ionization energy E of the lower ionization stage due to plasma effect, a0 is the Bohr radius and EH is the ionization energy of hydrogen.

6.1.3. Temperature from Doppler Profile The most reliable spectroscopic technique of measuring kinetic temperature of atoms and ions is based on the measurement of the widths of Doppler broadened spectral lines. In the case of Maxwellian velocity distribution of emitting species such lines have Gaussian profiles with FWHM given by Eq. (16b). One must make sure that the thermal Doppler effect is the major cause of line broadening in the source before using this method of temperature measurement. It has been found that even gross motions in the source could

44

S. N. Thakur

simulate a Gaussian profile. In order to avoid such misinterpretations it is advisable to observe the plasma from different angles to watch for shifts of the intensity maxima.

6.2. Determination of the Electron Density The most powerful spectroscopic technique of determining the electron density Ne of discharge plasma comes from the measurement of the Stark broadening of spectral lines. In this method absolute intensities of spectral lines are not required, merely line shapes and FWHM are sufficient. Since broadening is quite appreciable for electron density N ≥ 1015 cm−3 , standard spectrometers often suffice to record the spectra for measurements of line shape. The electron density Ne is extracted by matching the line width (or the entire line shape) with the calculated one. Details of line shape calculations can be found in a book by Griem [18]. In this section we will summarize only the salient points.

6.2.1. Hydrogen & hydrogen-like ions Hydrogen and hydrogen-like ions exhibit linear Stark effect. The broadening of spectral lines is found to be dependent on the optical transition and a judicious choice is important. At low plasma densities a spectral line with large Stark broadening is desirable but at high densities a line with relatively smaller broadening is useful so that its outer wings do not overlap with neighboring lines. The FWHM (in A) of the spectral line in the quasi-static approximation is given by:  = 816 × 10−19 1 − 07ND −1/3  02 n2 2 − n1 2  Zp1/3 /Ze  N2/3

(23a)

where ND = 4 /3N D 3 is the number of particles in the Debye sphere, N is in cm−3  0 is the line center, n2 and n1 are the principal quantum numbers of the upper and lower states respectively, Zp is, the nuclear charge on the perturbing ion and Ze that on the emitting particle (atom or ion) In the above discussion we have neglected contributions to FWHM from the plasma electrons. Although the line shapes do depend on the electron contribution, the FWHM are generally insensitive. Eq. (23a) represents a very good estimate of FWHM in those hydrogenic lines that do not have a strong undisplaced Stark component as for example, the Lyman , Lyman , Balmer  and Balmer  transitions. On the other hand the FWHM of hydrogenic lines with strong central Stark components are dominated by interaction of the electron with the emitting atom such as Lyman  and Balmer  transitions. Such lines have a Lorentzian line shape and FWHM for Lyman  transition in the impact approximation is given  ≈ 162 × 10−17 N/T 1/2 1376 − log N1/2 /T

(23b)

where  is in A, T is in K and N is in cm−3 . It is seen from Eq. (23a,23b) that the ion broadening, in the quasi-static approximation, varies as N2/3 and is virtually independent of temperature whereas the collisional broadening, in the impact approximation, varies approximately as N and it is very much

Atomic Emission Spectroscopy

45

temperature dependent. In case T is not reliably known, it is advisable to determine electron density N from Balmer  line who’s FWHM is ion- dominated. It is to be noted that electron densities determined from Eq. (23) are only crude estimates of N and one must compute the entire line profile to extract the total line width for an accurate value of N. Some very precise hydrogen line shape measurements have been reported by Wiese and coworkers [19,20].

6.2.2. Many-electron atoms & ions Plasma generated by high power lasers focused on gas or solid targets, have electron energies in the range of 50 eV to 10 keV and hydrogen as well as other low Z atoms are fully stripped and do not radiate. It is found that carbon is completely stripped when T ≥ 100 eV and copper is stripped when T ≈ 1keV [21] which suggests that sufficiently high-Z atoms should be involved in spectroscopic measurements on hot plasma. Spectral lines of neutral atoms and non-hydrogenic ions have Stark broadening mostly due to electron impacts. For such high-Z ions the Stark widths are too small to be resolved and they may be dominated by Doppler broadening. Non-hydrogenic high-Z ions are therefore not very suitable for determining electron densities in very hot plasma.

6.3. Qualitative Emission Analysis One of the major applications of atomic emission spectra is the identification of elements present in the source of light. The use of this technique for qualitative analysis of samples which could be fed into flames to emit characteristic light (yellow for Na, red for Ca) dates back to Bunsen and Kirchoff and other spectroscopists of 19th century. Major developments in astrophysics have resulted from the studies of the spectra of radiating stellar bodies which provided information about their chemical composition, temperature, distance, mass etc. Some elements are easily excited for their emission spectra to be observed and recorded, than others. Thus non-metal atoms are more difficult to excite than metal atoms because of their high ionization potential. It is found that presence of easy to excite atoms in a sample, suppress the emission from atoms that are relatively difficult to excite. Thus emission from helium is suppressed in the presence of nitrogen, that of nitrogen in presence of mercury and the emission from mercury is suppressed if the sample also contains potassium. When an element is excited in an arc or discharge source, a number of lines of varying intensities are observed over a wide range of the spectrum. As one dilutes the amount of the element in the arc, the number of lines observable is reduced and ultimately only a few lines of that element remain observable. These lines are known as persistent lines. It has been found that the persistent lines are also the lines of largest intensity. From this description of the persistent lines it is apparent that in the qualitative analysis for a particular element we need look only for the persistent lines of that element. If they are absent it may be safely assumed that the element is not present in the sample. Tables of persistent lines of many elements are to be found in a book by Brode [22] and in a publication by Meggers [23]. Persistent lines for some elements are given in Table 2. In most cases of qualitative analysis, the major constituents of the discharge are readily determined by inspection of the strongest lines and other lines belonging to

46

S. N. Thakur

Table 2. Wavelengths (in Angstrom) of Persistent Lines of some Elements Element Wave-length Element Wave-length Element Wave-length Element Wave-length Ag I

Ag II Al I

Al II

Au I

BI

328068 338290 520907 546549

228802 231284 257309 274858

224641 243780

326106 340365

303216 309271 394403 396153

346620 361051 643847 214438

16710 18560 18581 18625 263155 266917 281618 623176 624336

Cd II Cl II

Co I

Co II

226502 479454 481006 481946 345351 346580 352981 228616 230786

242795 267595 280219

236379 237862 238892

249678 249773

251982 340512

B II

345141

Ba I

230423 233527 307159 542462 551912 553555 577767

Ba II

Cd I

389179 413066 455404 493409

Cr I

Cr II

Be I

Be II Bi I

Bi II Br II

CI C II

234861 265078 332101 332109

Cu II Fe I

219226 224699 358120 371994

332134 313042

373713 374556

313107 206170 227658 278052

374590 374826 238204 239563

280963 289798 293830 298903 306772 190941 472255 470486 478550 481671 229689 247857

Fe II

Hg I

KI

283671 283760

240488 241052 241331 184968 253652 365015 365483 366328 404656 435835 546074 404414 404720 766491

425435

426702

427480 428902 520452 520604 520844 283563 284325

426727 422673 442544 443496 445478 315887 317933

Mg I

285213 382935 383231 383820 516734 517270 518362

393367 396847 521820 213595

Mn I

403076 403307 403449 257610

284984 285568 286092

Ca I

Ca II

Cu I Cu II

769898

Mn II

these constituents can then be identified by use of the tabulation of spectral lines of the elements [2,24, and 25]. After this procedure, the lines left unidentified are generally weak and they may be hitherto unobserved lines of the major constituents or lines of the unknown minor constituents. In the latter case, they must be among the stronger lines of these elements. The unknown element can then be identified by comparing these lines with the persistent lines of various elements.

Atomic Emission Spectroscopy

47

The qualitative analysis of a sample may be divided into two parts: (1) search for a definite element (2) identification of unknown elements In search for a definite element, the identification of the persistent lines should be sufficient to confirm the presence of the element. The identification of unknown elements requires a more complete identification of all spectral lines in the emission spectrum of the sample. At least three lines concerning whom there is no possible doubt as to origin should be identified so as positively identify the element.

6.4. Quantitative Emission Analysis The determination of the elemental composition of a gaseous or condensed phase sample by means of laser induced plasma requires the measurement of the intensities of those spectral lines that are characteristic of the individual elements present in the sample. The intensities must then be related to the number density of atoms or ions present in the plasma. The first step in the quantitative analysis is the preparation of standard samples in which the concentration of the element of interest is varied in a precisely known manner. It is obvious that the intensities of spectral lines of this element in the emission spectra recorded for different standard samples will be proportional to its concentration. The next step in the analysis is the measurement of intensities of one or more spectral lines of the said element from spectra of all the standard samples as well as the samples to be analysed. The last step of the analysis is to plot a working curve of line intensity against known concentration of the element for the standard samples. The concentrations of the element in the unknown samples are determined from the working curve from the measured intensities of their spectral lines. The procedure outlined in the previous paragraph for quantitative analysis is based on the assumption that the excitation conditions for the unknown as well as the standard samples are exactly identical. This is never realized in practice and fluctuations in emission from the plasma may cause the relative intensities of a line for two samples of different elemental concentration not to reflect their relative concentrations. This shortcoming due to uncontrolled random fluctuations of emission intensity arising from difference in excitation conditions leads to a less accurate working curve and consequent error in quantitative estimates of the element in unknown samples. To avoid this error it becomes necessary to refer the intensities of lines in the unknown and standard samples to some common line of another element which remains unchanged in both spectra. Such a spectral line is called internal standard because its intensity is also affected by the random fluctuations as that of the element under investigation. Two types of internal standard lines may be used: (1) a weak line of the element which is the major constituent of the sample whose intensity remains constant for all standard samples. (2) a persistent line of an added small amount of an element which is known not to be present in either the unknown or the standard samples.

48

S. N. Thakur

Since the lines from the element to be analyzed as well as the internal standard originate in the plasma from the same sample, it is obvious that any variation in the excitation conditions will not affect their relative intensities. The working curve is now plotted by using this relative intensity against the concentration of the element in the standard samples. This procedure reduces the errors in quantitative analysis by a very large factor. For further details of quantitative analysis the reader is referred to the books by Brode [22], Sawyer [26] and Radziemski and Cremers [27].

REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]

Transactions of International Astronomical Union, 12A (1964) 137 G. R. Harrison, M.I.T. Wavelength Tables, Wiley, New York (1959) H. C. Longuet-Higgins, J. Chem. Edu. 34 (1957) 30 G. Herzberg, Atomic Spectra and Atomic Structure, Dover, New York (1944) H. G. Kuhn, Atomic Spectra, Longman, London (1964) V. Kondratyev, The Structure of Atoms and Molecules, Foreign Language Publishing House, Moscow (1960) B. Cagnac, J.C. Pebay-Peyroula, Modern Atomic Physics: Fundamental Principles, The Macmillan Press, London (1975) P. Bousquet, Instrumental Spectroscopy, Dunod, Paris (1969) A. P. Thorne, Spectrophysics, Chapman & Hall and Science Paperbacks, London (1974) V. Weisskopf, Phys. Z. 34 (1933) 1 E. Lindholm, Arkiv Mat. Astron. Fysik. 28B (1941) no 3 H. M. Foley, Phys. Rev. 69 (1946) 616 P. W. Anderson, Phys. Rev. 86 (1952) 809 J. Holtsmark, Ann. Physik 58 (1919) 577 W. R. Hindmarsh, Prog. in Quantum Electronics 2 (1972) 143 D. D. Burgess, Space Science Reviews 13 (1972) 493 H. R. Griem, Plasma Spectroscopy, McGraw-Hill, New York (1964) H. R. Griem, Spectral Line Broadening by Plasmas, Academic Press, New York (1974) W. L. Wiese, D.E. Kelleher and D.R. Paquette, Phys. Rev. A6 (1972) 1132 W. L. Wiese, D.E. Kelleher and V. Helping, Phys. Rev. A11 (1975) 1854 D. Mosher, Phys. Rev. A10 (1974) 2330 W. R. Bride, Chemical Spectroscopy, Second Edition, John Wiley, New York (1952) W,F. Meggers, J. Optical Soc. Am. 31 (1941) 44, 605 C. E. Moore, Selected Tables of Atomic Spectra, Washington, N.S.R.D.S.-N.B.S. 3, Section 1 (1965) and Section 2 (1967) C. E. Moore, Bibliography on the Analysis of Optical Atomic Spectra, Section 1, H toV, N.B.S. Special Publ. 306, Washington (1968) R. A. Sawyer, Experimental Spectroscopy, Third Edition, Dover, New York (1963) L. J. Radziemski and D. A. Cremers (Ed), Laser Induced Plasma and Applications, Marcel Dekker, New York (1989)

Chapter 3

Laser Ablation R. E. Russo, X. L. Mao, J. H. Yoo and J. J. Gonzalez 1 Cyclotron Road, Lawrence Berkeley National Laboratory Berkeley, CA 94720, USA

1. INTRODUCTION The definition of ablation from the Merriam-Webster Dictionary is “loss of a part by melting or vaporization”. In order for ablation to occur, energy absorption is needed. The energy can be provided in the form of electrical discharges (e.g. an arc and spark) or in the form of light (e.g. as a laser). Laser ablation means using laser light energy to remove a portion of a sample by melting, fusion, sublimation, ionization, erosion, and/or explosion. Laser ablation results in the formation of a gaseous vapor, luminous plasma, and in the production of fine particles. By measuring the emission spectrum from the laser-induced plasma, qualitative and quantitative information about the sample’s chemical composition can be obtained. This measurement technology is known as Laser Induced Breakdown Spectroscopy (LIBS). LIBS is an exciting field of study, both theoretically and experimentally due to the wealth of diverse mechanisms underlying the physical processes and its significant potential for spectrochemical analysis. This chapter will discuss the fundamental mechanisms of laser ablation processes and their relation to LIBS. “The history of the interaction of high-power lasers with solid matter is as old as the laser itself”[1]. In 1917 Albert Einstein [2] first proposed that stimulated emission of light (process that makes lasing possible) should occur in addition to absorption and spontaneous emission. It then took over 40 years until the development of theoretical principles of lasers were established by Arthur Schawlow and Charles Townes [3] in 1958 and two more years to develop the first laser source by Theodore Maiman [4,5]. The laser was built using a rod of synthetic ruby as the active medium. In 1962 the first account of laser ablation was presented by Breech and Cross [6] at the International Conference on Spectroscopy held at the University of Maryland. A ruby laser was used to vaporize and excite atoms from solid surfaces, and the plasma spectrum was used to characterize the elemental composition of the sample. This paper began the field of laser microprobe emission spectroscopy, which was one of the first real applications of laser ablation. Throughout 1963 and 1964, about a dozen publications detailed early laser ablation experiments [1]. Many phenomena, which were first observed in those years, are still the subject of study today. During the 1970s and early 1980s, the Laser-Induced Breakdown Spectroscopy Jagdish P Singh, Surya N Thakur (Editors) © 2007 Elsevier B.V. All rights reserved.

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development of lasers and the understanding of laser ablation were incremental and steady. The fastest growing applications during the eighties were driven by the needs of materials science. Several laser ablation-based methods reached maturity in the late 1980, including pulsed laser deposition (PLD) [7] for making high-Tc superconductor thin films, micro-machining [8], and laser-based medical applications such as laserbased ophthalmology (LASIK) [9], removal of birthmarks, tattoos and smoothing of wrinkled skin in dermatology [10,11]; laser surgery for internal arthroscopic cutting and for arterial angioplasty [12]; and for dental applications [13]. For analytical purposes, laser ablation became widely used for a range of microanalysis applications. Matrix-assisted laser desorption/ionization (MALDI) [14] revolutionized the identification and study of large molecular weight bio-molecules and polymers. Laser ablation as a sampling technique coupled to established analytical techniques such as inductively coupled plasma atomic emission spectrometry (ICP-AES) [15] and inductively coupled plasma mass spectrometry (ICP-MS) [16], improved analytical capability for direct solid analysis by minimizing the time of analysis, reducing hazardous chemical exposure and waste, and by providing reliable alternative to acid dissolution for chemical analysis. Laser ablation has been recognized for its powerful advantages; rapid, in situ, multi-element analysis of any kind of sample with no sample preparation. Diagnostics and theoretical studies have advanced laser ablation research to a very active field supported by a wide range of applications. The basis of LIBS is rooted in laser ablation. Laser ablation is the first step in the LIBS process, and its influence will be reflected in the “figures of merit”, temporal and spatial resolution, sensitivity, precision, and accuracy. The influence of laser ablation on LIBS is addressed in this chapter.

2. FUNDAMENTAL ABLATION PROCESSES Laser ablation is governed by a variety of distinct nonlinear mechanisms. Once the laser beam illuminates the sample, mass leaves the surface of a sample in the form of electrons, ions, atoms, molecules, clusters, and particles, each of the processes separated in time and space. An understanding of the fundamental mechanisms involved in each of these processes is critical for efficiently coupling the laser beam to the sample and removing mass in the appropriate form for analysis. Understanding laser-material interaction will allow ablation of stoichiometric vapor and control of the laser-induced plasma properties for optimum LIBS performance. Laser ablation will be divided into three main processes for discussion in this chapter: bond breaking and plasma ignition, plasma expansion and cooling, and particle ejection and condensation. These laser ablation processes occur over several orders of magnitude in time, starting with electronic absorption of laser optical energy 10−15 sec to particle condensation 10−3 sec after the laser pulse is completed. Fig. 1, shows a summary of these three processes and various mechanisms occurring during each. During the plasma ignition process, the mechanisms and plasma properties strongly depend on the laser irradiance and pulse duration. For a nanosecond laser pulse with irradiances less than 108 W/cm2 , the dominant mechanism is thermal vaporization: the temperature of the solid surface increases, and a well defined phase transition occurs, from solid to liquid, liquid to vapor, and vapor to plasma. For a picosecond laser pulse with irradiance between 1010 –1013 W/cm2 , both thermal and non-thermal

Laser Ablation Plasma ignition fs laser (1012 – 1017 W/cm2)

51 Plasma expansion and cooling

Particles ejection and condensation

Electronic excitation and ionization Shockwave propagation Nano particles formation (10–4–10–3 s) (10–15–10–13 s) Plasma expansion (10–11–10–6 s) Ejection of liquid droplet (10–8–10–6 s) Coulomb explosion (10–13 s) –6 –4 Solid exfoliation (10–6–10–5 s) –12 s) Plasma radiation cooling (10 –10 s) Electron–lattice heating (10

ns laser (107 – 1011 W/cm2) Thermal vaporization (10–9–10–8 s) Non-thermal ablation (10–9–10–8 s) Plasma shielding (10–9–10–8 s)

Fig. 1. A summary of laser ablation processes and various mechanisms occurring during each process.

mechanisms such as Coulomb explosion exist. For irradiances higher than 1013 W/cm2 with femtosecond laser pulse, Coulomb explosion is the main bond breaking mechanism. When the laser pulse duration is in the nanosecond time region, the later part of laser pulse can be absorbed by the laser induced plasma, which is called plasma shielding. For picosecond pulsed laser ablation, the laser pulse is too short to be absorbed by the plasma. Plasma shielding will influence how much of the solid mass is converted into vapor and the properties of vapor. However, an air plasma can form during the pico second laser pulse duration due to seed electrons from the target surface; this air plasma can absorb part of the picosecond pulse. With femtosecond laser pulses, plasma shielding can be neglected because, to the best of our knowledge, no mass can be ejected from the surface during the short pulse duration. Plasma expansion begins after the plasma ignition process. The plasma expansion process will be governed by the initial plasma properties (at the end of the laser pulse) and the expansion medium. The properties (electron number density, temperature, and expansion speed) of the plasma initially are strongly dependent on the laser properties. Plasma expansion will be related to the initial mass and energy in the vapor plume, and the gas environment. Plasma expansion will be adiabatic until approximately 1 microsecond after the laser pulse. After that time, line radiation will be a dominant energy loss influencing the temperature decrease. Particle formation will be influenced by these primary processes. Nano-sized particles will be formed from condensation of the vapor. Condensation starts when the vapor plume temperature reaches the boiling temperature of the material (∼3000 K) and stops at the condensation temperature of the material ( Eg , where Eg is the bandgap energy of the dielectric material, and h is the photon energy. A second photoionization process, tunneling ionization, may come into play under an extremely strong laser electromagnetic field interaction with dielectrics. In the strong-field regime, the superposition of the nuclear Coulomb field and the laser electric field results in an oscillating finite potential barrier through which bound electrons can tunnel, thus escaping the atom. In dielectrics, this mechanism allows valence electrons to tunnel to the conduction band in a time period shorter than the laser pulse duration. Both multiphoton and tunneling ionization can be treated under the

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same theoretical framework developed by Keldysh [18]. The transition from multiphoton to tunneling ionization is characterized by the Keldysh parameter  [18].  1/2  2me Eg (1) = eEA where me and e are the effective mass and charge of the electron and EA is the amplitude of the laser electric field oscillating at frequency . When  is much larger than one, which is the case for high intensity laser interactions with dielectrics, multiphoton ionization dominates the excitation process. For semiconductor samples, where the photon energy is larger than the bandgap, single photon absorption is the dominant mechanism for exciting valence electrons to the conduction band [19,20]. In the case of semiconductors with an indirect bandgap, such as silicon, single photon absorption can still occur with photons of energy greater than the gap, but phonon assistance is necessary to conserve momentum. Once an electron-hole plasma is formed inside the solid, the carriers can absorb additional laser photons, sequentially moving to higher energy states. The absorption coefficient 0 depends on the imaginary part of the refractive index , which is related to the dielectric function . According to the Drude model [21], can be expressed as:  

2

2 (2) + i = 1 − 2p 1 + 2 2  1 + 2 2  where  the scattering time is typically a fraction of a femtosecond and depends on the conduction electron energy. p is the plasma frequency defined by  e2 N (3) p = 0 m e where N is the carrier density and 0 is the electric permittivity. Photon absorption increases the carrier energy of the electron–hole plasma; when the energy of carriers is well above the bandgap (or Fermi level in a metal), collisional ionization generates additional excited carriers. A high energy electron can ionize another electron from the valence band, resulting in two excited electrons with lower energy at the conduction band [22,23]. These electrons can be heated by the laser through free carrier absorption and impact additional valence band electrons. This process can repeat itself as long as the laser electromagnetic field is present and intense, leading to the socalled electronic avalanche. Avalanche ionization requires seed electrons to be present in the conduction band, which can be excited by photoionization. The following rate equation can describe the injection of electrons into the conduction band of dielectrics using femtosecond to picosecond laser pulses, under the combined action of multiphoton excitation and avalanche ionization [24]: dN = aIN + NI n dt

(4)

where a is a constant. Within the fs timescale, a large number of excited electrons can leave the solid, the lattice modes remain vibrationally cold and the irradiated solid consist of charged ions and an electron-hole plasma. After about 10% of the valence electrons

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are removed, the lattice is weakened and begins to melt. This is called non-thermal melting (Coulomb explosion) caused by high energy electrons and ions [25–29]. Coulomb explosion processes were studied by time resolved reflectivity and x-ray diffraction, using pump-probe experiments [30]. Optical reflectivity spectra for femtosecond laser-excited silicon are plotted in Fig. 2. Negative times mean that the probe beam arrived at the surface before the pump ablation laser pulse. The dashed–dotted and the dashed lines represent the well-known reflectivity spectra of crystalline and molten silicon, respectively. When the probe pulse occurred 120 fs before the ablation pulse, the reflectivity spectrum was similar to that of crystalline Si. An abrupt increase in the optical reflectivity occurred within three hundred femtoseconds after the ablation pulse, the reflectivity spectrum changes from crystalline to liquid character within this short time. High energy femtosecond laser material interactions can produce x-ray radiation with duration comparable to that of the laser pulse. This x-ray pulse can be used to detect lattice changes by x-ray diffraction using a pump-probe method with femtosecond time resolution. Fig. 3 shows time-resolved x-ray diffraction [30] measured from the (111) lattice planes of Ge as a function of the delay time between the x-ray probe and the laser excitation pulse for two different laser fluences. Negative times indicate that the x-ray probe pulse arrived before the laser excitation pulse. The diffraction intensity remained unchanged when the x-ray pulse arrived before the excitation (ablation) pulse. A sharp decrease in the diffraction was observed after the arrival of the ablation pulse. The initial decrease in reflectivity takes approximately 300 femtoseconds and indicates lattice melting. Conventional electron phonon interactions occur on the picosecond time scale. Therefore, the fast decrease in diffraction (within a few hundred femtoseconds) and optical reflectivity demonstrate that a portion of the Ge crystal underwent non-thermal melting.

2.1.2. Picosecond laser ablation For picosecond laser ablation, the lattice could be melted through thermal and/or nonthermal processes, depending on the laser irradiance. Electrons are ejected from the 1.0 liquid-Si

Reflectivity

0.8

300 fs

0.6 0.4

50 fs –20 fs

cryst-Si

–120 fs 0.2 0.0 200

400

600

800

1000

Wavelength (nm)

Fig. 2. Spectra of the optical reflectivity of silicon. Dash–dotted curve: crystalline silicon. Dashed line: molten silicon. Data points: spectra measured at various time delays between the pump pulse and the optical probe pulse (optical pump/optical probe measurements). Ref [30]

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Integrated reflectivity

300 fs

1.0

0.2 J/cm2 0.4 J/cm2 0.8

0.6 –1

0

1

∞

2

Delay time [ps]

Fig. 3. X-ray diffraction from the (111) lattice planes of Ge versus time delay for two different energy fluences of the pump pulse. Ref [30]

target surface during the laser pulse. The free electrons can interact with the air and absorb laser energy to initiate an air plasma during the ps laser pulse duration. The plasma forms long before the plume forms [31–33]. Fig. 4 shows the measured air plasma electron number density Ne as a function of distance z from the target surface, at 150 ps delay time between the pump and probe beams. The electron density at z was measured from the interference pattern (insert of Fig.4) using the expression Ne z =

2 0 me 2 qz e2 lz

(5)

2.0 × 10 20

Ne (cm–3)

1.5 × 10 20

z 1.0 × 10 20

Target 5.0 × 1019

0.0 0

50

100

150

200

250

z (μm)

Fig. 4. Electron number density profile along the incident laser axis. The solid curve is a least square fit of the experimental data showing exponential decay. The inset is an interferogram of the picosecond laser ablation plasma. Ref [33]

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where q(z) and l(z) are, respectively, the average phase shift and width of the plasma at location z. and  are circular frequency and wavelength of the probe beam. The electron number density of this air plasma was on the order of 1020 cm−3 which is higher than the air density. The air plasma was observed immediately and expanded longitudinally during the laser pulse. Its longitudinal extent remained approximately constant after about 100 ps, after which the plasma expanded principally in the lateral (radial) direction. The evolution of the laser-ablated air plasma was simulated with a two-fluid plasma model [31,34]. The air plasma above the sample would absorb a part of the incoming laser beam radiation. Unlike ns laser ablation, plasma shielding is not caused by absorption from the vapor plume; on the picosecond time scale, plasma shielding is caused by the air plasma. To confirm this plasma shielding mechanism in ps laser ablation, the lateral expansion of early stage ablation plasma induced by a 1064 nm, 35 ps laser pulse on a copper target was measured. A relation of t1/2 was found for the lateral expansion of the air plasma. Measurements of energy absorption by the air plasma (∼10% of incoming laser energy) confirmed plasma shielding for picosecond laser ablation.

2.1.3. Nanosecond ablation When the pulse duration is on the order of a few nanoseconds, and laser irradiance is on the order of 107 –1011 W/cm2 , some of the mechanisms involved in ablation are: melting, fusion, sublimation, vaporization, ionization, etc. If the laser irradiance is high enough, non-thermal ablation is also important and can co-exist with these thermal mechanisms. When the laser irradiance is less than 108 W/cm2 , thermal processes are dominant. The temperature at the target surface will rise during the laser pulse, and eventually the target will melt and vaporize. The temperature distribution in the target can be calculated with the heat conduction equation [35]. T x t  = t x



 C p s



 T x t  + I x t x C p s

(6)

where T represents the temperature inside the target, x is the position from the surface, , Cp , s and  denote the thermal conductivity, heat capacity, mass density and absorption coefficient of the solid target material, respectively. The thermal evaporation rate Jv is the function of surface temperature. Assuming thermal equilibrium, 

L Jv = 106 × 10 exp − v kB 6



1 1 − Ts TB

 

M 2kB Ts

(7)

where Lv is the heat of vaporization and M is mass of vapor. kB is the Boltzmann constant. Tb and Ts are the boiling-point temperature and surface temperature of the sample, respectively. Vaporized mass can be ionized by absorbing the incoming laser beam, forming a plasma. Laser radiation is absorbed primarily by inverse Bremsstrahlung, which involves

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the absorption of a photon by free electrons during the collision with heavy particles (ions and atoms). The inverse Bremsstrahlung absorption coefficient is given by: 1/ 2     hc 4e6 3 Ne Z2 Ni 2 × 1 − exp − × IB = QNe N0 + 3hc4 me 3me kB Te kB Te

(8)

where Q is the cross section for photon absorption by an electron during the collision with atoms, c is the speed of light, h is Planck’s constant and Z is the charge on ions. Ne  N0 and Ni are number density of electron, atoms and ions, respectively. Te is the electron temperature. The first term on the right side of Eqn. (8) is the electron atom interaction and the second term is related to electron ions interaction. Multi-photon ionization in the vapor also can contribute to this process, if the laser intensity is high and laser wavelength is short. When the plasma plume is near the critical density, the later part of the laser beam pulse energy would be partially absorbed before it reaches the target. Plasma shielding was observed by the transmitted laser-pulse temporal profile through a glass sample (Fig. 5). The temporal profiles of the transmitted laser pulse were similar to the original laser pulse at low laser irradiance. When the laser irradiance was greater than 03 GW/cm2 , the later part of laser pulse became truncated. [36].

0.05 GW/cm2

Transmitted laser intensity (Arb. units)

0.1 GW/cm2

0.15 GW/cm2

0.3 GW/cm2

0.6 GW/cm2

5 GW/cm2 57 GW/cm2 0

100

Time (ns)

Fig. 5. Transmitted laser beam temporal profiles through a glass sample at varies power densities. Ref [36]

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2.2. Plasma Expansion Processes After the laser pulse ends, the induced plasma plume will continue to expand into the ambient. The electron number density and temperature of the plasma changes as the plasma expands. Plasma expansion depends on the amount and properties of the ablated mass, how much energy was coupled into the mass, the spot size of laser beam, and the environment (gases, liquid, and pressure). Most LIBS spectra are recorded from several hundreds of nanoseconds to several microseconds after the laser pulse. Understanding plasma expansion during this time period is critical for optimization of LIBS measurements.

2.2.1. Expansion of the evaporated material plume and shockwaves After the laser pulse ends, hot electrons, atomic, and ionic mass leave the sample surface. The expansion of the evaporated material into vacuum can be described by the Euler equations of hydrodynamics, expressing the conservation of mass density, momentum and energy [35]:    =− t x     =− p + 2 t x       2 p 2   + IB I =−  Ed +  Ed + +  2 2 t x

(9) (10) (11)

where  is the mass density,  is the velocity, Ed is the internal energy density, and p is the local pressure. This theory governing plasma expansion can be used for both ns and fs laser ablation. In a vacuum, the laser induced plasma-plume expands adiabatically. The expansion speed can be expressed by [37]  p =

4 + 10 E 3 M

(12)

where p is the velocity,  is the specific heat ratio, E is the energy supporting the expansion, and M is the total vaporized sample mass within the vapor plume. Most of the plume energy is kinetic energy. When ablation occurs into a gas or liquid environment, the ejected mass compresses the surrounding media and produces shockwaves. The plume is the ablated mass from the sample target. The plasma is a mixture of atoms and ions, and mass from both the ablated target material and the ambient gas. The interaction between the plume and surrounding media slows the expansion of the plasma. At the same time, the ambient media performs work on the vapor. The vapor temperature will be higher than that for free expansion; temperature and number density of ablated mass depend on the properties of the surrounding media.

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Once the external shockwave is formed, its expansion distance can be described by Sedov’s theory. The expansion distance H, representing the location of the shockwave front, can be calculated as a function of time [38]:  H = 0

E0 1

1/2+d t2/2+d

(13)

where the parameter d is the dimensionality of the propagation (for spherical propagation d = 3, for cylindrical propagation d = 2, and for planar propagation d = 1). 0 is a dimensionless constant. E0 has the unit of “energy per area” in the case of one dimensional expansion (planar propagation), “energy per length” for two-dimensional expansion (cylindrical propagation), and “energy” for three-dimensional expansion (spherical propagation). 1 is air density. By fitting the experimental data using Eqn. (13), the dimensionality of expansion can be determined. Early stage plasma expansion from femtosecond laser ablation of stainless steel targets was investigated by time resolved shadowgraph imaging (Fig. 6). At early times, the femtosecond laser-induced plasma expanded primarily in the direction perpendicular to sample surface; the expansion distance was approximately 10 m after 130 ps. There was no lateral expansion until nanoseconds after the ablation laser pulse, after which the lateral expansion slowly increased. If the laser has a nano-second pulse duration, the perpendicular expansion distance of the plasma is proportional to t2/5 , and can be predicted by Sedov’s blast wave theory for spherical propagation (three dimension expansion). The perpendicular expansion of the plasma generated by the femtosecond laser pulsed ablation was proportional to t2/3 , corresponding to one dimensional expansion (measured in the shadowgraphs of Fig. 6 which showed no lateral expansion at early times).

(a)

470 ps

3.2 ns

17 ns

Perpendicular expansion distance (micron)

500 130 ps

100

10 0.1 1130 ps

100 μm fs laser

43 ns

fs plasma ns plasma

(b)

Hns ~ t 0.4

Hfs ~ t 0.66

1

10

100

Time (ns)

500 μm ns laser

Fig. 6. (a) Sequence of shock wave images obtained by laser shadowgraph for femtosecond and nanosecond laser ablation. Please noted the scale for fs laser and ns laser is different. (b) Perpendicular expansion distance of shockwave as a function of time for femtosecond and nanosecond laser ablation. Ref [39]

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Once the plume pressure equalizes to the pressure of the surrounding media the expansion stops. The stopping time and distance of the vapor plume can be expressed as [40]:  ts = st  Rs = st

E pg E pg

1/3 1/3

1 cg

(14) (15)

where st and st are constants, pg is pressure, and cg is the sound velocity of the gas. The stopping time is in the range of microseconds. LIBS is usually measured after microseconds, a time at which vapor plume expansion has stopped. The final distance determines the volume of the vapor plume. LIBS performance depends on the electron number density and temperature of the plasma, which strongly depends on the plume volume.

2.3. Plasma Emission Spectra 2.3.1. Femtosecond pulsed laser plasma emission When a femtosecond laser pulse is focused in air, optical emission of nitrogen molecular lines will exist in the air plasma several picosecond after the laser pulse; molecular structure is preserved for this time period [41]. A spectrum, measured using gated integration with a delay slightly after the laser pulse (1 ns), is presented in Fig. 7. The laser energy was 200 mJ. The two electronic systems observed are the 2+ N2 C 3 !u → B3 !g 

2+ system N2

Amplitude

600

2-1

2-0 400

0-0

1-0

800

3-1

0-1

1-1 3-2

1-2 2-2 2-3

200

0 280

1– system N2+

300

320

340

0-0 0-2 1-3 2-4 3-5

360

380

0-1 1-2 0-3 2-3 1-4 3-7

400

420

440

Wavelength (nm)

Fig. 7. Molecular line emission spectrum obtained at laser energy of 200 mJ with the maximum of the gate pulse ∼1 ns before the laser pulse. The upper–lower vibrational levels of the transitions are indicated. Ref [41]

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61

→ X 2 "+  bands. No oxygen lines were observed in this early and the 1− N2 + B2 "+ g u time period. The bands are identified in Fig. 7 with notation indicating the upper (v ) and lower (v ) vibrational levels. The estimated vibrational temperature Tv  of the 2+ system was between 7000 and 8000 K with accuracy ∼1000 K. A streak camera was used to obtain a fast time-resolution measurement of the total emission, which is shown in Fig. 8. A narrow 60-picosecond duration peak was observed just after the laser pulse followed by a slower decrease in amplitude lasting 0.5 nanosecond. The peak is due to molecular line emission, as there were no other spectral features measured at this short delay. Conventional LIBS measurements are made using nanosecond to microsecond delays after the laser pulse. Emission spectra at these times depend on the laser-induced plasma properties; when the plasma is hot and dense, the spectrum is mostly composed of continuum emission. During plasma expansion, the temperature and number density decrease; ionic lines then atomic emission lines appear. Continuum emission was observed within one nanosecond after the fs laser ablation pulse; Fig. 9 shows continuum emission spectrum measured at increasing time delays [41]. Within the time measurement resolution (4.5 ns), the amplitude of continuum emission increased ∼10 times after the laser pulse, and decreased by about two orders of magnitude in 80 ns. Ionic lines began to appear about 10 ns after the laser pulse. The femtosecond pulsed laser induced plasma has a shorter overall lifetime compared to those plasmas initiated using longer laser pulses.

2.3.2. Nanosecond pulsed laser plasma emission For the ns pulsed laser induced plasma, continuum emission appears during the laser pulse and lasts for several hundred nanoseconds. Ion emission also dominates on the ns timescale. Atomic and molecular line emission occurs after ∼1 microsecond. Molecular line emission measured at later times is from the recombination of species in the plasma. 5000

Amplitude

4000

3000

60 psec 2000

1000

0 –0.1

0

0.1

0.2

0.3

0.4

0.5

Time (ns)

Fig. 8. Streak camera traces line-outs of the light emitted on a fast time scale. The laser pulse is at t = 0. Streak speeds are 15 mm/ns. Ref [41]

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(b)

100 6 ns 5 ns 4.5 ns 3 ns

100

10

Amplitude

Amplitude

(a)

10 ns 16 ns 23 ns 35 ns

10

60 ns

2 ns 90 ns

1 300

400

500

600

1 300

700

400

500

600

700

Wavelength (nm)

Wavelength (nm)

Fig. 9. Continuum emission spectra at successive delays after the laser pulse. The spectra in both (a) and (b) are all plotted to the same scale. The time values indicate the delay of the sampling pulse with respect to the laser pulse. (a) 2–6 ns; and (b) 10–90 ns. Ref [41]

C-N and C-C swan bands are often observed on the microsecond time scale. To the best of our knowledge, there are no reports for the original molecular structure being preserved for ns-pulsed plasma emission.

2.4. Electron Density and Plasma Temperature Plasma temperature and electron number density can be estimated from the continuum emission and peak width of atomic and ionic emission lines. A Lorentz function can be used to fit the line spectra (Fig. 10). Stark line broadening from collisions of charged species is the primary mechanism influencing the emission spectra in conventional LIBS experiments. [42]

Intensity (a.u.)

1.5 × 104

1.0 × 104

FWHM

5.0 × 103

y0 0.0 286

288 x0

290

292

Wavelength (nm)

Fig. 10. Lorentzian fitting of the Stark broadened profile. The full width half maximum (FWHM) was used for the calculation of the electron number density.

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The FWHM of Stark broadened lines is related [42,43] to the electron number density Ne by Eq. (10):

#1/ 2 = 2W

     3 −1/3 Ne 1/4 Ne 1 − ND 1 + 175A 1016 4 1016

(16)

where ND is the number of particles in the Debye sphere and is estimated from ND = 172 × 109

Te3/2 Ne1/2

(17)

W is the electron impact parameter in nm and A is the ion impact parameter; W and A are functions of temperature and can be obtained approximated by second-order polynomials from reference [43]. Under the assumption of local thermal equilibrium (LTE), the plasma temperature T can be determined by the line-to-continuum intensity ratio c / l , where c is the continuum emission coefficient and l is the integrated emission coefficient over the line spectral profile. The line emission coefficient l can be expressed in terms of the electron temperature and density [44]:  l =

   g2 hl Eion − E2 h3 −3/2 N NT exp A21 4 2Zion T  2me kB 3/2 e i e kTe

(18)

where A21 is the Einstein transition probability of spontaneous emission, and Eion is the ionization potential. E2 and g2 are upper level energy and degeneracy, respectively. l is the frequency of the emission line. Zion T is the partition function for ions, which is given by [45]: Zion T  =

 i

  E gi exp − i kTe

(19)

with gi the degeneracy or statistical weight of the i-th energy level Ei . At early times (10–100 ns), the plasma temperature is relatively high and the second ionization state can be important at this time. By including the second ionization contribution, the expression for continuum radiation can be rewritten as [44,45]:

c =

16e6 3c3 6m3e kB 1/2



     −hc −hc +G exp  Ne Ni+ +4Ni++ Te−1/2  1−exp kB T e kB T e (20)

where Ni+ and Ni++ are the number density of single and double charged ions. G is the free-free Gaunt factor, which is assumed to be unity by Kramer’s rule [29]. c is the frequency of the continuum emission.  is the free-bound continuum correction factor.

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From Eqns. (18) and (20), the line to continuum ratio is:   Eion −E2 exp kB Te l g 1        ++  Cr A21 2 = N −h c Zion l Te  1 − exp c c + G exp −h 1 + 4 Ni + k T k T 2c

B e

i

(21)

B e

c  l are the continuum and center wavelength of the spectral line, respectively. Cr is N ++ a constant. The ion ratio Ni + can be calculated using the Saha equation [46]: i

 ++  2me k Te 3/2 2Z++ T 1 Eion Ni++ = exp − h3 Z+ T Ne kB T e Ni+  3/2 + +  + Ni 2me kB T  2Z T 1 Eion = exp − k B Te Na h3 Z0 T Ne

(22) (23)

+ ++ where Eion and Eion are the first and second ionization potentials, respectively. Z++ T Z+ T and Z0 T are the partition function for second ionized, first ionized, ions and atoms, respectively. The evolution of the plasma temperature and electron number density were evaluated from the Si(I) line emission versus time (Fig. 11). Since continuum emission dominates initially, the electron number density and temperature could not be obtained for delay time less than 30 ns. After 30 ns, but before 300 ns, the line and continuum intensities were comparable; good measurement precision for the temperature calculation could be obtained. For later times t > 300 ns the continuum was very weak, and the lineto-continuum ratio would be sensitive to the errors of continuum determination. The data in Fig.11 show that in the early stage of plasma evolution (30–300 ns), temperature and electron number density decreased rapidly with time. Within 300 ns, the plasma temperature and electron number density decreased following a power-law dependence,

4 × 1019 5 × 104

6 × 104

T = A2 t –0.77 T (K)

4 × 104

104

3 × 1019 1019

ne (cm–3) ne = A2 t –0.99 100

30

2 × 104

3 × 1019

2 × 1019 300

1 × 1019

0

Electron number density (cm–3)

Plasma temperature (K)

8 × 104

0 0

500

1000

Delay time (ns)

Fig. 11. Temporal evolution of plasma temperature (T) and electron number density ne . The inset shows in log/log scale that in the early phase(30 ns–300 ns) of the plasma.

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proportional to t n (insert of Fig. 11). The exponent n, obtained by experimental data regression, was −070 and −099 for the temperature and electron number density, respectively. The analytical performance of LIBS is determined by the temperature and electron number density of laser induced plasma. When the continuum emission becomes weak, the plasma temperature calculated from ratio of line to continuum is not accurate. The plasma temperature can be estimated by using a Boltzmann plot based on measuring the relative intensities of lines with known transition probabilities and degeneracies. Temperature can be determined from the slope of a Boltzmann plot using ln I/gA vs. the upper energy level of the transition, where I is the line intensity, g is the statistical weight, and A is the transition probability.

3. PARTICLE FORMATION PROCESSES A significant quantity of the ablated mass is not excited vapor, but in the form of particles. Particle formation occurs from condensed vapor, liquid sample ejection, and solid-sample exfoliation. The mass ablated as particles does not contribute to a LIBS measurement unless these particles can be re-evaporated and excited by the plume itself. Particles are important for laser ablation ICP analysis. For LIBS, laser parameters must be established to eliminate particle formation.

3.1. Particle Ejection In most cases, laser ablated mass consists of primarily particles. Kelly et al. reported that homogeneous boiling within the molten layer was a significant mechanism responsible for particle removal during high-power nanosecond pulsed laser ablation [47,48]. Timeresolved shadowgraph images show that the violent ejection of particles occurs on the microsecond time scale (Fig. 12). After the laser pulse, there is a time period in which no particle ejection is observed; approximately 400 ns after the laser pulse, mass leaving the silicon surface begins to appear. The ejection of these particles lasts for about 30 s. The largest particles were estimated to be on the order of 10 m in size [49]. Much of the theoretical foundation on explosive boiling was established by Martynyuk [50,51]. A rapid heating rate is required to induce explosive boiling. For a 3-ns laser pulse duration, the heating rate can exceed 1012 deg/s. Since thermal diffusion takes place on the order of 10–11 s, a melt layer readily forms and propagates into the bulk sample (silicon in this case) during the laser pulse. The liquid silicon is heated above its boiling temperature and becomes metastable. Near the critical point, density fluctuations can generate vapor bubbles in the superheated liquid silicon. Vapor bubbles greater than a critical radius, rc, will grow; bubbles of size less than rc will collapse [17]. Once vapor bubbles of size rc are generated in the superheated liquid, they undergo a rapid transition into a mixture of vapor and liquid droplets. Rapid expansion of the high-pressure bubbles in the liquid leads to a violent ejection of molten droplets from the target surface [52]. This phase explosion process is detrimental to LIBS in that a significant portion of ablated mass is not utilized for analysis.

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1100 ns

12100 ns

Fig. 12. Liquid ejection from Si single crystal sample.

3.2. Nanoparticle Formation When the laser induced vapor plume cools to below the boiling point of the sample material, atoms begin to condense and form nano-particles. The size of particles is determined by the cooling time and the density of the vapor plume. Currently, numerous groups study particle formation mechanisms because of their influence for ICP-MS and nanotechnology applications [53–60]. For LIBS, the particles represent a loss of signal. However, by understanding particle formation mechanisms, laser parameters can be established to minimize this loss and ultimately increase LIBS sensitivity.

4. LASER ABLATION PARAMETERS As discussed above laser ablation involves complex non-linear phenomena that depend on the laser and sample material properties. The experimental isolation and effect of each laser and material parameter is very difficult without considering the interaction with other parameters. A general description of each of these parameters and their influence on ablation behavior is presented in this section.

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The primary mechanisms operative during ablation depend on the laser irradiance. In general, for low intensity nanosecond laser pulses, the dominant mechanism is thermal vaporization. For picosecond laser pulses and high intensity nanosecond laser pulses, both thermal and non-thermal mechanisms exist. For high intensity femtosecond laser pulses, Coulomb explosion is the primary ablation mechanism. Therefore, the laser pulse duration and irradiance are the most important factor for defining experimental conditions. In addition to the pulse duration, some of the other parameters considered during the selection of the laser are wavelength, energy, beam profile, repetition rate, fluence, etc. The influence of environmental ambient (gas and pressure) and the sample’s properties on laser ablation processes also will be discussed.

4.1. Nanosecond Pulsed Lasers Nanosecond pulsed lasers are the most commonly used for analytical applications, especially LIBS. Analytical performance using nanosecond lasers for LIBS has been studied in many papers, including the influence of the wavelength, energy, repetition rate, dual and multi-pulse regime, and the ambient gas [61–63].

4.1.1. Laser wavelength The laser wavelength effect on LIBS could be addressed from two points of view; a) the laser-material interaction (energy absorption) and b) plasma development and properties (plasma-material interaction). a) Laser-sample interaction: Shorter wavelengths offer higher photon energies for bond breaking and ionization processes. For example, the UV wavelength 193 nm has photon energy of 6.4 eV compared to 1064 nm that provides 1.16 eV. For most materials, bonding energy is a few eV. When the photon energy is higher than the bond energy, photon-ionization occurs and non-thermal mechanisms will play an important role in the ablation process. In addition, shorter optical penetration depth exits with UV-wavelengths, providing more laser energy per unit volume for ablation. In general, the shorter the laser wavelength, the higher the ablation rate and lower the elemental fractionation. Ablation rate is a parameter used to describe the amount of ablated mass per laser pulse per unit area. Ablation rate also is an indirect indicator of the coupling efficiency between the laser energy and the target material, and a measurement of the spatial resolution (lateral and depth resolution). An example of different ablation rates was presented by Gunther et al. [64]. They studied the ablation rate in metals using a 266nm-Nd:YAG laser and 193nm-excimer laser (Table 1). The ablation rate depended on the laser wavelength for samples with low optical absorbance; Fig. 13 shows the rate behavior for NIST standard reference materials 610–614 (series of glasses). Different ablation rates using 266 nm wavelength (Fig. 13a) implies different laser-material coupling efficiency for each of these samples. By using 193 nm wavelength, the three glass samples exhibited the same ablation rate (Fig. 13b); optical penetration depth was very shallow for these three samples at this wavelength. During the interaction of nanosecond laser pulses with materials, there is enough time for a thermal wave to propagate into the sample and create a relatively large molten layer. Evaporation occurs from the molten liquid, which can cause preferential evaporation

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Table 1. Ablation rates for metals and CaF2 (IR grade) and NIST silicate glasses at 23 J/cm2 Metal or compound

Ablation rate (mm/pulse) Nd:YAG

Ablation rate (mm/pulse) Excimer

104 039 020 012 013 013 055 014 018 054 02 20 034 060 067

07

92 71

013 030 029 065

457 475 355 35 61 445 356

Al Si Ti Cr Ni Cu Zn Mo Pt Au ZnSe CaF2 SRM NIST 610 SRM NIST 612 SRM NIST 614

025 075

028 029 029

160

266 nm 23 J/cm2(3.8 GW/cm2) Helium

NIST 610

100 80 60 40

(b)

NIST 614

25

NIST 612

120

Depth (μm)

Depth (μm)

30

(a)

NIST 614

140

Reflectance at 260 nm

NIST 612

20

NIST 610

193 nm 23 J/cm2(1.9 GW/cm2) Helium

15 10 5

20

0

0 0

100

200

300

400

Number of pulses applied

500

0

50

100

150

Number of pulses applied

Fig. 13. (a) Depth vs. applied number of pulse for the widely used standard reference materials NIST 610, 612 and 614 (266 nm Nd:YAG). Despite their similarity in their major element composition largely different ablation rates have been found ranging from 0.34 to 067 m/pulse using energy of 23 J/cm2 and from 0.49 to 096 m/pulse at 35 J/cm2 (inlet −54GW/cm2 ). (b) Depth vs. number of laser pulses for the standard reference materials NIST 610, 612 and 614 using the 193 nm Excimer LA system with an energy density of 23 J/cm2 38 GW/cm2 . Ref [64]

or elemental fractionation. Elemental fractionation is due to many factors, including wavelength, laser energy, pulse duration, sample properties, etc; wavelength is not the most critical parameter influencing fractionation [65]. Fractionation can be minimized or enhanced in any sample, depending on the laser beam irradiance. b) Plasma development and properties: The initiation of the plasma and its properties also depend on the laser wavelength. Plasma formation requires vaporization of the sample surface as a first step. The initiation of the nanosecond laser-induced plasma over the target surface could be promoted by two different photon absorption processes. One

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is inverse Bremsstrahlung absorption by which free electrons gain kinetic energy from the laser beam and during collisions among sample atoms and ions, electron and gas species. The second mechanism is photoionization of excited species and, at sufficiently high laser intensity, multiphoton ionization of excited or ground state atoms. Laserna et al. [66] studied the influence of wavelength (1064, 532 and 266 nm) on the plasma formation threshold for metal samples. Although laser energy coupling is more effective at shorter wavelengths, the fluence threshold for plasma formation was greater for 266 nm compared to 532 and 1064 nm. These studies agree with assumption of plasma ignition by inverse Bremsstrahlung, which is approximately proportional to 3 , (see Eqn. 8) considerably more favorable for IR than UV wavelengths. Russo et al. [65] studied the influence of laser wavelength on fractionation in laser ablation by comparing three different UV wavelengths (266 nm, 213 nm and 157 nm). It was found that the shorter the wavelength, the more controlled and reproducible was the ablation rate. Also, the shorter the wavelength, the lower was the fluence required to initiate ablation. These data support the proposed mechanism of photoionization and/or multiphoton ionization due to the higher photonic energy provide by UV wavelengths (7.9 eV, 5.8 eV, and 4.7 eV for 157 nm, 213 nm and 266 nm, respectively). For these wavelengths, the Inverse Bremsstrahlung process was less important. When the inverse Bremsstrahlung process occurs, part of the laser beam heats the plasma. Reheating the plasma can increase lifetime and the intensity of the emission lines, which would be beneficial to LIBS. However, an increase in the broadband background emission also can occur. The overall effect of the inverse Bremsstrahlung plasma reheating needs to be better investigated and understood. At high fluence, the efficiency of inverse Bremsstrahlung can be such that the plasma acts to shield the laser pulse energy (plasma shielding), from reaching the sample surface. Longer wavelengths favor the inverse Bremsstrahlung plasma shielding processes, but lower the ablation rate and increase the chances of elemental fractionation[37,67–69]. In general, most LIBS studies are based on the 1064 nm (IR) Nd:YAG wavelength, whereas most laser ablation ICP-MS studies are based on the fourth or fifth harmonic (266 or 213 nm) of the Nd:YAG laser.

4.1.2. Laser energy The primary energy related parameters influencing the laser material interaction are fluence (energy per unit area, J/cm2 ) and irradiance (energy per unit area and time, W/cm2 ). Laser ablation processes (i.e. melting, fusion, sublimation, erosion, explosion, etc) are dependent on the laser energy and the pulse duration, these different processes have different fluence (or irradiance) threshold [70–74]. These processes define the characteristics of the laser-induced plasma (temperature and number electron density) and the characteristics of the ablated mass. The effect of the laser energy alone is difficult to quantify. In general, the ablated mass quantity and the ablation rate increase with increase of the laser energy (when compared to same pulse duration and spot size). Russo et al. [37,75–79], investigated the plasma shielding effect on the mass ablation rate. By using different lasers, increased laser irradiance lead to increased ablated mass, as shown in Fig. 14 (∼03 GW/cm2 for a copper target). However, plasma shielding caused saturation in mass removal and constant ablation rate with further increase in irradiance. Studies of mass ablation rate dependence on experimental parameters (laser

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Cu ICP intensity/area

1012 1011 1010 109 108 107 106 105 0.01

0.1

1

10

100

Power density (GW/cm2)

Fig. 14. ICP emission intensity / area versus laser power density showing the two distinct mass ablation rate and roll-off at 0.3 GW/cm2 . Ref [36]

irradiance, spot size, pulse width, surface condition, etc) at atmospheric pressure showed that, in general, the mass ablation rate increased with increasing irradiance and decreasing pulse duration. Shorter pulses (on the order of picoseconds or femtosecond) produce higher mass ablation rates, likely because they are not affected by plasma shielding. Also, the fraction of the pulse energy lost by thermal diffusion (heat effected zone) in the sample is lower for shorter pulses [39,79,80]. Theoretical studies have been conducted to model the processes underlying nanosecond laser pulsed ablation. Bogaerts and Chen [81] presented a model to describe nanosecond pulsed laser ablation with “typical” experimental conditions; wavelength of 266 nm, a Gaussian-shaped laser pulse, pulse duration of 10 ns. Copper (Cu) was used as the sample. The model supported that laser-material interactions during nanosecond laser pulsed ablation include heating, melting, and vaporization of the material. For a laser irradiance of 107 W/cm2 , target heating was moderate, and neither melting nor vaporization took place. When the laser irradiance increased to 108 W/cm2 or 2 × 108 W/cm2 , target heating and melting became more pronounced, but evaporation was still limited; the vapor plume was short and cool and no plasma was formed. At higher laser irradiance 5 × 108 W/cm2 , target evaporation became much more significant, the plume became longer and hotter and a plasma was established. Plasma shielding was predicted at this irradiance. For laser irradiances between 5 × 108 and 1010 W/cm2 , target heating, melting and vaporization increased, and the plume lifetime became longer. The vapor density, temperature, and the degree of ionization increased with irradiance. Although this model did not include all mechanisms underlying nanosecond pulsed laser ablation, it qualitatively supported measured behavior. Dumitru et al. [82] developed a numerical model based on enthalpy to describe nanosecond laser ablation. The authors reported that vaporization stopped before the end of the laser pulse. The effect was related to shielding by the plume, which is continuously growing and whose adsorption is increasing. Differences in ablated volumes were reported when the calculation was performed with different pulse durations (1 to 100 ns); the lower the pulse duration, the higher the ablated volume.

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4.1.3. Ambient The simplicity of LIBS derives from the fact that sample preparation is not required and in situ analysis at atmospheric pressure can be performed. However the interest of seeking new applications for LIBS, for example, designing an analytical instrument that can operate in Venus or Mars atmospheres [83,84], has led to studies of LIBS performance under different ambient conditions. The laser induced-plasma size, propagation speed, stability, energy and emission properties depend strongly on the gas ambient into which the plasma expands. Plasma properties have been studied in different gases [81,85–89], as a function of pressure [85,90], even in liquids [91–93]. The ambient gas either helps or prevents the coupling of laser energy into the plasma. For example, the ambient can shield the sample from the laser beam (plasma shielding) if gas breakdown occurs before sample vaporization occurs [85]. The ambient also can quench the luminous plume by collisional cooling; a shorter plasma lifetime with lower temperatures was found in air atmosphere compared to argon [94]. This observation was explained by lower conductivity and specific heat of argon with respect to air. Wisbrun et al. [95] found that the collisional translation energy also was dependent on the atomic mass of the ambient gas, being less effective when the atomic mass increased, thereby causing a plasma with longer lifetime. These opposite effects may be less significant for gaseous or aerosols samples, but will be more important for solid samples in the form of reduced ablation rates (plasma shielding), higher continuum background, and shorter-lifetime (fast dissipation). The ambient gas also can be helpful as to confine the expanding plume and minimize background emission by atmospheric elements. In a particular case where the UV spectrum ranged from 100–200 nm, the use of inert gas avoided the absorption by oxygen molecules [96]. Ambient pressure will influence plasma expansion and LIBS emission intensities. For low ambient pressure (> n . De Michelis and Mattioli [31] and references there-in have discussed this subject in detail.

4.2. Line Emission The emissivity of a spectral line (i.e. number of photons emitted per unit time and per unit volume) is equal to the product of its radiative transition probability times the emitting excited level density. On the other hand, the absorption coefficient for line radiation is obtained from the radiative transition probability by using the Einstein relations. For the interpretation of the line emission, the excited level population should be known.

4.3. Temporal and Spatial Resolution of Emission The emission from laser produced plasma in general and optical emission in particular has been studied extensively using temporally and spatially resolved spectroscopy [1,9–16]. It has been observed that initially after plasma formation an intense continuum is emitted,

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which remains close to the target surface. This emission from the dense plasma is like a black body continuum radiation. However, as the plasma expands away from the surface of the target, it cools and the emission is dominated by the spectral lines. During this expansion the spectral lines from highly ionized atoms are observed close to the target surface whereas those from neutral atoms are observed in the plume away from the target. Thus line emissions are superimposed on the continuum emission. The line emission from the multiply ionized species occurs at the time of plasma formation, whereas emission from singly ionized and neutral species are observed nearly 500 ns after the plasma-formation. The rates of decay of the continuum and line emissions are different. The continuum emission due to Bremsstrahlung from hot plasma decays faster in comparison to the line emission. Thus it is necessary to record the emission after a certain time delay to get clear information about the line emission from the cold plasma for the purpose of elemental analysis.

5. THEORETICAL MODELS FOR PLASMA Interpretation of the radiation emitted by the plasmas requires knowledge of both the charge state distribution and the excited level populations of different ions. This is possible by obtaining the solution of a complex system of rate equations, describing the population and depopulation of all the levels by the processes such as ionization, recombination, collisional excitation and de-excitation, radiative decay and absorption as well as the stimulated emission. Any given charge state is connected with its two neighbouring states by the processes of ionization and recombination. Considering the difficulties associated in solving these equations, the approximations used in order of increasing electron density are: the corona model (CM), the collisional-radiative model (CRM), the local thermodynamic equilibrium (LTE) and models suitable for ultra high density Ne ≥ 1024 cm−3  plasmas [29,31,33].

5.1. Corona Model In this approximation there is a balance between collisional ionisation (and excitation) and recombination (and spontaneous decay). This model, therefore, depends critically on the knowledge of atomic cross sections. Assuming that free electrons have a Maxwellian velocity distribution, it is required that only a negligible number of ions be in excited levels (as compared to the ground level). Two neighbouring ionisation states, of charge Z and Z + 1, are then connected by NZ Ne SZ Te  = NZ+1 Ne Z+1 Te  Ne  or  T  N  NZ = Z+1 e e SZ Te  NZ+1

(3)

where Sz and Z+l are the ionisation and recombination rate coefficients, respectively. Eq. (3) is, to a first approximation, independent of Ne . In this approximation a balance

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between the rate of collisional excitation from the ground level and the rate of spontaneous radiative decay determines the population densities of excited levels. This model requires that the electron density be sufficiently low for collisions not to interfere with radiative emission.

5.2. Local Thermodynamic Equilibrium Model In the LTE model it is assumed that mainly particle collision processes determine the distribution of population densities of the electrons, which are so fast that the latter take place with sufficient rapidity and the distribution responds instantaneously to any change in the plasma condition. In such circumstances each process is accompanied by its inverse and these pairs of processes occur at equal rate by the principle of detailed balance. Thus the distribution of population densities of the energy levels of the electrons is the same as it would be in a system in complete equilibrium. The population distribution is determined by the statistical mechanical law of equipartition among the energy levels and does not require the knowledge of atomic cross sections. Actually the plasma temperature and density vary in space and time, but the distribution of population densities at any instant and point in space depends entirely on the local values of temperature, density and chemical composition of the plasma. The uncertainty in prediction of spectral line intensities from LTE model plasma depend mainly on the uncertainty in the values of these plasma parameters and atomic transition probabilities. For analytical plasmas the condition of LTE is considered very much vital for getting any reliable quantitative information. In the case of thermal equilibrium all the processes in the plasma are collision dominated as discussed above and the plasma can be considered as having a single temperature Ti = Te . However in the expanding plasma this is possible only locally and for specific time segment during the evolution. The following criterion must be satisfied by the plasma to be in local thermodynamic equilibrium [33–34]: Ne ≥ 1 6 × 1012 E 3 Te1/2

(4)

where E eV is the largest observed transition energy for which the condition holds, and Te is the excitation temperature (K). It should be noted that the choice of the time delay is crucial for obtaining the best operating conditions in the LIBS plasma to ensure that LTE prevails during the measurements for obtaining the quantitative results. However, it has also been found that LTE is not an indispensable condition for qualitative analysis, once the measurement conditions are kept constant and are exactly reproducible.

5.3. Collisional Radiative Model In the case of high density plasma both of the above models can not be used safely (although LTE is sometimes marginally satisfied). Salzmann [35] has addressed the

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applicability of the CM and LTE approximations to laser-produced plasmas. In the intermediate density range the rate equations have to be solved up to the thermal limit. The total ion densities of two successive ionisation states of charge Z and Z + 1 are connected by eff NZ SZeff = NZ+1 eff Z+1 + Ne Z+1 

(5)

eff where SZeff  eff Z+1 and Z+1 are the effective (or net) ionisation, recombination and three-body recombination rate coefficients, respectively. The last term dominates only at high densities, when NNZ tends to the Saha equation value. At low electron densiZ+1 ties the three-body recombination is negligible, and Eq. (5) tends to Eq. (3) (excited state populations become negligible with respect to the ground-state population). The CRM system can be solved locally for given Ne and Te values. It can however also be coupled to a hydrodynamic code describing the plasma evolution. Capitelli et al. [36] have studied non-equilibrium and equilibrium problems in laser induced breakdown plasma. Particularly they focused on the problem associated with the fluid dynamics of the expanding plume with time dependent collisional-radiative models for describing the population densities of excited states and with the time dependent Boltzman equation for characterizing the electron energy distribution function in the LIBS plasmas. It was found that the violation of equilibrium conditions in laser-produced plasma near the surface could be caused by the decrease in the plasma temperature due to expansion. This can occur if the characteristic time of such temperature decrease is less than or comparable with that corresponding to the ionization balance, which is esti−1 mated as ion ≈ Na kion −1 , where Na cm−3  and kion cm3 s  are the number density of heavy particles and ionization rate coefficients respectively. For typical laser plasma conditions Na ≈ 3 × 1019 cm−3  Te ≈ 2 eV ion is in the range ≈ 10−6 − 10−5 s. This quantity has to be compared with the characteristics time of the laser plasma expansion, i.e. exp ≈ d/v, where d is the laser spot diameter and v ≈ 105 − 106 cm/s is the plasma expansion velocity. It can be concluded that the equilibrium conditions are violated, whenever

ion ≥ exp

(6)

which takes place at laser spot sizes d < 1 cm. In this condition, the deviation from the equilibrium state has a recombination character. LIBS plasmas, characterized by large electron densities and electron temperatures, apparently seem to satisfy the LTE conditions. However, the characteristics equilibrium times for the different phenomena can occur in the same temporal scale during which LIBS measurements are performed. Dedicated experiments and the development of a unitary theory, which takes into account the fluid dynamics and the kinetic aspects, is necessary to completely master the experimental conditions for the development of a calibration–free LIBS [37].

6. MEASUREMENT OF PLASMA PARAMETERS During the LIBS experiments some important parameters such as emission line shape functions, electron density and plasma temperature are required in order to produce

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reliable spectral features. Some relevant information about the measurement of these parameters has been given in the following sections.

6.1. Line Broadening In the spectroscopic study of line emission from the plasma, intensity of the spectral line as well as its profile plays an important role. The emission line profile is important because it contains information related with the emitter and surrounding plasma environment. An introduction to the theory of line broadening can be found in the books by Griem [29,38] and Sobelman et al. [39]. Various types of line broadening have been observed in the plasma emission. Natural broadening occurs due to the finite lifetime of excited states and results in a Lorentzian profile. It must be considered in particular cases of highly ionised ions like Fe25+ . The Doppler broadening occurs due to thermal or directed motion of the emitting ions and has a Gaussian line shape with a width proportional to the square root of the emitter temperature. This is the dominant broadening mechanism at low electron densities. Stark broadening which is also known as pressure broadening is observed because the emitting ions experience an electric field due to the presence of plasma electrons and ions around them. This field varies statistically for individual emitters and fluctuates in time. The net result is an ensemble averaged line shape with an overall width related to the average strength of the perturbation in the bound states. During the early stage of plasma formation in the LIBS experiment the electron density remains very high ne ∼1015 −1018 cm−3 . As a consequence, the line profiles are dominated by Stark/pressure broadening for a considerable period of time [14]. Doppler as well as natural broadening is generally negligible during this period. When the expanded plasma cools and the electron density decreases, the dominance of the Stark broadening is reduced and finally Doppler broadening starts playing a leading role. It is well known that measured line profiles normally contain contribution from instrument resolution width also, if spectrometers are used for wavelength analysis. This indicates that measured profiles must be deconvoluted prior to their analysis for the extraction of plasma parameters. The instrument width needs to be measured experimentally for a given spectrometer, which is dependent on the parameters such as slit width, the grating dispersion, and the dynamic behavior of the photon detector. The central peak of the spectrometer slit function can be approximated to a good degree of accuracy by a Lorentzian profile. Since both (the Stark-broadened) spectral line and the spectrometer exhibit Lorentz shape functions, convolution and deconvolution become easy. In this case line widths can simply be added or subtracted in the convolution/deconvolution process as given below Total = line + spectrometer

(7)

This shows that actual line width can easily be extracted from the measured line width by simply subtracting the instrument width. Samek et al. [14] fitted the line shape for CaI and CaII for the highest concentration in liquid sample and nearly perfect Lorenzian profiles were obtained even for delay time of 5 ps, a significant fraction of the laser energy absorbed by the plasma expanding in front of the target should contribute to increasing the excitation temperature. This higher initial temperature lengthens the plasma cooling phase as compared with the sub-picosecond regime. In the latter case, the absorbed laser energy is fully deposited in matter at the solid density and no further plasma heating takes place. In both cases (short and long pulses), at the end of the laser pulse, the plasma cools down by the same mechanisms, namely: (i) thermal conduction with the ambient air and the unablated target, (ii) the work done by the expanding plasma against the ambient air and (iii) radiative losses [41]. Fig. 8 shows the time evolution of the Mg (I) 285.2 nm, Al (II) 281.6 nm and the continuum for three different laser pulse durations. One observes significant differences in the temporal evolution of the line emission for the various laser pulse durations

1

1

b: 5 ps Normalized intensity (arb. units)

Normalized intensity (arb. units)

a: 500 fs 0.1

0.01

1E–3

1E–4

1E–5 0.01

Continuum Mg (I) 285.21 nm Al (II) 281.60 nm

0.1

1

10

0.1

0.01

1E–3

1E–4

Continuum Mg (I) 285.21 nm Al (II) 281.60 nm

1E–5 0.01

100

0.1

Delay (µs)

1

10

100

Delay (µs)

1

Normalized intensity (arb. units)

c: 270 ps 0.1

0.01

1E–3

1E–4

1E–5 0.01

Continuum Mg (I) 285.21 nm Al (II) 281.60 nm

0.1

1

10

100

Delay (µs)

Fig. 8. Time evolution of the line intensity of Mg (I) 285.2, Al (II) 281.6 nm and the continuum for three different laser pulse durations (500 fs, 5 ps, 270 ps).

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considered, while the rate of decrease of the continuum emission is constant. As the pulse duration increases the plasma takes longer to decay so that the radiation emission lasts longer and the emergence of the spectral lines above continuum occurs later. Nevertheless, it is likely that laser-produced plasmas evolve through similar transient states, the only difference being that these states, close to LTE, are reached at different delays after the laser shot. Therefore, temporal gating parameters appear to be key parameters for LIBS performance optimization and must be appropriately chosen for each laser pulse duration. As we will see later, once this optimization of the delay and integration time is made, the laser pulse duration itself seems not to appear as a critical point in LIBS science. These results mentioned here for the temperature and electron density in the ultrashort plasma produced in air at atmospheric pressure are in agreement and in line with other values published in the literature [40–53] in similar conditions. It should be mentioned that the values of temperature and electron density presented here are space-averaged values where no Abel inversion was used. The emission signal can be taken from the side of the plasma or from the top or over the complete plasma volume from different views.

4. SPECTROCHEMICAL ANALYSIS BY ULTRA-SHORT LASER-INDUCED PLASMA For spectrochemical analysis by laser-induced plasma, or any other sources of excitation, there are two important parameters considered by the chemist in the evaluation of a spectroscopic technique from an analytical point of view: the limit of detection and the precision. In emission spectroscopy, the spectral line intensities, which are related to the species concentrations, are strongly influenced by various parameters that cannot be usefully controlled. Among these parameters, are the quantity of vaporized matter, the degree of ionization, which depend on the laser pulse parameters (pulse duration, wavelength, energy, beam quality, focusing conditions) and on the target characteristics (thermal conductivity, reflectivity, melting and vaporization temperature, etc.). Another important parameter is the surrounding atmosphere (pressure and composition). In this section, we will compare the LIBS spectrochemical analysis performances by using ultra-short pulse and long pulse laser-induced plasma based on the recent literature in the field. (Here we will deal only with ambient air as a surrounding atmosphere.). Although pulse duration is known to strongly affect the laser ablation dynamics as shown in Section 3.3, only a few studies have compared the characteristics of plasmas generated with nanosecond and sub-picosecond or picosecond pulses and discussed to what extent pulse duration will affect the analytical performances of the LIBS independently from the tools used to get the analytical signal [45–53]. It is natural to examine and evaluate to what extent for a given fluence the analytical performances of LIBS could be significantly improved by a suitable choice of laser pulse duration. In the last Section 3.3.3, we discussed the influence of the laser pulse duration on the plasma characteristics, in this section, we will be focusing on the analytical aspects of LIBS based on the literature on the subject. For example, Le Drogoff et al. [45,50] studied the quantitative analysis of minor elements in aluminum and copper alloys for three typical laser pulse durations, namely 90 fs, 2 ps and 270 ps, chosen to represent the three regimes of laser-matter ablation. The 90 fs pulse represents the regime of pulses so

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short that there is negligible conduction of heat into the target or movement of heated material out of the target during the laser pulse. The 2 ps pulse is representative of the situation where, during the laser pulse, significantly more energy is conducted away from the deposition (skin-depth) area, but where outward motion during the laser pulse is still negligible. Finally the 270 ps pulse, although much shorter than usual LIBS laser pulses, still represents the same situation, namely, that during the laser pulse there is considerable heat transfer to the interior and excessive laser-heating of the expanding plasma. Le Drogoff et al. used experimental conditions similar to Sabsabi et al. [54] with the more conventional Q-switched Nd:YAG laser operating at the fundamental wavelength of 1064 nm and a pulse width of 8 ns. Fig. 9 presents the calibration curves for silver in copper alloys for the two extreme laser pulse durations considered here (90 fs and 270 ps). One clearly sees that the calibration curves do not vary linearly with the Ag concentration in the matrix, the nonlinearity problem being much worse for the shortest pulse. It is only at very low Ag concentration, typically below 75 ppm, that the signal intensity varies linearly with the concentration and falls to zero with the element concentration. This deviation at higher concentration of the calibration curves from the linear relationship results from self-absorption due to the resonant character of the lines considered here. Similar behaviors were observed for any of the temporal windows considered for other pulse durations. The self-absorption mechanism depends on the concentration of the minor element studied in the lower level of the transition (most often the ground level) as well as on the plasma thickness and on the oscillator strength of the transition. In the examples of Fig. 9, self-absorption is so strong that a self-reversed effect of the line occurs, this phenomenon being considerably more pronounced as the laser pulse duration is reduced.

3.0 × 104

Intensity (cps)

2.5 × 104

2.0 × 104

1.5 × 104

1.0 × 104 2

20 J/cm

5.0 × 103

0.0

τ = 90 fs τ = 270 ps

0

50

100

150

200

250

300

350

400

450

Ag in copper alloys (ppm)

Fig. 9. Calibration curve of silver vs. its concentration in copper alloys for the two extreme laser pulse durations (90 fs and 270 ps). The temporal gating parameters were: delay time td = 2 s and gate width tw = 5 s. Note the severe nonlinear behavior for the short pulse and the significantly nonlinear behavior for the long pulse.

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This latter observation is consistent with the fact that when  decreases, the discrepancy between the ideal linear calibration curves and the leveling of the experimental calibration curves increases (see Fig. 9). In addition, it is worth mentioning that using 8 ns laser pulses with similar samples, Sabsabi et al. [55] did not observe self-absorption for this Ag line  = 338 nm. The increase of self-absorption for sub-picosecond pulses may be understood as a consequence of both higher ablation efficiency [13–33] and lower plasma temperature [40–45], which tend to make the population density of the ground state species higher thus enhancing the absorption of the resonant lines. The use of non-resonant lines could help to minimize self-absorption and to increase the dynamic range of the detection method to higher concentrations. Again this is the fact that non-resonant lines are usually much less intense than resonant ones (as one should expect since excitation to the non-resonant lower state is likely to be rare). Clearly, when the concentration is very low, resonant lines should be used for analyte detection since nonlinearity is not then a problem. Since these lines are the most intense, they also naturally yield the lowest limits of detection (LOD). As far as LOD is concerned, the results in the literature show the critical importance of optimizing the observation delay and the integration time [45]. Despite this, tremendous variation in LODs obtained as gate windows are changed, in general, for a given element, comparable LOD values can be obtained for each of the different pulse durations provided the best temporal window is chosen. However, provided the best gate is used for each pulse duration, the results do not show significant evidence of an optimum pulse duration as far as LODs are concerned Le Drogoff et al. [45] showed that, gate-optimized LOD values as low as ∼2, 14, 2, and 10 ppm were found for Cu, Si, Ag and Ni, respectively. Their values of LOD are consistent with those obtained by Sabsabi et al. [54] of 10, 14, 1, and 10 for Cu, Si, Ag and Ni respectively, for an optimal time window with td = 10 s and tg = 10 s, using a 8 ns pulse duration and an energy density similar to that used in the present work. From all these observations, it appears that, for a given pulse duration and element, the optimum gate and integration time need to be found for optimizing the LOD (see Fig. 10). Once this window is determined, there is no evidence of a particular pulse duration that would optimize the LOD, so the choice of pulse duration can be made on other grounds. Recently, similar finding were obtained by Stavropoulos et al. [52] in comparing the LOD of Al, Fe, and Si in metallic samples under nanosecond (6 ns) and picosecond laser excitation (35 ps).

5. NON-GATED ANALYSIS BY ULTRA-SHORT LASER PULSES Considering the lower continuum emission of plasmas produced by sub-picosecond rather than nanosecond laser pulses, it might be possible to perform LIBS analysis without any detector gating (i.e. no delay and very long integration time), the protocol to be referred to here as “non-gated”. Recent studies have shown that the non-gated spectra obtained with picosecond and sub-picosecond laser produced plasmas show relatively low background and better line thinness in comparison with nanosecond lasers [13,45,50–53]. However, according to the authors of those works, the signal-to-background ratio of the non-gated spectrum emission is significantly poorer.

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167 Si(I) 288.16 nm

120.00

80.00

60.00

40.00

Limit of detection (ppm)

100.00

20.00

0.00

0.5–2.5

2–5 Dela y - In 8–10 100 fs t. tim e (µs )

500 fs

5 ps

1 ps

200 ps

n

duratio Laser pulse

Fig. 10. Limit of detection obtained for silicon in aluminum alloys by using different laser pulse duration and different temporal conditions.

In order to test the analytical performances of LIBS in absence of gating, Le Drogoff et al. [45] examined this approach for three laser pulse durations representing the three regimes (sub-pico, pico and nanosecond). For this purpose, the plasma emission was integrated over 200 s from the laser trigger signal. Their results showed, as expected, that the continuum and line emission increase with . This results from the longer plasma lifetime. The non-gated detection limits obtained for Si and Cu in aluminum alloy and Ag in copper alloy are summarized in Table 1 for each of the three pulse durations and for the

Table 1. Non-gated limits of detection (LOD) and calibration plot data for Cu and Si in aluminum alloy samples and silver in copper alloy samples, together with best-gated LOD values  

Minor Element

Best gated LOD (ppm)

LOD (ppm)

80 fs 2 ps 270 ps

Cu

32 17 20

106 ± 29 80 ± 19 83 ± 25

80 fs 2 ps 270 ps

Si

305 141 172

3512 ± 538 1826 ± 254 1949 ± 318

80 fs 2 ps 270 ps

Ag

14 13 22

37 ± 11 43 ± 09 86 ± 22

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best gate. The best non-gated LOD results (i.e., ∼182, 8, and 3 ppm for Si, Cu and Ag, respectively) are typically obtained for  = 2 ps, which presents the best compromise between a high signal-to-background ratio and a relatively low noise level. However, a comparison with the best-gate results indicates that in the case of Cu and Ag, the values for limits of detection (LODs) are higher (i.e. worse) by a factor of 3–4 than those obtained by optimally gating the signal. In the case of Si, gated measurements allow a much greater improvement (by about one order of magnitude).

6. CONCLUSIONS While use of long pulse laser in LIBS analysis has matured over many years, femtosecond LIBS is still in its infancy. The general conclusions to be drawn from the literature in the benefits of using femtosecond pulses include: • • • •

Ultrafast excitation can improve the material interaction Ultrafast absorption of energy reduces post ejection interactions Heat affected zone is confined to smaller region – less vaporization of substrate Potential for highly selective desorption-ionization

It is too early for a sound general assessment of the potential of femtosecond laser for LIBS analysis. This chapter is a first step for investigating whether these specific features of ultra-short laser–matter interaction may offer advantages with respect to longer pulses when LIBS applications are concerned. To this regard, we tried to identify the specific potential advantages of the use of ultrshort laser pulses in LIBS analysis based on the finding of the few works published on the subject. We have discussed the effect of laser pulse duration on the ablation rate, ablation threshold, plasma characteristics and analytical performance of the laser-induced plasma for spectrochemical analysis. We have also discussed the sensitivity of LIBS analysis to pulse duration for a few minor elements embedded in aluminum and copper alloys. It appears that the dependence of the time decay of the plasma emission on the laser pulse duration requires optimizing the temporal gating parameters. The results indicated that, providing the best gate window is chosen, LIBS performance for most applications is almost independent of the laser pulse duration (at least for the representative values used of 80 fs, 2 ps, 270 ps). In this context, the choice of the laser system should be dictated by considerations such as cost and robustness. To this regard, the ns laser is more robust for field application and it is much cheaper than sub-picosecond laser. Considering the lower continuum emission of plasmas produced by sub-picosecond rather than nanosecond laser pulses, we have examined the possibility of performing LIBS analysis without any detector gating. The results indicated that gated LIBS spectra using picosecond or sub-picosecond lasers provide better LOD for the elements studied in this chapter than the non-gated spectra. The degree of improvement of the LIBS sensitivity achieved with optimal gating rather than non-gated spectra is element-dependent for all three laser pulse durations (80 fs, 2 ps, 270 ps). We believe that LIBS could, however, benefit from using ultra-short laser pulses in the framework of microanalysis and profilometry, but for reasons other than sensitivity or limits of detection. There is a growing demand for the development of new microanalysis

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techniques compatible with industrial requirements and applicable to routine analysis or industrial applications. Femtosecond laser pulses, which provide high lateral and depth resolution of the ablation process could be advantageous for microanalysis, since the lateral and transverse thermal effects induced by nanosecond laser pulses can be avoided. Furthermore, since there is no thermal heating and no plasma shielding are involved we believe that the accuracy of LIBS could be improved. This approach has just begun to be investigated and further studies are needed to determine just how important these advantages may actually be. Finally, it seems to me too early to establish a general assessment on the comparison of the ns and ultra-short laser-pulse duration for LIBS analysis, however the results obtained by several authors in the field can be summarized as the following: • The short pulses provide better ablation efficiency and lower threshold. • The temperature increases slightly with laser pulse duration while the electron density is independent of it. • The optimal integration time varies with the laser pulse duration. • The performances of the LIBS in terms of sensitivity are almost independent from the laser pulse duration if we chose appropriate optimal time conditions. • The spatial resolution obtained by fs pulses is better than ns pulses. • LOD are worse for non-gated than gated arrangements. The accuracy is expected to be better, however, the cost of the fs laser is 10 times higher than ns laser.

ACKNOWLEDGMENTS The author wishes to thank his colleagues S. Laville, F. Vidal, M. Chaker, J. Margot and L. Radziemski for their help in reading the manuscript and useful discussion. Support from the NRC is also gratefully acknowledged.

REFERENCES [1] D. Gunther, S.E. Jackson, and H.P. Longerich, Spectrochim. Acta Part B 54 (1999) 381. [2] R.E. Russo, X. Oleg, V. Borisov, and H. Liu, Laser ablation in atomic spectrometry. Encyclopedia of Analytical Chemistry: Instrumentation and Applications, Wiley, Chichester, (2000). [3] N. Bloembergen. IEEE J. Quant. Elec.,Vol. QE-10, (1974) 375. [4] A. Penzkofer, D. von der Linde, A. Laubereau and W. Kaiser, Appl. Phys. Lett. 20 (1972) 351. [5] L. Radziemski, Spectrochim. Acta Part B 57 (2002) 1109. [6] W. Kaiser, Ed., Ultrashort Laser Pulses: Generation and Applications, Topics in Applied Physics, Vol. 60, Springer, Berlin, (1993). [7] S.L. Shapiro, Ed., Ultrashort Light Pulses: Picosecond Techniques and Applications, Topics in Applied Physics, Vol. 18, Springer, Berlin, (1977). [8] D. von der Linde, K. Sokolowski-Tinten and J. Bialkowski, Appl. Surf. Sci. 109/110 (1997) 1. [9] R. Russo, Appl. Spectrosc. 49 (1995) 14A [10] B. Rethfeld, K. Sokolowski-Tinten, D. von der Linde, S.I. Ansimov, Appl. Phys. A 79 (2004) 767.

170

M. Sabsabi

[11] E.G. Gamaly, A.V. Rode, B. Luther-Davies and V.T. Tikhonchuk, Phys. Plasmas, 9 (2002) 949. [12] S. Laville, F. Vidal, T.W. Johnston, O. Barthélemy, M. Chaker, B. Le Drogoff, J. Margot, and M. Sabsabi, Physical Review E, 66 (2002) 066415 1–7. [13] Vanja Margetic, Ph.D thesis, Dortmund University, (2002). [14] D. von der Linde and H. Schüler, J. Opt. Soc. Am. B 13 (1996) 216. [15] S. Preuss, A. Demchuk, and M. Stuke, Appl. Phys. A 61 (1997) 33. [16] K. Furusawa, K. Takahashi, H. Kumagai, K. Midorikawa, and M. Obara, Appl. Phys. A 69 [Suppl.] (1999) 359. [17] S. Nolte, C. Momma, H. Jacobs, A. Tünnermann, B.N. Chichkov, B. Wellegehausen, and H. Welling, J. Opt. Soc. Am. B. 14 (1997) 2716. [18] D. Günther, I. Horn, and B. Hattendorf, Fresenius J. Anal. Chem. 368 (2000) 4. [19] F. Vidal, S. Laville, M. Chaker, T.W. Johnston, B. Ledrogoff, J. Margot, and M. Sabsabi, Phys. Rev. Lett. 86 (2001) 2573. [20] N. Arnold, J. Gruber, and J. Heitz, Appl. Phys. A 69 [suppl.] (1999) 87. [21] D. Strickland and G. Mourou, Opt. Commun. 56 (1985) 219. [22] B. Le Drogoff, F. Vidal, Y. von Kaenel, M. Chaker, M. Sabsabi, T.W. Johnston, S. Laville, J. Margot, J. Appl. Phys. 89 (2001) 8247. [23] B.N. Chikov, C. Momma, S. Nolte, F. von Alvensleben and A. Tünnermann, Appl. Phys. A 63 (1996) 109. [24] A. Semerok, C. Chaléard, V. Detalle, J.-L. Lacour, P. Mauchien, P. Meynadier, C. Nouvellon, B. Sallé, P. Palianov, M. Perdrix and G. Petite, Appl. Surf. Sci. 138/139 (1999) 311. [25] V. Margetic, A. Pakulev, A. Stockhaus, M. Bolshov, K. Niemax and R. Hergenröder, Spectrochim. Acta B55 (2000) 1771. [26] P.P. Pronko, S.K. Dutta, D. Du, and R.K. Singh, J. Appl. Phys 78 (1995) 6233. [27] B.C. Stuart, M.D. Feit, A.M. Rubenchik, B.W. Shore, and M.D. Perry, Phys. Rev. Lett. 74 (1995) 2248. [28] P.B. Corkum, F. Brunel, N.K. Sherman, and T. Srinivasan-Rao, Phys. Rev. Lett. 61 (1988) 2886. [29] H. Jacobs, B.N. Chichkov, C. Momma, S. Nolte, F. von Alvensleben and A. Tunnermann, Appl. Phys. A 63 (1996) 109. [30] C. Momma, S. Nolte, B.N. Chichkov, F. von Alvensleben, and A. Tünnermann, Appl. Surf. Sci. 110 (1997) 15. [31] A. Semerok, C. Chaléard, V. Detalle, J.-L. Lacour, P. Mauchien, P. Meynadier, C. Nouvellon, B. Sallé, P. Palianov, M. Perdrix, and G. Petite, Appl. Surf. Sci. 139 (1999) 311. [32] B. Sallé, O. Gobert, P. Meynadier, M. Perdrix, G. Petite, and A.Semerok, Appl. Phys. A 69 (1999) S381. [33] B. Le Drogoff, F. Vidal, S. Laville, M. Chaker, T. Johnston, O. Barthélemy, J. Margot, and M. Sabsabi, Appl. Opt. 44 (2005) 278. [34] S. Yalcin, Y. Y. Tsui and R. Fedosejevs, J. Anal. At. Spectrom. 19 (2004) 1295. [35] D.A. Rusak, B.C. Castle, B.W. Smith and J.D. Winefordner, Crit. Rev. Anal. Chem. 27 (1997) 257. [36] E. Tognoni, V. Palleschi, M. Corsi and G. Cristoforetti, Spectrochim. Acta B57 (2002) 1115. [37] E.H. Evans, J.A. Day, W.J. Price, C.M.M. Smith, K. Sutton and J.F. Tyson, J. Anal. At. Spectrom. 18 (2003) 808. [38] W.-B. Lee, J. Wu, and Y-I. Lee, J. Sneddon, Appl. Spectrosc. Rev. 39 (2003) 27. [39] J. Vadillo, and J.J. Laserna, Spectrochim. Acta Part B. 59 (2004) 147. [40] B. Le Drogoff, J. Margot, F. Vidal, S. Laville, M. Chaker, M.Sabsabi, T.W. Johnston, and O. Barthélemy, Plasma Sources Science & Technologies. 13 (2004) 223. [41] S. Laville, F. Vidal, T.W. Johnston, M. Chaker, B. Le Drogoff, O. Barthélemy, J. Margot, and M. Sabsabi, Phys. of Plasmas. 11 (2004) 2182.

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[42] R.E. Russo, X.L Mao, C. Liu and J. Gonzalez, J. Anal. At. Spectrom. 19 (2004) 1084. [43] V. Margetic, K. Niemax, and R. Hergenröder, Spectrochim. Acta B56 (2001) 1003. [44] F. Vidal, S. Laville, T.W. Johnston, O. Barthélemy, M. Chaker, B. Le Drogoff, J. Margot and M. Sabsabi, Spectrochim. Acta B56 (2001) 973. [45] B. Le Drogoff, J. Margot, M. Chaker, M. Sabsabi, O. Barthélemy, T.W. Johnston, S. Laville, and F. Vidal, Spectrochim. Acta B56 (2001) 987. [46] V. Margetic, T. Ban, F. Leis, K. Niemax and R. Hergenröder, Spectrochim. Acta B58 (2003) 415. [47] S.M. Angel, D.N. Stratis, K.L. Eland, T. Lai, M.A. Berg, and D.M. Gold, Fresenius J. Anal. Chem. 369 (2001) 320. [48] K.L. Eland, D.N. Stratis, D.M. Gold, S.R. Goode, and S.M. Angel, Appl. Spectrosc. 55 (2001) 286. [49] G.W. Rieger, M. Taschuk, Y.Y. Tsui, and R. Fedosejevs, Spectrochim. Acta B58 (2003) 497. [50] Le Drogoff, M. Sabsabi, M. Chaker, J. Margot, O. Barthélemy, T.W. Johnston, S. Laville, and F. Vidal, Appl. Spectrosc. 58 (2004) 122. [51] J.-B. Sirven, B. Bousquet, L. Canioni and L. Sarger, Spectrochim. Acta B59 (2004) 1033. [52] P. Stavropoulos, C. Palagas, G.N. Angelopoulos, D.N. Papamantellos and S. Couris, Spectrochim. Acta B59 (2004) 1885. [53] D. Grojo, J. Hermann and A. Perrone. J. Appl. Phys. 97 (2005) 063306 (1–9). [54] M. Sabsabi and P. Cielo, Appl. Spectrosc. 49 (1995) 499.

Chapter 8

Micro-LIBS M. T. Taschuk, I. V. Cravetchi, Y. Y. Tsui and R. Fedosejevs Department of Electrical and Computer Engineering, University of Alberta, Edmonton, Alberta T6G2V4, CANADA

1. INTRODUCTION MicroLIBS (LIBS) is a new growing area of Laser Induced Breakdown Spectroscopy which employs J energy laser pulses for excitation of plasma emission. Such J energy pulses are required to carry out 1D, 2D or 3D microanalysis of material surfaces with spatial resolutions approaching micron scale sizes laterally and nm scale sizes in depth. These pulses allow sampling of very small volumes ∼10–1000 m3  and masses ∼10 pg−ng. LIBS is also applicable where modest limits of detection, low cost or portable systems are required. The use of femtosecond pulses for LIBS over the past decade has in some cases also employed J laser pulses [1–4] and many of the advantages of LIBS are also observed using such ultrashort pulses as described in Chapter 7 on femtosecond LIBS. In this chapter we will review the capabilities of LIBS as one scales to microjoule laser pulse energies and progress to date in the application of such systems. The development of LIBS has been driven by two factors: 1) the desire to obtain higher spatial resolution when carrying out 2D scans of material surfaces and 2) the development of high repetition rate compact microchip lasers leading to ideal sources for very low energy LIBS applications. It has been found that the plasma and continuum emission decreases significantly with lower pulse energies and thus one can obtain reasonable performance without using temporal gating. The term LIBS has also been used in the context of applications with ablation spots of micron scale size [5,6]. In most cases, the definitions based on J energy and micron resolution spot sizes are equivalent. This chapter will focus on those studies which have used pulse energies less than 1 mJ. In the early 1990s Zayhowski developed the microchip laser [7–9] and he and other researchers started applying it to material analysis using both LIBS and laser-induced fluorescence detection [9–11]. Bloch et al. were able to achieve limits of detection Laser-Induced Breakdown Spectroscopy Jagdish P Singh, Surya N Thakur (Editors) © 2007 Elsevier B.V. All rights reserved.

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(LOD) of the order of 100 to 1000 ppm without temporal gating for Pb, Cu and Fe in soils using pulse energies of several J [10]. More recently J fiber laser sources with pulse lengths of 10 ps [12] to several nanoseconds [13] have been developed. By the mid 1990s several groups had started to investigate the application of J pulses to LIBS in variety of studies [9,14–17]. In 1996, Geertsen et al. demonstrated the use of 30–70 J pulses for the microanalysis of aluminum alloys [15]. The authors used 266 nm pulses for a variety of studies including LOD of minor elements down to 10 ppm, relative standard deviation (RSD) of ∼10% and lateral resolution to 6 m. Sallé et al. [18,19] carried out studies of the crater diameters, ablation volumes and expansion plumes for the interaction of 248 nm and 266 nm pulses with energies of 65 to 130 J and at 532 nm with energies of 10 J to 4 mJ. Semerok et al. extended these studies to look at the scaling of ablation craters with pulse length for durations of femtoseconds, picoseconds and nanoseconds and energies down to 10 J [20,21]. LIBS has also been combined with scanning probe fiber tip microscopy to achieve micron scale size ablation spots by Kossakovski et al. [22]. However, in the later case the emission was not strong enough to give good species identification for submicron ablation spots using a non-gated and non-intensified camera. Rieger et al. [23] explored the scaling and optimization of measuring trace constituents in aluminum alloy for 50 to 300 J laser pulses at 248 nm as a function of gate time. They achieved LODs of 2 to 450 ppm for elements of Mg to Fe for the case of optimized gate times and 200 J pulses. Further studies by the same group [24] compared LIBS signals for picosecond versus nanosecond 248 nm pulses. Scaling of line emission, continuum emission and emission lifetime with pulse energy were characterized with reported energy thresholds for observable line emission of 1 J for ns pulses and 01 J for 50 ps pulses. Above energies of 3 J the characteristics of the LIBS emission were reported to be comparable for both pulse lengths when identical laser and focusing conditions were used. In recent work by Gornushkin et al. [25], studies were carried out with a 7 J 1064 nm microchip laser using a non-gated and non-intensified detector. The authors highlight many of the advantages of using microchip lasers, such as good mode quality of the beam, high shot-to-shot reproducibility, the high repetition rate, the low continuum emission and the possibility of using ungated detectors. Observation of line reversal in the emission spectra of Zn demonstrate that optically thick plasma conditions can exist for major constituents even for low energy microplasmas and observed signals and crater sizes for several metals were reported [25]. LODs of the order of a few percent were obtained for metallic samples but poorer sensitivity was observed for pelletized graphite samples. Studies of surface mapping using LIBS also began in the mid-1990s. Häkkänen et al. [14] used 200 J 308 nm pulses to map Ca and Si concentrations in surface coatings of paper. They found good correlation with measurements of the surface using laser induced fluorescence. The group of Laserna et al. started their studies on surface mapping using LIBS with the investigation of depth profiling of a TiO2 antireflection coating on silicon [16] and 2D mapping of carbon impurities [26] using 400 J pulses at 337 nm. Further investigations indicated depth resolution of the order of 40 nm for carbon impurity and demonstrated 3D scans of carbon contamination with 70 m lateral resolution and 160 nm depth resolution [27]. Recently Menut et al. [5] demonstrated 2D surface scans with 3 m spatial resolution using 5 J pulses at 266 nm and LODs in the percent range for mapping of the concentration of minor constituents on the surface of steel. They also reported that ablation probe spots down to 1 m were possible but

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that there was not sufficient signal to allow for measurements of the trace elements at the 1% level. Cravetchi et al. [28] studied crater size, line emission and RSD of signals from trace elements in aluminum demonstrating that LIBS can resolve different types of micron scale size precipitates in aluminum. RSDs less than 10% were obtained with 7 J pulses at 266 nm. Full 2D scans mapping precipitate distribution in aluminum were subsequently demonstrated by the same group with a lateral resolution of 10 m [29]. Redeposition of ablated material and cross contamination of a scanned surface has been noted by a number of different groups [15,19,22,25,27,29,30]. In the late 1990s, the analysis of elemental chemical content of liquid samples using J pulses with the goal of measuring the elemental contents of single cells was demonstrated by Ho and Cheung et al. [17,31]. Using 80–250 J pulses at wavelengths of 532 nm and 193 nm they demonstrated a LOD of 50 ppm for Na in water. An acoustic normalization which corrected for shot-to-shot variations in pulse energy and careful spatial sampling of the expansion plume improved sensitivity to the few ppm range. During the first decade of work in the J energy regime many features of LIBS have been identified and characterised as described in more detail below. In the following sections, microjoule laser sources and their application to LIBS are briefly reviewed in Section 2, the scaling of LIBS to J pulse energies is discussed in Section 3, and finally, a review of the demonstrated applications of LIBS to date is given in Section 4.

2. MICROJOULE LASER SOURCES While traditional lasers can be operated in the microjoule range, one of the earliest sources specifically designed as a microjoule pulse source was the microchip laser developed by the group at MIT [7–9,11,32–35]. Additional sources for the J energy regime have also been developed by other groups [13,36–40]. At the same time femtosecond laser sources were developed, many of which also operate at microjoule energy levels. Studies of femtosecond LIBS are covered in Chapter 7 and thus femtosecond laser sources will not be discussed here. Recently, fiber optic oscillators and amplifiers have been developed to the point that J output energies are obtainable in pulsed operation mode and offer a potential new option for robust sources which can be used in field portable LIBS systems.

2.1. Microchip Lasers In 1989 Zayhowski et al. reported on the development of a single frequency microchip laser in various different lasing materials [7]. Q-switched operation of the laser was developed using piezoelectric, electro-optic and passive techniques [8,33,34]. The output at the fundamental wavelength is polarized and frequency conversion of the output and Nd:YAG laser harmonics down to 213 nm have been demonstrated [9]. When using ∼1 W pump power, output pulse energies of 8 J at the fundamental wavelength, 35 J at 532 nm and 07 J at 266 nm were reported. Higher pulse energies have since been reported with 10 W of diode pump power resulting in pulse energies of up to 250 J and 310 ps pulsewidths at the fundamental wavelength of 1064 nm and 12 J at 266 nm output with kHz repetition rates [11,35]. The layout of a low power harmonically

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Fig. 1. Schematic of a UV harmonically converted passively Q-switched microchip laser. Entire device is about a cm across. (Reproduced with permission from Zayhowski [11]).

converted microchip laser is shown in Fig. 1. It is fabricated by bonding the gain medium to a saturable absorber and harmonic conversion crystals. In a typical configuration a 0.75 mm thick Nd:YAG gain medium is coupled to a 0.5 mm thick Cr:YAG saturable absorber [34]. Diode pump laser light of ∼1 W at 808 nm is coupled to the gain medium by a butt coupled fiber. The resonator is formed between a dichroic dielectric mirror at the fiber input face, with a high reflectivity at the laser wavelength and high transmissivity at the pump wavelength, and a partially transmitting mirror at the output face of the saturable absorber. KTP and BBO crystals ∼5 mm long are butt coupled to the output face to generate 2nd and 4th harmonic output respectively. The laser output is a single frequency TEM00 Gaussian mode with a diameter of the order of 50 m. The short cavity length ensures single longitudinal mode operation since only one axial mode has sufficient gain to exceed the lasing threshold within the laser bandwidth. Other groups have also developed similar microchip lasers with various gain media and geometries. Fluck et al. [41] passively modelocked an Er-Yb:Glass gain medium using a semiconductor saturable absorber mirror (SESAM) to achieve 4 J pulses at 1535 nm with a repetition rate of 320 Hz. Spuhler et al. [42] applied a SESAM to a Yb:YAG laser, producing 11 J pulses at 1030 nm with a repetition rate of 12 kHz. Feldman et al. [43] used a 4 mm Nd:YAG microchip crystal bonded to a 2 mm Cr:CaYAG saturable absorber crystal in a 31 mm external resonator to produce 50 J pulses at 1064 nm. Karlsson et al. [44] have produced 12 J at 1535 nm with a 1 mm Er-Yb:Glass microchip laser with an external acousto-optic Q-switch and cavity mirror. Druon et al. [45] achieved ∼9 J pulses at 106 m and ∼08 J pulses at 355 nm with pulse durations of 300 ps using a Nd:YAG microchip laser together with a double-pass microchip amplifier. Higher repetition rate picosecond to subnanosecond pulsewidth microchip lasers with submicrojoule output energies have also been developed using SESAMs at 106 m [46] and 134 m [47]. Hansson et al. used a low voltage multiple quantum well electro-absorption Q-switch system applied to an Er-Yb:Glass laser to generate output pulses up to 470 nJ at a repetition rate of 10 kHz [48]. Further scaling in pump energy or addition of an amplifier chip should allow an increase in the output energy for some of these systems.

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Microchip lasers have several attractive features for LIBS as pointed out by a number of authors [11,25]. They are compact, robust and relatively inexpensive. Because of the very short cavity length, the longitudinal mode spacing can be larger than the gain medium bandwidth and only a single narrow linewidth longitudinal mode will be generated. High repetition rates of 1 to 20 kHz can be obtained by passive Q-switching which can result in sub-nanosecond pulses making it easier to achieve the breakdown threshold for materials compared to several nanosecond pulses with the same energy. Active Q-switching can be used to set exact repetition rates and synchronize to external events at the cost of somewhat longer pulse durations. The pulse to pulse stability of microchip lasers is in the range of 0.05 to 0.5% [11,49]. Single transverse TEM00 mode output is readily achieved via gain guiding and M2 values of 1.0 to 1.3 have been obtained [32,35]. This leads to low divergence output which can be focused to diffraction limited spot diameters. Various output wavelengths in the range of 1030 nm to 1550 nm have been demonstrated. With low energy pulses, 1550 nm pulses can fall in the eye safe operation range which is an important advantage for system use in public areas. There remain some disadvantages of microchip lasers with respect to their use for LIBS. When converted to UV wavelengths microchip lasers still have limited energies, on the order of 1 to 10 J per pulse. Further, when the simplest technique of passive Q-switching is used the laser output is free running, making it difficult to synchronize gated detectors. Additionally, the repetition rates of passively modelocked microchip lasers may be too fast for some applications. It has been reported that in the case of graphite the damage from one pulse may modify the surface for the subsequent pulse, decreasing the reliability and sensitivity of the measurement [25]. Commercial versions of microchip lasers are currently available with output energies of the order of ten microjoules at 1064 nm. It is expected that output energies from commercially available microchip lasers will soon be sufficient to exploit the full capabilities of LIBS. Such lasers should lead to the design of compact LIBS units.

2.2. Microjoule Fiber Lasers High power modelocked fibre lasers offer another potential laser excitation source for LIBS. Fiber lasers have undergone intense development for applications in communications and recently with the advent of cladding-pumped large mode area (LMA) fibers it is possible to achieve J to mJ pulse energies. Erbium doped fibers at 1550 nm are of particular interest since they are eye safe at low microjoule pulse energies. Recently, acousto-optic Q-switching of LMA Er-doped [37] and Yb-doped fibers [38] have demonstrated close to diffraction limited transverse mode quality output pulses with pulse durations of 100 ns, pulse energies of 500 J and 700 J, and repetition rates of 400 Hz and 2 kHz, with output wavelengths of approximately 1550 nm and 1060 nm respectively. Passive Q-switching of Er-Yb co-doped fiber has also been demonstrated yielding shorter 3.5 ns, 60 J output pulses at around 1550 nm with 0.6 to 6 kHz repetition rates [13]. Active seeding with a pulsed CW diode laser injected into a multi stage erbium fiber amplifier has led to 118 J 1550 nm pulses with a duration of few nanoseconds [36] and more recently seeding with a thin disk laser source yielded longer but higher energy diffraction limited pulses of 4 mJ and 50 ns duration at 1060 nm from LMA Yb-doped fiber [40]. In the latter case undoped plain fused silica end caps were fused onto the ends

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of the fiber to allow the mode to expand before exiting to avoid damage on the fiber end faces. Harmonic conversion of the nanosecond output pulses from both acoustooptically modelocked and CW diode-laser-seeded Er-doped fiber systems has also been demonstrated using periodically poled Lithium Niobate crystals yielding peak 2nd harmonic conversion efficiencies to 768 nm pulses of 83% and 62% respectively, a peak 2nd harmonic energy of 80 J in 45 ns pulses and 3rd harmonic conversion efficiencies of 15% [50]. Generally the acousto-optically Q-switched systems have pulse lengths of tens of nanoseconds which is longer than the optimum pulse length of picoseconds to a nanosecond for LIBS applications. The alternative approaches of passively modelocking and amplification of a short seed pulse allow much shorter pulses. However, the damage fluence levels of the fibers in the nanosecond regime scale with the 0.5 power of pulse length given by heat diffusion scaling. Thus, shorter pulses are limited to lower maximum energies. Even so, the amplification of 0.8 ns pulses to 1.2 mJ has been demonstrated in a high power chirped-pulse-amplification femtosecond laser system at 1055 nm [39] and 60 J pulses have been generated by passive modelocking at 1550 nm [13], indicating that sources with nanosecond duration are possible at the 100 J level. Recently work has started on the development of high-pulse-energy high-repetition-rate picosecond fiber sources with 06 J, 10 ps pulses at 1064 nm amplified at an 80 MHz repetition rate in a Yb-doped holey fiber system. These pulses were also frequency doubled with 50% efficiency to 532 nm. It is expected that by using lower repetition rate seed sources the pulse energy should increase leading to 10 ps pulse sources with energies in the range of microjoules. Recently a guide fiber has been employed for coupling light to micron scale size spots onto a sample for LIBS analysis [22]. While fiber laser systems have not yet been applied to LIBS studies it is expected that they will soon become useful in LIBS microanalysis.

3. SCALING LIBS TO MICROJOULE ENERGIES Over the past decade a basic understanding has been developed of the scaling of the performance of LIBS systems to J energies. It has been found that the duration of the line and continuum emission along with the relative amount of continuum radiation decreases as one goes below 1 mJ excitation energy. In many cases the signal to noise ratio (SNR) is a weak function of energy and thus it is still possible to obtain good sensitivity if care is taken in collecting the emission light. This means that working with ungated detectors becomes possible which greatly simplifies the detector requirements and reduces system cost. However, the highest sensitivities are achieved using gated systems. Due to the submillimeter size of the plasmas obtained in LIBS a large fraction of the plasma emission can be coupled to the narrow input slit of grating spectrometer systems. As the pulse energy decreases, the craters produced in LIBS decrease in diameter and depth. The smaller sample areas achieved allow the probing of much smaller features approaching a micron in size for microanalysis applications. However, there is a tradeoff between sensitivity and sample area that must be taken into account for any given application. In the following section the scaling of these properties is discussed in detail.

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3.1. Plasma Emission and Lifetime Many materials have been examined using LIBS, including Si photovoltaic cells, paper and paper coatings, and various metals. These studies have been performed across a range of energies from 400 J [16] down to 01 J [24]. A spectrum typical of what can be obtained using LIBS is given in Figure 2. This spectrum of aluminum was taken using a single 8 J pulse with zero gate delay and a gate width of 200 ns. Emission scaling with energy has been studied using a photomultiplier with a bandpass filter centered at 289 nm and a collection angle of f/6. Single line emission has been detected from Si down to 1 J with 10 ns 248 nm pulses and down to 01 J with 50 ps 248 nm pulses [24]. The resultant signal strengths are shown in Fig. 3 as a function of pulse energy for these two pulse lengths. It is seen that above an energy of approximately 3 J the signals are of the same strength. Only as the breakdown threshold is approached does one see a difference in the emission. Emission is observed for shorter pulses at lower energies while emission disappears for longer pulses because the intensity is no longer sufficient to breakdown the target surface. Thus, above several microjoules the important variable appears to be energy fluence rather than intensity. The focal spot diameter was approximately 5 m for these experiments leading to a fluence of approximately 5 J cm −2 for an energy of 1 J. The vertical scale units for Fig. 3 correspond approximately to photons per steradian except above ∼3 × 107 when the photomultiplier became weakly saturated. A more efficient optical collection system and more sensitive photomultiplier detector should be able to detect signals at even lower energies. The scaling of peak emission time versus pulse energy has been studied by Häkkänen et al. [14] and Rieger et al. [24] for dielectric and metallic targets respectively. The scaling for the former case is shown in Fig. 4 indicating that optimum measurement 7e+07

Time integrated spectral intensity [Photons Sr–1 nm–1]

6e+07 5e+07 4e+07 3e+07 2e+07 1e+07 0 385

390

395

400

405

Wavelength [nm]

Fig. 2. Background corrected spectrum of aluminum plasma emission using a single 50 ps, 8 J pulse at 248 nm, with a gate width of 200 ns and zero gate delay. The spectrum was obtained using a system for which an absolute calibration was performed. (Reproduced with permission from Rieger et al. [24]).

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PMT signal [A.U.]

108

50 ps 10 ns

107

106

105

104

0.1

1

10

Energy [µJ]

Fig. 3. Filtered photomultiplier detection of silicon line emission at 288 nm as a function of laser pulse energy for 10 ns and 50 ps pulses at 248 nm. The focal spot diameter was approximately 5 m, yielding a fluence of ∼5 J cm−2 for 1 J pulse energies. The horizontal line represents the noise floor of the PMT. (Reproduced with permission from Rieger et al. [24]). (b)

1.0

0.6

3 mJ 0.4

0.7 mJ 0.2 0.0

0.2 mJ 0

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600 500

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Decay time [ns]

Normalized SBR

(a)

Ca Si

400 300 200 100

400

600

Time [ns]

800

1000

0 0.1

1

10

Energy [mJ]

Fig. 4. (a) Smoothed and normalized time-resolved signal-to-background ratios of the silicon line at 251 nm at various excitation energies using a 308 nm laser. (b) Decay time for calcium and silicon signals at 422 nm and 251 nm respectively as a function of laser pulse energy. (Reproduced with permission from Häkkänen et al. [14]).

times for peak signal to background ratio decreases to about 100 ns for 200 J pulses in line with the decrease in plasma emission decay time with pulse energy. The results of Rieger et al. [24] shown in Fig. 5 indicate that the emission decay time reduces further to a few nanoseconds as the pulse energy is reduced below 2 J for 10 ns pulses and below 03 J for 50 ps pulses. For energies above 3 J an expanding spherical plasma with a lifetime of tens of ns is formed for both ps and ns pulses leading to similar decay time constants for the emission. Similar observations have been reported by other authors with plasma emission decay times of 8 ns [25] to 15 ns [10] for 10 J 1064 nm subnanosecond microchip laser pulses. Gornushkin et al. also observed a prompt emission which was only slightly longer than the subnanosecond laser pulse [25].

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Time constant [ns]

10 ns

10

1

0.1

1

10

Energy [µJ]

Fig. 5. Decay time constant for silicon line emission at 288 nm as a function of pulse energy for 10 ns and 50 ps pulses at 248 nm. The horizontal line represents the rise time of the PMT used. (Reproduced with permission from Rieger et al. [24]).

It has been observed that the decreasing decay time of the line emission is matched by an even faster decay of continuum emission. As a result, the ratio of line to continuum emission improves as one reduces pulse energy and thus one can detect the LIBS signal even in the absence of a gated detector [16,24,25].

3.2. Crater Size – Lateral and Depth Resolution One of the important advantages of LIBS for microanalysis is the size of the ablation spot as compared to conventional LIBS. Several groups have studied the scaling of crater diameter or crater volume as a function of pulse energy [15,18,19,21,25,27,28,51]. However, one must distinguish between the detection region, which is ionized sufficiently to yield emission signals, and the total region, which is ablated by the laser pulse. Much of the ablated material is removed after the laser pulse by the shock wave and melt wave propagating into the target. The crater size will therefore represent an upper bound to the actual region probed in composition measurements. A careful test of the lateral resolution obtainable by LIBS was performed by Geertsen et al. [15] using a specially fabricated test sample with a sharp Cu/Al interface. Using pulse energies in the range of 35–40 J at 266 nm, a series of shots were spaced at 2 m intervals measured perpendicular to the interface. The sample was displaced parallel to the interface by 15 m between each shot to prevent previously ablated material from being resampled. The data from the experiment is shown in Fig. 6. The reported lateral resolution was ∼6 m. Kossakovski et al. [22] used 125 J pulses at 337 nm coupled to an etched fiber probe tip in a scanning probe microscope to investigate the surface of basalt and meteorite samples. They were able to produce submicron ablation spot diameters but noted that the corresponding emission signals were too weak to obtain useful LIBS signals using a

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Normalized intensity [A.U.]

Cu signal

Al signal

1 0.8 0.6 0.4 0.2 0 0

1

2

3

4

5

6

7

8

Relative position of the focal spot [µm]

Fig. 6. LIBS 1D scan using pulses in the range of 35–40 J at 266 nm across a Al-Cu interface for determination of lateral resolution. Special care was taken to prevent resampling of ablated material, and a lateral resolution of 6 m was reported. (Reproduced with permission from Geertsen et al. [15]).

20X microscope objective and viewing the emission plasma from the side. Useful LIBS signals were obtained when using spots greater than a micron in diameter. The scaling of crater size and volume is an important variable in LIBS applications. Aluminum is one of the most thoroughly studied materials in the LIBS literature, and probably represents the clearest dataset with which to investigate the scaling laws for sample volume. Measured crater diameters and volumes for aluminum using nanosecond J pulses are shown in Fig. 7 and Fig. 8. Given the different methods of defining crater 35

Crater diameter [µm]

30 25 20 15 10

Cravetchi Geertsen Gornushkin Salle

5 0

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150

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Pulse energy [µJ]

Fig. 7. Single-shot crater diameter as a function of energy for Al. Open circles represent shots using 248 nm pulses as described in [23]. Data from Cravetchi et al. [28], Geertsen et al. [15] and Sallé et al. [18] taken with ∼10 ns pulses at 266 nm and data from Gornushkin et al. [25] taken with 1064 nm are shown as solid points.

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Crater volume [µm3]

10000 8000 6000 4000 2000

Geertsen Salle

0 0

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Pulse energy [µJ]

Fig. 8. Single-shot crater volume as a function of energy for Al. Open circles represent shots using 248 nm pulses as described in [23]. Data from Geertsen et al. [15] and Sallé et al. [18] taken with ∼10 ns pulses at 266 nm. The line is a linear regression to the 248 nm data points which are above 25 J and below 300 J.

diameter and of measuring crater volume used in the literature the agreement between groups is quite good. Ablation efficiency m3 J−1  has been measured for a variety of metals under different conditions. Results which used nanosecond pulses are given in Table 1, and picosecond results are given in Table 2. There are significant variations which may be due to the different focal geometries and intensities employed. While reasonable agreement for crater size scaling in the range applicable to LIBS has been achieved in the literature between a number of groups, further work will be required to reach a consensus on the scaling of ablation efficiency. The volume which actually contributes to LIBS emission is expected to be smaller and shallower than the final ablation crater. These effects will depend on the focal spot profile as well as the material characteristics. Redeposition both immediately surrounding the

Table 1. Nanosecond Ablation Efficiencies m3 J−1  Author



Pulse-width

Al

Cu

Fe

Ni

Rieger [51] Geertsen [15] Semerok [20] Semerok [21]

248 nm 266 nm 266 nm 266 nm

10 ns 6 ns 4 ns 6 ns

30 98 293 6

65 2

31 1

3

Sallé [19] Semerok [20] Semerok [21]

532 nm 532 nm 532 nm

6 ns 4 ns 6 ns

49 124 5

193 31 2

15

2

09

Gornushkin [25] Semerok [21]

1064 nm 1064 nm

0.55 ns 6 ns

5

Pb

Mo

457 9

14

611 186 6 07

200 6

06

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Table 2. Picosecond Ablation Efficiencies m3 J−1  Author Semerok [21] Semerok [21] Gornushkin [25] Semerok [21]



Pulse-width

Al

Cu

Fe

Ni

Pb

Mo

266 nm 532 nm 1064 nm 1064 nm

25 ps 25 ps 550 ps 25 ps

57 4

28 09

06 04

09 07

0.5 0.7

1

04

045

06

213 125 200 20

0.3

crater and at further distances is an additional phenomenon which affects the effective lateral resolution that may be achieved by LIBS. Under some conditions ablated material may redeposit and contaminate unsampled areas as discussed in Section 4.2 below. Further work will be required to ascertain the actual source volume contributing to the LIBS signal observed, and the ultimate spatial resolutions that will be possible in LIBS.

3.3. Limits of Detection Only a few studies have reported LODs for elements using LIBS. Geertsen et al. used a frequency-quadrupled Nd:YAG with pulse energies of ∼40 J to measure LODs of minor elements in aluminum [15]. Specially prepared homogeneous aluminum targets were used for this work. In an alternative approach Rieger et al. [23] took advantage of the small probe spot size to probe only the matrix material in standard aluminum alloys for LOD measurements. The concentration of minor constituents in this matrix region was calibrated using electron probe microanalysis of the matrix region of the alloys. In the latter measurement the SNR was obtained by taking the peak line emission compared to the 3 noise in nearby regions of the spectrum without line emission. For trace elements at concentrations below ∼1% it was assumed that the signal scales linearly with concentration. A comparison with the traditional technique for determining LOD was performed and reasonable agreement between the techniques was obtained. The SNR was measured versus gate delay times and the optimum gate delay found for the given plasma conditions for a number of trace elements. An example of the 3 LOD for Cu in aluminum alloy as a function of pulse energy and gate delay is given in Fig. 9. It is seen that the LOD and optimum gate time are weak functions of pulse energy. The optimum LODs for a number of elements were determined and are presented in Table 3 together with values reported by Geertsen et al. [15]. A typical set of values for mJ energy LIBS measurements from Sabsabi et al. [52] is also given for comparison. To compare values taken with different number of shots it was assumed that the LOD scales with the inverse square root of the number of shots. The values presented have all been scaled to single shot values using this scaling. It is seen that the optimized values are not greatly different from those reported for 60 mJ pulses, and single-shot LODs are mainly in the range of 20 to 400 ppm for 40 to 200 J pulses, depending on the element and line observed. Using more shots improves the LODs that are possible, as in the case of Geertsen et al. who report a LOD of 3 ppm for Mg using an accumulation of 150 shots [15]. The detector used in Rieger et al. [23] and Sabsabi et al. [52] were both similar gated intensified photodiode arrays, with similar spectrometer characteristics, including

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LOD [ppm]

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Delay [ns]

Fig. 9. LOD as function of gate delay for Cu emission at 324.8 nm in Al 7075 alloy for 100 J (open squares) and 200 J (solid circles) laser pulse energy. (Reproduced with permission from Rieger et al. [23]).

Table 3. LOD for minor elements in aluminum alloys. All values are scaled to −1/2 equivalent single shot acquisitions values, using an Nshots scaling Element

Emission Wavelength

Cr Cu

425.4 nm 324.8 nm 327.4 nm 438.4 nm 285.2 nm 279.5 nm 403.1 nm 251.6 nm 288.2 nm 334.5 nm

Fe Mg Mn Si Zn

Geertsen [15] 40 J 266 nm

Rieger [23] 200–240 J 248 nm

Sabsabi [52] 60 mJ 1064 nm

204 ppm 22 ppm 245 ppm 37 ppm

71 ppm 447 ppm ≤2 ppm 35 ppm 67 ppm

3 ppm 14 ppm 99 ppm

141 ppm 281 ppm

the slit widths. The only major difference in the experiments besides the energy is the laser wavelength used: Sabsabi et al. used 1064 nm whereas Rieger used 248 nm. The absorption will be better at 248 nm for metals, and plasma shielding will be a greater issue for the 1064 nm at higher energies which may affect the comparison somewhat. At still lower energies, Bloch et al. reported obtaining hundreds of ppm LODs for metals in soil [10]. In the case of a high energy LIBS emission plasma, the entrance slit of the spectrometer represents the limiting aperture for acceptance of light. The entrance slit and the collection optics limit the spatial region of the plasma plume that may be observed at any given time. In the case of LIBS, the spatial expansion of the plasma is much

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Intensity [A.U.]

Cu 324.8 nm Cu 327.4 nm Average 103

102

101

101

102

103

104

Concentration [ppm]

Fig. 10. Calibration curve of copper in aluminum matrix. Each point is an average of ten 240 J pulses at 248 nm. Gate width is 300 ns and gate delay is 200 ns. The 3 LOD is 12 ppm, and the straight line is a linear fit to the averaged data points below 1000 ppm. (Reproduced with permission from Rieger et al. [23]).

smaller with a size comparable to the slit widths used for spectral measurements. Thus, a larger fraction of the plasma emission can be collected by the spectrometer as compared to traditional mJ pulse energy LIBS. The result is that the LODs reported in the LIBS literature are often comparable to those reported by more traditional LIBS systems using mJ pulse energies.

3.4. Signal Linearity with Concentration An important issue in the application of LIBS to analytical measurements is the scaling of the signal with concentration. For a small, optically thin plasma it is expected that the line emission strength should scale linearly with concentration for minor constituents. This linear scaling is observed in the emission of Cu in aluminum targets for concentrations below 0.1% as seen in Fig. 10. For the dominant species self reversal, indicating strong optical opacity, can be observed at energies as low as 7 J as reported by Gornushkin et al. [25]. Signal linearity depends on the characteristics of the line under observation, the focal conditions of the laser and the observation time. However, it appears that for many elements at concentrations less than ∼1000 ppm signal linearity can be assumed.

4. APPLICATIONS To date, the main application areas of LIBS have been in the analysis of very small sample volumes ∼10–1000 m3  and in the scanning microanalysis of surface composition. Microanalysis can be carried out in 2D or 3D with depth resolution by using repeated scans over the surface. Initial reported results in these areas are presented below.

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4.1. Microanalysis of Small Volumes Microanalysis of metallic samples has been reported by Geertsen et al. [15], Rieger et al. [23,24], Cravetchi et al. [28] and Gornushkin et al. [25]. Geertsen et al. demonstrated the use of J pulses for the microanalysis of aluminum alloys [15]. The authors used 30 to 70 J pulses at 266 nm focused onto the samples using a 25X reflective microscope objective. Due to the very small focal spot used in these experiments, the signals obtained were very sensitive to inhomogeneity on the m scale size. In studies reported by Cravetchi et al. [28,29] it was shown that the small probe spot could be positioned on individual precipitate crystals and used to analyze the composition of individual precipitates. The placement of a probe spot either on the precipitate or in the surrounding homogeneous matrix region is illustrated in Fig. 11. It was shown that statistically significant determination of precipitate type could be made with single shot spectra by detecting emission lines which were more than 3 higher than the same line for the homogeneous matrix [29]. It is essential for any application on the micron scale that the analysis be obtained in a single shot since the features being measured may be ablated in a single shot. Broadband LIBS signals, covering a large spectral range using J pulses have recently been demonstrated by Gornushkin et al. [25]. The use of broadband LIBS is seen as a major step forward for material analysis since one to two orders of magnitude more data can be obtained on each laser shot, thereby making optimum use of the limited photon emission from a single laser shot. Using an ungated detector, Gornushkin et al. measured the LIBS spectra of a number of metals with clear observation of emission lines but with significant continuum for the 1064 nm 7 J probe pulses. The authors observed that a moving target was necessary since the melting from previous laser shots left a

(a)

(b)

M2

M1

P2

10 µm

Observed signal [counts]

5000

P1

Al

Mn-Fe-Cu precipitate Al-alloy matrix

4000

Mn

3000 Cu

Mg

Fe

2000 1000 0 320

340

360

380

400

420

Wavelength [nm]

Fig. 11. (a) Scanning electron microscope image of precipitates on the surface of aluminum alloy and individual single-shot LIBS craters produced with 7 J pulses at 266 nm. Matrix (dark area) shots are labeled M1 and M2, while LIBS shots that sampled precipitates (bright areas) are labeled P1 and P2. (b) Representative single-shot spectra from matrix and precipitate regions of the aluminum surface. Clear differences are observed with a pulse energy of 7 J. (Reproduced with permission from Cravetchi et al. [28]).

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more reflective surface and the LIBS signal would disappear after the first shot. This indicates that the use of 1064 nm wavelength is not optimum for measurements of many metals because of high reflectivity at this wavelength. For this reason many LIBS investigations have been carried out using UV wavelength lasers to take advantage of the improved coupling. Bloch et al. studied soil samples to detect Cu, Fe and Pb contamination using 10 J pulses at 1064 nm from a microchip laser [10]. The plasma emission was measured using an ungated and unintensified compact diode-array based spectrometer. The authors noted that the plasma continuum radiation decays quite rapidly with a time constant of ∼15 ns. As a result, they were able to measure concentrations at the hundreds of ppm level without the need for temporal gating. Kossakovski et al. probed a meteorite sample comparing probing with a focal spot from a 50 mm quartz lens with that through an etched fiber probe showing the signals were similar for similar power densities [22]. In both cases light was collected from the side with a 20X microscope objective and an ungated unintensified spectrometer was used. Good signals were obtained when higher energies per pulse were used leading to probe craters on the order of 2 m in diameter or greater. Pelletized graphite targets impregnated with magnesium hydroxide powder were studied by Gornushkin et al. using 7 J pulses with limited success [25]. They attributed the lack of success to the fragility and roughness of the target surface which eroded easily under the 5 kHz repetition rate laser. Due to the high repetition rate, the target was scanned in a spiral pattern in order to present a new spot to the sampling laser for every pulse.

4.2. Scanning Microanalysis of Material Surfaces One main application of LIBS is in the scanning microanalysis of material surfaces. To achieve high spatial resolution and small ablation depths, very small energies and small focal spots are desired. It has been reported that as the spot size approaches one micron, the signal becomes too weak for material composition analysis [5,22]. However, these observations were made for nanosecond pulses and without optical gating in one case. Using shorter picosecond or femtosecond pulses and better light collection efficiency it may be possible to obtain LIBS signals in cases where the ablation crater is less than 1 m. Häkkänen et al. have studied the application of LIBS to the mapping of surface coatings on paper [14,30]. This work also represents one of the earliest uses of LIBS as a surface mapping tool. The LIBS results were compared with laser-induced fluorescence (LIF) and found to give good agreement. The results of the 2D LIBS scan and 2D LIF scan are presented here in Fig. 12. 2 J pulses at 308 nm were scanned over a 10 mm by 10 mm section of paperboard while monitoring the fluorescence signal at 422 nm. The same scan was performed after increasing the energy to 200 J pulses at a fluence of ∼109 W cm−2 , leading to plasma emission. This fluence was sufficient to remove the paper coating, and generated craters 30 m in diameter, and 2 m deep. The Si I 251 nm line was monitored using a PMT with a delayed boxcar integrator. 8 such shots, each displaced 32 m, were averaged to generate a single data point corresponding to a pixel 30 m × 250 m. 40 such pixels were taken to make a single row, and 40 such rows make up the entire image presented in Fig. 12b. The LIF and LIBS images are

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(a)

(b)

Fig. 12. 2D scan of a 10 mm by 10 mm piece of coated paper board. (a) Laser-induced fluorescence of underlying paper at 422 nm and (b) LIBS scan at 251 nm for Si in paper coating. (Reproduced with permission from Häkkänen et al. [14]).

expected to be negatives of each other since the Si line for the LIBS signal is sensitive to the coated regions of the paper while the fluorescence measurements are sensitive to organic compounds visible in the less coated regions. Further improvements to the measurement technique were reported in a subsequent investigation [30]. Using a similar setup as previously the authors employed 80 J pulses at 308 nm and a 40 mm lens to generate focal spot sizes ∼100 m in diameter. The resulting craters were also 100 m in diameter and 05 m deep. Using a series of 40 shots for each location on the target surface, the authors were able to measure the depth profile distribution of the pigment layers that make up the smooth surface of modern paper. A 2D depth resolved scan through the topcoat, precoat and into the base layer of paper, is shown in Fig. 13. Scanning LIBS of anti-reflection coated and uncoated silicon surfaces has been studied by Laserna’s group in several reports [16,26,27,53]. In the initial investigation of Hidalgo et al. TiO2 anti-reflection coatings for photovoltaic cells were studied using large, low fluence spots in order to achieve better depth resolution [16]. Using pulse energies of 400 J delivered to the target and focal spots of 160 m × 40 m, depth profiling of the TiO2 coating was performed, and the coating was distinguishable from the Si substrate. However, a depth resolution was not estimated by the authors. One interesting feature noted by the authors was a dependence of the emission signal strength on the coating thickness which also correlated with the film reflectivity. The peak field

Depth [µm]

0

10

20

0

10

20

30

40

50

Length (mm)

Fig. 13. LIBS 2D depth profile scan of the composition of paper using 80 J pulses at 308 nm. Gray indicates top coat, composed of a 50:50 mix of calcium carbonate:kaolin. Black indicates precoat, composed of a 80:20 mix of calcium carbonate:kaolin. White indicates the base paper. (Reproduced with permission from Häkkänen et al. [30]).

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strength in the coating which causes breakdown and emission depends on the interference between the reflected wave and incident wave and is a sensitive function of the layer thickness and thus a dependence on coating thickness is to be expected. Using the same setup, Vadillo et al. applied LIBS to a full 2D and 3D mapping of photovoltaic cell structures on silicon. The pulse energy was approximately 40 J [26]. Using these conditions, it was possible to produce a 2D surface map of carbon contamination distribution. By taking multiple shots to obtain depth profiling at each of the surface map locations, a 3D map was also produced, giving carbon distribution not only at the surface, but at layers further down. The mapping work was extended to simultaneous monitoring of multiple wavelengths for mapping of Si photovoltaic cells in Romero et al. [27]. The setup was similar to that of Hidalgo et al. [16] where pulses of 100 to 400 J were used. Using this setup, the authors were able to generate a set of spectrally resolved images from their data, with a lateral resolution of about 80 m. Moving on to a full three dimensional analysis of the photovoltaic cells Romero et al. studied the distribution of carbon in the solar cells, using a series of 2D scans over the same area [53]. The resulting lateral resolution obtained by Romero et al. was 70 m, and the depth resolution was approximately 016 m. In this work the goal was to achieve good depth resolution and thus the focal spot size was increased to give the low fluences necessary. Menut et al. combined a LIBS system with an optical microscope and generated a 2D surface scanning instrument with a lateral resolution of 3 m using an Ar buffer gas [5]. Crater sizes down to 1 m are reported, though at such low energies the SNR was insufficient for analysis of minor constituents. The setup described by Menut et al. [5] detected signals at a pulse energy of 5 J, resulting in craters approximately 3 m in diameter for their steel sample. The system was able to acquire signals at 20 Hz, and has been used to map the surface composition of various samples. In Fig. 14 a multi-elemental map of a single inclusion in a steel alloy is shown. Cravetchi et al. reported 2D mapping of aluminum surfaces and identification of precipitates using 8 J pulses at 266 nm [29]. Particular attention was directed towards improving the statistical validity of the precipitate identification technique. A Gaussian function was fit to the signal intensity distribution of all shots in the mapped region to derive the average and standard deviation for signals corresponding to the background matrix. Only signals 3 above this level were deemed to be regions of precipitates. Correlations between various elements in a given type of precipitate can easily be

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Fig. 14. Scanning LIBS image of a single inclusion on the surface of steel as seen in the emission of elements Mn, Fe, Ti and Ni. (Reproduced with permission from Menut et al. [5]).

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Fig. 15. Correlation plots of peak intensity values for minor elements in aluminum alloys. (a) Mn vs Fe shows a positive correlation as they appear in the same precipitates, (b) Mn vs Mg yields a negative correlation in both lobes, as they do not appear in the same precipitates. Dashed lines are 3 values from the nonlinear Gaussian fit to all available data. Solid lines are linear regressions within their respective quadrants. (Reproduced with permission from Cravetchi et al. [29]).

observed as shown in Fig. 15. The densely populated region in the lower left hand corner of the plots represents the matrix background. Based on the standard deviation of signals observed, it was possible to set detection thresholds for various trace elements and map out the two dominant precipitates in Al 2024 alloy with 10 m lateral resolution [29]. A few of the groups studying LIBS have noted the issue of cross contamination from material redeposited onto the surface from previous ablation spots. Romero et al. [27] and Häkkänen et al. [30] measured the single-shot contamination range from an ablated location for silicon and paper targets using the LIBS signal itself, giving values of 80 m and 200 m respectively. Their results are shown in Fig. 16. Several other groups also refer to visible redeposition of target material on the sample surface [15,18,22,25]. Clear evidence of material redeposition was found in the 2D mapping of aluminum surfaces experiment of Cravetchi et al. [29]. Redeposition of Al2 O3 on the target surface was observed. The resulting coating of the target was quite pronounced when a large number of shots was taken, as can be seen in Fig. 17a. The left image is a SEM image which shows a smooth coating over the original aluminum surface. However, as can be seen in Fig. 17b, the redeposited layer ceases abruptly as one approaches the mapped area and around the isolated shots at the bottom of the images. This can be understood by considering the blast wave in air and shock wave in the material created by the ablation plasma. As a LIBS plasma is created, it launches a shock wave that expands with a quasi-spherical symmetry and the force of this wave near the ablation spot is sufficient to remove the deposited material from the surface. The radius of this cleaned area is larger than the distance to the subsequent shot in the scanning analysis and thus the original target surface is probed by the scanning LIBS measurement. The dynamics of material deposition and cleaning will depend on the sample being scanned and the conditions being employed. Redeposition may be reduced if one carries out the scans in vacuum but detailed studies need to be carried out to quantify the reduction.

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Fig. 16. (a) Ratio of peak intensities of the Ti line measured at 626 nm from two adjacent points on the surface of a TiO2 coated Si sample. A ratio of 1.0 indicates the second shot has sampled an undisturbed surface. (b) Silicon intensity of the first ablation layer of coated paper as a function of distance between sampling points and number of shots at each sampling point. In this case, Si is a contaminant from buried layers in a paper coating. (Reproduced with permission from (a) Romero et al. [27], and (b) Häkkänen et al. [30]).

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Fig. 17. (a) Scanning electron microscopic image of macroscopic redeposition of Al2 O3 and shock cleaning near the perimeter of a 2D LIBS scan area 300 m by 900 m in size with probe spot separation of 10 m. The edge of the scanned area is visible at the left edge of the image. (b) Isolated crater created using single 15 J pulses at 266 nm. (Reproduced with permission from Cravetchi et al. [29]).

In order to compare the various mapping experiments, we define a surface mapping rate (SMR) as the sample area per shot multiplied by the sample rate. In this case, the sample area per shot is defined by the crater diameter. These are plotted for published reports of LIBS surface analysis in Fig. 18. Included for comparison is the use of linefocused beam scans with milli-joule energy pulses, as applied by Mateo et al. [55,56] and Rodolfo et al. [57]. In such line focused beams, the irradiance applied to the target can be in the same range as that of LIBS. This plot demonstrates the current capabilities of LIBS scanning rates for 2D multi-elemental surface mapping.

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6 266 nm 308 nm 331 nm 532 nm 1064 nm

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Fig. 18. Surface scanning rate as a function of applied power. 1 = Menut et al. [5], 2 = Cravetchi et al. [29], 3 = Vadillo et al. [26], 4 = Romero et al. [53], 5 = Romero et al. [27], 6 = Häkkänen et al. [30], 7 = Romero et al. [54], 8 = Mateo et al. [55], 9 = Mateo et al. [56], 10 = Rodolfo et al. [57]. For comparison, surface scans carried out using millijoule laser pulses in a line focus geometry are also shown in the upper right area of figure [55–57].

4.3. Liquid Samples In the early 1990s a series of experiments applying LIBS to the detection of elements in water jet samples were performed by Cheung et al. [58,59]. This work has been extended to the LIBS regime in more recent work by Ho and Cheung et al. [17,31] for detection of Na and K. One of the goals was to demonstrate sufficient sensitivity to measure the chemical content of single cells. 532 nm Nd-YAG and 193 nm ArF laser pulses were used as excitation sources, with both a photomultiplier tube and ICCD detection. To increase the absorption of the liquid water, a solution of 12 mM methyl violet was used. Using 240 J pulses at 532 nm a detection limit of 50 ppm was achieved. The reported detection limit using the ArF excitation beam was 230 ppb. In further work by the same group, Cheung et al. note that the plasma generated by the ArF beam is significantly cooler than that generated by the 532 nm beam at short delay times. Plasma temperature and electron density were determined by line intensity ratios and line widths.

5. CONCLUSIONS In the past decade there has been good initial progress in the development and understanding of LIBS. Pulsed microchip laser sources with energies of 1 to 240 J have been demonstrated and are beginning to be commercially available. The primary sources demonstrated to date are in the infrared region while the optimum wavelength for LIBS is most likely in the UV region to give smaller, diffraction limited focal spots and better target absorption. The energy of harmonically converted UV sources is still limited to

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less than ∼10 J. However, at 1550 nm (erbium based lasers) one also has the advantage of eye safe sources at microjoule energy levels making practical systems easier to implement. More work needs to be done on the wavelength scaling issues to determine how effective these mid-infrared sources could be for LIBS. In microanalysis applications the ability to achieve single shot LODs of 10 to 100 ppm has been demonstrated with ∼100 J energy pulses using gated detector systems and 100 to 10,000 ppm with ∼10 J energy pulses using ungated detectors. The ability to analyze sample volumes of 10 to 1000 m3 has been demonstrated. 2D surface scans have been carried out with 3 m lateral resolution on steel and 10 m lateral resolution on aluminum. However, issues of determining the exact region of LIBS emission sensitivity within the ablation volume and cross contamination remain to be addressed in detail. It is likely that cross contamination is very much material and laser parameter dependent and particular attention should be paid to this issue in any scanning microanalysis system. Microanalysis of water samples has also been demonstrated achieving sub ppm sensitivities under optimized conditions. While there is much that remains to be done in the study of LIBS, particularly in the area of wavelength and pulselength scaling of LODs achievable, the use of LIBS already appears as a promising new regime which should soon lead to cost effective portable systems.

REFERENCES [1] V. Margetic, A. Pakulev, A. Stockhaus, M. Bolshov, K. Niemax and R. Hergenröder. Spectrochim. Acta B55 (2000) 1771. [2] V. Margetic, M. Bolshov, A. Stockhaus, K. Niemax and R. Hergenröder. J. Anal. At. Spectrom. 16 (2001) 616. [3] K.L. Eland, D.N. Stratis, D.M. Gold, S.R. Goode and S.M. Angel. Appl. Spectrosc. 55 (2001) 286. [4] S. ¸ Yalçin, Y.Y. Tsui and R. Fedosejevs. J. Anal. At. Spectrom. 19 (2004) 1295. [5] D. Menut, P. Fichet, J.-L. Lacour, A. Riovallan and P. Mauchien. Appl. Opt. 42 (2003) 6063. [6] B. Al Ali, D. Bulajic, M. Corsi, G. Cristoforetti, S. Legnaioli, L. Masotti, V. Palleshi, A. Salvetti and E. Tognoni. SPIE 4402 (2001) 25. [7] J.J. Zayhowski and A. Mooradian. Opt. Lett. 14 (1989) 24. [8] J.J. Zayhowski. Opt. Lett. 16 (1991) 575. [9] J.J. Zayhowski. Opt. Lett. 21 (1996) 588. [10] J. Bloch, B. Johnson, N. Newbury, J. Germain, H. Hemond and J. Sinfield, Appl. Spectrosc. 52 (1998) 1299. [11] J.J. Zayhowski, J. Alloys Compd. 303–304 (2000) 393. [12] J. Limpert, A. Liem, M. Riech, T. Schreiber, S. Nolte, H. Zellmer, A. Tünnermann, J. Broeng, A. Petersson and C. Jakobsen. Opt. Express 12 (2004) 1313. [13] M. Laroche, A.M. Chardon, J. Nilsson, D.P. Shepard and W.A. Clarkson. Opt. Lett. 27 (2002) 1980. [14] H. Häkkänen and J.E.I. Korppi-Tommola. Appl. Sectrosc. 49 (1995) 1721. [15] C. Geertsen, J.-L. Lacour, P. Mauchien and L. Pierrard. Spectrochim. Acta B51 (1996) 1403. [16] M. Hidalgo, F. Martin and J.J. Laserna. Anal. Chem. 68 (1996) 1095. [17] W.F. Ho, C.W. Ng and N.H. Cheung. Appl. Spectrosc. 51 (1997) 87. [18] B. Sallé, C. Chaléard, V. Detalle, J.-L. Lacour, P. Mauchien, C. Nouvellon and A. Semerok. Appl. Surf. Sci. 138–139 (1999) 302.

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[19] B. Sallé, M.N. Libenson, P. Mauchien, G. Petite, A. Semerok and J.-F. Wagner. SPIE 3822 (1999), 56–67. [20] A. Semerok, C. Chaléard, V. Detalle, J.-L. Lacour, P. Mauchien, P. Meynadier, C. Nouvellen, B. Sallé, P. Palianov, M. Perdix and G. Petite. Appl. Surf. Sci. 138–139 (1999) 311. [21] A. Semerok, B. Sallé, J.-F. Wagner, G. Petite, O. Gobert, P. Meynadier, M. Perdix. SPIE 4423 (2001), 153–164. [22] D. Kossakovski and J.L. Beauchamp. Anal. Chem. 72 (2000) 4731. [23] G.W. Rieger, M. Taschuk, Y.Y. Tsui and R. Fedosejevs. Appl. Spectrosc. 56 (2002) 689. [24] G.W. Rieger, M. Taschuk, Y.Y. Tsui and R. Fedosejevs. Spectrochim. Acta B58 (2003) 497. [25] I.B. Gornushkin, K. Amponsah-Manager, B.W. Smith, N. Omenetto and J.D. Winefordner. Appl. Spectrosc. 58 (2004) 762. [26] J.M. Vadillo, S. Palanco, M.D. Romero and J.J. Laserna. Fresenius J. Anal. Chem. 355 (1996) 909. [27] D. Romero and J.J. Laserna. Anal. Chem. 69 (1997) 2871. [28] I.V. Cravetchi, M. Taschuk, G.W. Rieger, Y.Y. Tsui and R. Fedosejevs. Appl. Opt. 42 (2003) 6138. [29] I.V. Cravetchi, M. Taschuk, Y.Y. Tsui and R. Fedosejevs, Spectrochim. Acta B59 (2004) 1439. [30] H. Häkkänen, J. Houni, S. Kaski and J.E.I. Korppi-Tommola, Spectrochim. Acta B56 (2001) 737. [31] N.H. Cheung, C.W. Ng, W.F. Ho and E.S. Yeung, Appl. Surf. Sci. 127–12 (1998) 274. [32] J. Zayhowski. Lincoln Laboratory Journal, 3 (1990) 427. [33] J. Zayhowski and J. Keszenheimer. IEEE J. Quantum Electron. 28 (1992) 1118. [34] J.J. Zayhowski and C. Dill III. Opt. Lett. 19. (1994) 1427. [35] J.J. Zayhowski. Laser Rev. 26 (1998) 841. [36] D. Taverner, D.J. Richardson, L. Dong, J.E. Caplen, K. Williams, R.V. Penty. Opt. Lett. 22 (1997) 378. [37] H.L. Offerhaus, N.G. Broderick, D.J. Richardson, R. Sammut, J. Caplen and L. Dong. Opt. Lett. 23 (1998) 1683. [38] C.C. Renaud, H.L. Offerhaus, J.A. Alvarez-Chavez, J. Nilsson. W.A. Clarkson, P.W. Turner, D.J. Richardson and A.B. Grudinin. IEEE J. Quantum Electron. 37 (2001)199. [39] A. Galvanauskas. IEEE J. Sel. Top. Quantum Electron. 7 (2001) 504. [40] J. Limpert, S. Höffer, A. Liem, H. Zellmer, A. Tünnermann, S. Knoke and H. Hoelckel. Appl. Phys. B: Lasers Opt. 75 (2002) 477. [41] R. Fluck, R. Häring, R. Paschotta, E. Gini, H. Melchior and U. Keller. Appl. Phys. Lett. 72 (1998) 3273. [42] G.J. Spühler, R. Paschotta, M.P. Kullberg, M. Graf, M. Moser, E. Mix, G. Huber, C. Harder and U. Keller. Appl. Phys. B: Lasers Opt. 72 (2001) 285. [43] R. Feldman, Y. Shimony, Z. Burshtein. Opt. Mater. 24 (2003) 393. [44] G. Karlsson, V. Pasiskevicius, F. Laurell, J.A. Tellefsen. Opt. Commun. 217 (2003) 317. [45] F. Druon, F. Balembois, P. Georges and A. Brum. Opt. Lett. 24 (1999) 499. [46] B. Braun, F.X. Kärtner, U. Keller, J.-P. Meyn, G. Huber. Opt. Lett. 21 (1996) 405. [47] R. Fluck, B. Braun, E. Gini, H. Melchior, and U. Keller. Opt. Lett. 22 (1997) 991. [48] Björn T. Hansson and Ari T. Friberg. Opt. Lett. 26 (2001) 1057. [49] J.J. Zayhowski. J. Commun. Res. Lab. 46 (1999) 385. [50] D. Taverner, P. Britton, P.G.R. Smith, D.J. Richardson, G.W. Ross and D.C. Hanna. Opt. Lett. 23 (1998) 162. [51] G.W. Rieger, M. Taschuk, Y.Y. Tsui and R. Fedosejevs. SPIE 4087 (2000), 1127. [52] M. Sabsabi and P. Cielo. Appl. Spectrosc. 49 (1995) 499. [53] D. Romero and J.J. Laserna. J. Anal. At. Spectrom. 13 (1998) 557. [54] D. Romero and J.J. Laserna. Spectrochim. Acta B55 (2000) 1241.

196

M. T. Taschuk et al.

[55] [56] [57] [58] [59]

M.P. Mateo, L.M. Cabalín and J.J. Laserna. Appl. Spectrosc. 57 (2003) 1461. M.P. Mateo, L.M. Cabalín, J.M. Baena and J.J. Laserna. Spectrochim. Acta B57 (2002) 601. K. Rodolfo and D. Cremers. Appl. Spectrosc. 58 (2004) 367. N.-H. Cheung and E.S. Yeung. Appl. Spectrosc. 47 (1993) 882. N.-H. Cheung and E.S. Yeung. Anal. Chem. 66 (1994) 929.

Chapter 9

LIBS Application to Off-Gas Measurement F. Y. Yueh and J. P. Singh Institute for Clean Energy Technology, Mississippi State University 205 Research Boulevard, Starkville, MS 39759, USA

1. INTRODUCTION Laser induced gas breakdown is the results of the interaction of high intensity laser beam with a gas. Typically, an irradiance corresponding to an electric field strength on the order of 105 volt/cm with a gas near atmospheric pressure can produce gas breakdown through multiphoton ionization or electron avalanche [1]. The gas breakdown thresholds in the atmospheric pressure are proportional to the ionization potential of each gas divided by the collision frequency. Due to the presence of micron-sized aerosols and impurity particles, the observed breakdown threshold in gas samples is generally lower than that from the theoretical prediction. This is because the particles acting as seeds can significantly lower the breakdown threshold of clean gas. Laser-induced breakdown in gas has been studied extensively [2]. Beside the particle size, the laser-induced gas breakdown thresholds also depend strongly on the gas pressure and the laser wavelength. Typically, laser-induced air breakdown has a plasma temperature of 20,000 K and an electron density of 1017 −1018 cm−3 after the plasma is formed [1]. The application of LIBS for gas analysis involved a focused high-energy pulsed laser to produce the breakdown in the gas medium. The high temperatures and electron density laser-induced plasma prepares and excites the sample in single step. The emission from the laser plasma can be used directly to measure the composition of gas, eliminating the need for sample preparation. Schmieder et al. were first to show that LIBS can be applied as a combustion diagnostics for monitoring the elemental constituents of a combustion product [3,4]. They used a time-integrated photographic technique and diode array to detect N and O in gas mixtures and to measure the C/N ratio of the flame. Radziemski and Loree pioneered LIBS applications on gas measurements using time-resolved detection [5]. They used a time-gated optical multichannel analyzer or a PMT-boxcar detection system and found the detection limits for P and Cl in air as 15 and 60 ppm, respectively. Cremers and Radziemski were later able to detect Cl and F in air with a detection limit of 80 ng and 2,000 ng, respectively [6]. They also found that the absolute detection limit for Cl and F can be improved in a He atmosphere. Radziemski et al. have used LIBS to detect Be, Na, P, As, and Hg in air [7]. Due to the Laser-Induced Breakdown Spectroscopy Jagdish P Singh, Surya N Thakur (Editors) © 2007 Elsevier B.V. All rights reserved.

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small sample volume and possible sample inhomogeneity, LIBS measurement precision in a gas sample is generally poor. The various size particulates in the gas can cause the breakdown to be generated at different locations along the axis of the laser beam and lead to significant signal variations. The most common interference found in the air breakdown is the CN emission. CN is produced from the reaction of C and N, which are produced in the spark. The intensity of CN bands depends on the concentration of a C-containing compound in the gas stream. The analyte lines in the CN band covered spectral region have less sensitivity due to the spectral interference. Toxic metals from thermal processing units, is a great concern for environment protection agency. More strict limits on toxic metals from many process streams will be put on future regulations. Since LIBS is able to complete vaporization of aerosol particles up to roughly 2–10 m in diameter, and atomizes molecular species, it has great potential for detection of Toxic metals especially in the forms of fine particles (i.e. PM 2.5). Hahn has used LIBS for sizing and elemental analysis of sub-micrometer to micrometer-sized aerosol particles [8]. Buckley has studied the effects of experimental configuration, potential interferences and oxygen quenching to LIBS application to toxic metal emission measurements [9]. Biological warfare agent is identified as a great threat to general public due to the potential bioterrorism. The feasibility of using LIBS for rapid detection and identification of various biological aerosols has been demonstrated [10–12]. Hybl et al. have used a broadband LIBS system for laboratory measurements on some common biological agent simulants and a narrow band LIBS system to detect single simulant (Bg) particles in the size range 1–5 microns [12]. The optical characteristics of LIBS in gas measurements have been discussed in detail and can be found elsewhere [13,14]. This chapter explores the calibration techniques and various LIBS applications for gas samples using a mobile LIBS system which was developed at Institute for Clean Energy Technology (ICET), Mississippi State University, USA. This versatile mobile system was originally developed to monitor toxic metal concentrations in the off-gas emission of a plasma hearth process system. It has been used to conduct various laboratory studies and field measurements for different applications.

2. EXPERIMENTAL SETUP The experimental arrangement of the LIBS system requires a laser system that can deliver high pulse energy (e.g. 100–300 mJ/pulse) to produce a spark in the gas medium. A frequency-doubled Nd:YAG laser is directed and focused on the desired gas sample with a lens of proper focal length (generally 10–20 cm). The emission from the spark was collected with a UV optical fiber bundle and sent to the detection system. Usually one Czerny-Turner spectrometer that can cover a spectral region of 20–40 nm simultaneously is used for gas measurements. In some cases, two detection systems were needed to monitor two spectral regions simultaneously or two different measurement locations. The detection systems employed in the present study include a SPEX 500M spectrograph equipped with a 1024-element intensified diode array detector (Model IDAD-1024, Princeton Applied Research) and an optical spectrograph (Model HR 460, Instrument SA, Inc., Edison, NJ) equipped with a 1024 × 256 element intensified charge-coupled device (ICCD, Princeton Instrument Corporation, Princeton, NJ). A fiber bundle with the output

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end of the optical fiber bundle splitting into two bundles was coupled to two spectrographs for the measurements of two spectral regions. The detectors were operated in gated mode with the control of a high voltage pulse generator (PG-10, Princeton Instruments Corporation, Princeton, NJ) and was synchronized to the laser output. Data acquisition and analysis were performed using a personal computer and a notebook computer (Model T-4700CS, Toshiba). The gate delay time and gate width were adjusted to maximize the signal-to-background (S/B) and signal-to-noise (S/N) ratios, which are dependent on the emission characteristics of the elements as well as the experimental configuration. A gate delay of 5–10 sec and a gate width of 10–20 sec were used in most of the work. To quantify the LIBS data of gas sample, LIBS instrument needs to be calibrated with the samples of known concentration. The gas phase sampling generally uses a nebulizer to produce aerosol from the solution of standard reference materials. In the present case an ultrasonic nebulizer (USN, Cetac U-5000AT+ ) is used to produce the dry aerosols of selected metals. Two possible setups that were used for LIBS calibration in open and closed system are shown in Fig. 3 of chapter 5. Calibration has been performed by injecting known concentrations of dry aerosols from an ultrasonic nebulizer into either a sample cell (closed system) or air (open system). Volumetrically diluted plasma emission standard solutions (Spex Industries) were injected into the USN with a peristaltic pump at a rate of 1.9 ml/min. A 0.8-ml/min flow rate of air was used as a carrier gas flow to transport the aerosol through the USN. The aerosol in the USN was first dried by a heated (140 C) tube and then passed through a chilled (3 C) condenser to remove water. In the open system, the dry aerosol from the USN was sent to a stainless steel sample injection tube, and the laser beam was aligned 2 mm above the end of the tube and focused on the center of the tube to achieve reliable calibration. The sample injection tube was enclosed in a Pyrex cylinder to reduce interference from the surrounding air. In the closed system, the metal aerosol was injected continuously to the sample cell that is made of Polyvinyl Chloride (PVC). LIBS calibration data were collected after the composition equilibrium in the cell was reached. The waste solution was collected during the nebulization procedure. The USN system was later operated with the collected waste solution. A comparison of the LIBS signal from the stock and waste solution can be used to determine the efficiency of the nebulizer [15]. In the laboratory, both the open and closed systems can be used to calibrate the LIBS system, but in the field measurement only the open system is more suitable for on-site calibration. The on-site calibration should be performed before the field test started and after the test ended each day to verify the system response. The on-site calibrations have been carried out by injecting metal aerosol generated from a USN into the gas stream with a probe. The sample injection probe was mounted on the opposing port across the gas stream. Each day, the LIBS spectra need to be recorded before the metal injection for zero check.

3. CALIBRATION LIBS is an atomic emission spectroscopy. For quantitative analysis with LIBS, either internal standard calibration or external calibration method is needed. However, calibration is the most difficult issue in the development of LIBS, especially for the field measurement. This is because the calibration procedure should keep the

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same experimental conditions for the known sample used in the calibration and the unknown sample. The parameters, which can affect the characteristics of the LIBS spectra in gas, include particle size, gas pressure, temperature, and laser energy. Due to shot-to-shot laser fluctuations, it is hard to maintain the excitation condition for the calibration data and data from the unknown sample in laboratory environment. LIBS is being considered as a non-sampling technique for on-site measurement, this implies an extra difficulty for calibration. An extensive LIBS calibration study has been performed by Singh et al. [16–18]. They have compared two calibration methods using a hydride generator and a USN in LIBS experiments. LIBS spectra have been recorded using a hydride generator (Fig. 1) and a USN with a mixture solution of As, Sb, and Sn in N2 and He to study interference effects among different metals. Since HCl concentration plays an important role on hydride generation efficiency, different HCl concentrations in the mixture solution have also been used in this experiment. The spectral interferences were not significant in this study. However, the results from the hydride generation were quite sensitive to the acid concentration in the mixture. Comparison of metal generation from metal oxide particles produced by an ultrasonic nebulizer shows that, the actual gas stream metal distribution is close to that from the USN. Efficient metal hydride generation requires different acid concentrations for different metals. A USN, on the other hand, is easy to use, and works for all resources conservation and recovery act (RCRA) metals. Therefore, the calibration curves for every RCRA element have been obtained using a USN. Based on the data collected from the USN and, after averaging 50 laser pulses, the precision for most of the RCRA metals was ∼10% or better, and the accuracy was ∼5−10%. Studies of relative calibration were also performed to implement on-line calibration in field measurements. From the experimental results, it is recommended to use the USN to conduct the LIBS calibration for gas samples. The calibration data used here is obtained by injecting known concentrations of dry aerosols from the USN into air. Generally, LIBS data from four or more concentrations of an element were used to obtain the calibration curve. The calibration curve is based on either peak height or peak area of each analyte line. The slope of the calibration curve is used as the calibration factor to infer metal concentration. Reagents

Rubber septum

To sample cell Gas inlet Reactor

NaBH4 + 3H2O + HCl

H3BO3 + NaCl + 8H

Em+

EHn + H2(excess)

Fig. 1. Schematic diagram of the hydride generator. (Reproduced with permission from Ref. [16]).

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The peak area (or peak height) of an analyte line from a demonstration test on LIBS spectra was normalized using its calibration factor to obtain the metal concentration. In general, peak height calibration and peak area calibration give about the same result for an interference-free line. For different types of spectral interferences, either peak height or peak area must be selected for best results. From the obtained experimental results it was found that the peak area analysis yielded better results than did the peak height analysis for the self-absorbed spectral line, and the peak height analysis yielded better results than did the peak area analysis for a line overlapped with other lines. The limits of detection (LOD) of selected analyte lines of seven RCRA metals determined in the laboratory just before the CEM test are listed in Table 1. The precision of these measurements estimated from the calibration data are also listed in Table 1. The precision and accuracy greatly depends on pulse-to-pulse laser fluence fluctuations at focal volume and the concentration variation in the aerosol flow from the ultrasonic nebulizer (USN). The accuracy and precision of LIBS measurements can be improved by increasing the signal integration time because some lines have spectral interferences and the actual field detection limits may be slightly higher than the reported detection limit, depending on the concentration of the interfering elements. The LODs depend on the experimental conditions and can be reduced by improving the optical design and detection system. In some cases, if the absolute concentration calibration is too difficult to obtain due to the variation of the environmental conditions, relative concentrations may be considered. One can either use the calibration based on the intensity ratio of the analyte line and reference line or fit the observed spectra with a theoretical model. Analysis of LIBS data using spectral fitting requires the knowledge of spectroscopic constants such as plasma temperature, and degree of ionization. These two parameters, however, are not easy to be determined accurately. Alternately, Ottesen et al. used reference line intensity Table 1. Limit of detection of some selected metals in gas [23] Element As Be Co Cr Cr Zn Cd Hg Sb Sn Mn Mn

Ni Pb Fe ∗

Analyte Line (nm)

Relative STD (%)

LOD g/acm∗

278.02 234.80 345.35 425.44 359.30 330.30 228.80 253.65 259.81 283.99 257.61 403.08, 403.31, 403.45 341.48 405.78 404.58

9 3 8 5 5 15 5 13 9 10 4 8

600 1 24 78 12 570 45 680 120 190 4 75

acm = Actual cubic meter

9 6 6

30 90 140

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information from the NIST collections to perform spectral fitting to obtain relative concentration [19]. However, the excitation condition needs to be verified for this simple method they used, since the intensities in these reference collections are obtained under certain conditions, which may be very different from those in laser induced plasma. Ciucci et al. have developed an algorithm for calibration-free quantitative LIBS analysis and seem to have had great success with laboratory data [20]. However, such an approach relies on some basic assumptions, such as laser-induced plasma (LIP) is in local thermodynamic equilibrium (LTE); LIP is an optically thin; plasma composition is representative of the actual sample composition. It also requires measuring the emission lines of all the elements presented in the sample. The selected analyte lines should be free from the spectral interferences such as spectral overlapping, saturation or selfabsorption. To measure all the analytes simultaneously, a broadband spectrometers or multi-spectrometers are required for the calibration-free analysis. This technique still needs to be evaluated with more practical data to be widely accepted for most LIBS work. The practical environments are quite different from those in a laboratory. The transfer of the LIBS calibration obtained in a laboratory to field measurements is a great

(a)

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Fig. 2. (a) Cd calibration slope versus laser energy (b) Cd calibration slope versus LIBS background. (Reproduced with permission from Ref. [21]).

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challenge. To establish a calibration scheme for quantitative measurement in practical environments, a series of studies were made to correlate LIBS backgrounds with changes in excitation conditions [21]. A linear relationship between the LIBS calibration slope and the backgrounds for Cd and Be was found. These data were obtained from spectra recorded in the 230-nm spectral regions with different laser energies, gate windows, and test cells. Figure 2 shows the linear relation between the laser energy and the Cd calibration slope. The LIBS background was also found to be linearly proportional to the calibration slope (see Fig. 2). The data were recorded with gate delays of 20 s and 15 s with a fixed gate width of 30 s. These results imply that the background can be used to correct the changes in plasma conditions. However, the same experiment in the 415-nm spectral region shows a linear relationship between background and calibration slope only when laser energy is below a certain limit (see Fig. 3). At higher laser energy, the CN interference is dominant in this spectral region, and the intensities of the analyte lines of Pb and Cr are possibly saturated. The results of the background study show that background normalization can be used to correct the calibration factor due to minor changes in the plasma condition. However, this approach demands great care due to its limitations. (a)

Slope for Cr 425 nm

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Fig. 3. (a) Cr calibration versus laser energy (b) Cr calibration slope versus LIBS background. (Reproduced with permission from Ref. [21]).

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4. APPLICATIONS LIBS measurements of off-gas have a wide range of applications from the process control of a production plant to thermal waste treatment process. The detection of trace metals in the off-gas of various industrial plants such as coal-fired power plants, cement kilns, incinerators are a great public-health and environmental concern. Conventional analytical techniques involved getting sample from sites and sent to laboratory for analysis. It had to deal with sampling problems in off-gas system. LIBS can perform off-gas measurements by focusing the laser beam on the gas stream through a window and collecting the signal through an optical fiber. It offers a technique to perform remote and in-situ measurements. Radziemski and Cremers have applied LIBS to analyze effluent gases from a prototype fixed-bed coal-gasifier at the DOE Morgantown Energy Technology Center [1]. They demonstrated that LIBS has the capability for near real-time monitoring of the concentrations of major and minor species in the off-gas emission. Neuhauser et al. tested their on-line Pb aerosol detection system with aerosol diameters between 10 and 800 nm. A detection limit of 155 g/m3 was found [22]. Singh et al. have used LIBS as a process monitor and control tool for waste remediation [23]. They monitored the toxic metals from three plasma torch test facilities and proved that LIBS can be integrated with torch control systems to minimize the toxic metal emission during plasma torch waste remediation. Ferioli et al. have used LIBS to measure the equivalence ratio of a spark-ignited engine in a laboratory setting. They used C/N and C/O peak ratios to quantify the equivalence ratio over a range from  = 08 to  = 12 [24]. Ball et al. investigated the feasibility to apply LIBS as hydrogen-sensing technique to detect hydrogen leak for real-time monitoring. Using hydrogen 656.28-nm line, they obtain a limit of detection of about 20 ppm (mass) [25].

4.1. Analysis of Air-Sampling Filters with LIBS Filter collection with a sampling pump is widely used in environmental monitoring and personal protection. This technique can detect very low species concentrations through time accumulation. Conventionally, the filter is analyzed via chemical laboratory work that includes two main steps: chemical washing of the filter to produce a solution and, thereafter, performing a solution analysis. This is a time-consuming procedure and can require several hours. With LIBS, the collected species mass on the filter can be determined rapidly by laser sparks across the filter surface. Since the laser induced sparks vaporize and excite the sample without any sample preparation, the analysis time is reduced to a few minutes. The direct detection of trace elements in air with LIBS is very difficult due to its insufficient sensitivity. For trace metals below the LIBS detection limits, an air sample can be collected on a filter, which is then analyzed by LIBS. This method results in a lower detection limit and provides a quasi-on-line measurement. Arnold and Cremers have used this technique to determine metal particles on an air damping filter [26]. They used a cylindrical lens to form a long spark on the filter to increase the sample volume and reduce the filter damage. Using the calibration curve for Tl line at 535.05 nm, a LOD of 40 ng/cm 2 for Tl in filter paper was obtained. Later, Yamamoto et al. determined LODs of 21 ng/cm2 and 5.6 g/cm2 for Be and Pb on a filter [27]. They also noticed that particle size can affect the detection limit for filter

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analysis. Cremers and Radziemski have measured Be on a filter surface and found that Be particles greater than 10 m on the filter were not completely vaporized [28]. The particle size dependence of the LIBS signal restricts LIBS applications to air sampling through a filter [28]. The method has been tested using filter sampling and LIBS analysis to monitor particulate emission in a mini test stand. Two types of filters, PVC membrane filters and glass fiber filters, were evaluated for this application. The PVC membrane filters were burned by a couple of laser-induced sparks. Therefore, it is not suitable for LIBS application. Glass fiber filters made of borosilicate glass fiber have a maximum operating temperature of 500 C in air. Since it does not burn under the laser-induced spark, it is ideal for LIBS applications. This type of filter was then used in all the experiments conducted at the ICET mini test stand. The mini-test stand was operated at a 10-lb/hr air flow during the study. The dry metal aerosol of desired concentration produced by an ultrasonic nebulizer was injected into the test stand. A 1.5-inch diameter, 1-m pore size glass fiber filter was put in a closed-face air-monitoring cassette. The sampling device with the filter cassette was installed on a sampling port, which is 1.2 m downstream from the aerosol injection port, to collect the aerosol sample in-line. In most of the experiments, the sampling flow was set to 1.5 L/min to match the velocity of the sampling flow with the velocity of the gas stream. Each filter was used to collect samples for 10 to 20 minutes before analysis by LIBS. Since there was no absolute concentration measurement available at the sampling position, LIBS measurements in the gas stream were also performed in a port 15 cm upstream from the sampling port to provide a reference. For LIBS filter analysis, a filter was placed on a platform that rotated around the vertical axis with a speed of 300 rpm. During the LIBS measurements, the platform was translated to let the focused laser beam scan at different radii on the filter surface. The experimental parameters for filter analysis are: a laser energy of 10–15 mJ, a gate delay of 1.5–2 s, and a gate width of 5–10 s. The experimental parameters for off-gas analysis are: a laser energy of 120–130 mJ, a gate delay of 10–20 s, and a gate width of 10–30 s. To verify the performance of the filter, some sampled filters were sent to a laboratory for conventional chemical analysis to obtain the Be mass collected on the filter. The absolute Be concentration in the gas stream was then calculated from the sampling rate and Be mass deposited on the filter. The Be concentration in the gas stream was also calculated based on the concentration of the solution injected, nebulizer operation parameters, and mini test stand operation parameters. A comparison of metal concentration in the gas stream inferred from these two methods is shown in Table 2. The relative differences between these two methods are 4 to 15%. It is noted that the filter’s sampling ability may be decreased as the aerosol concentration increased. This is because this type of filter is initially designed to work with a very low concentration. To evaluate the analytical performance of LIBS on a filter, some sampled filters were analyzed by LIBS and compared with the on-line LIBS analysis. Fig. 4 shows the LIBS signals obtained from filters and from the on-line measurements versus the Be concentration of the solution injected. The on-line LIBS measurements are used to monitor the performances of the solution injection system and mini test stand. Since the intensities of LIBS obtained from the gas stream are linear to the solution concentration, a linear relationship is expected against the solution concentration from the LIBS filter data. At low Be concentrations (1, 2, and 4 g/ml), the intensities of LIBS on filters

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Table 2. Comparison of the concentration of Be aerosol calculated from filter analysis and from the solution concentration Be concentration in solution (g/ml)

6 8 10

Be concentration in gas stream (g/m3 

Be collected on filter (g)

calculation based on the filter analysis

calculated from the solution concentration, gas flow rate, etc.

3137 3529 3922

3268 3953 4621

08 09 10

Relative difference

40% 107% 151%

Be (313.04 nm) intensity (thousands)

70 LIBS in flow

60

LIBS on filter

50 40 30 20 10 0 0

2

4

6

8

10

12

Be solution concentration (µg/ml)

Fig. 4. LIBS signals obtained from filter and off-gas measurements.

do appear fairly linear with solution concentrations. The data taken at Be concentration of 6 g/ml shows a reduced intensity. It could be due to self-absorption in the plasma, incomplete vaporization of the particles, or lower sampling efficiency at high metal concentration. However, it can be found that the reproducibility of LIBS on filters is fairly good (10–20% RSD), as compared to LIBS in flow (20–30% RSD). Since the filter moves during data collection, a small standard deviation suggests that the mass distributed on the filter is fairly uniform. Similar experiments for Cr and Pb have been conducted. The results are very similar to those shown in Fig. 4. The signal from low concentration filter data increases as the solution concentration increases. The behavior of LIBS filter data at higher concentrations is more complicated to explain. More systematic experimental study is required. To evaluate the sampling performance at different sampling times, the gas stream was continuously monitored while a Cr solution of 30 g/ml was injected into the mini test stand. The results of these measurement are shown in Table 3. Here, the intensities from LIBS filter data are normalized by the sampling time. The intensity ratio of the LIBS filter data and concurrent off-gas data is about the same for a sampling time of 25 minutes. A higher intensity ratio was found in shorter sampling times (e.g., an

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Table 3. LIBS signals from filters at different sampling times Filter LIBS Measurement Sampling Time (Minutes) 25.0 25.0 8.5

Concurrent Off-gas LIBS Measurement

Time averaged Intensity I1 

Averaged LIBS signal I2 

353 309 468

8366 7460 6638

I1 /I2

0042 0041 0070

8.5-minute sampling). This shows that the longer sampling time for this Cr concentration level reduced the Cr detection sensitivity. Therefore, sampling time needs to be adjusted for different concentration levels. Although this quasi-continuous emission monitor method is very useful for low emission environments, it is difficult to achieve reliable quantitative results due to its sensitivity to the particle size, sampling time (concentration dependent), and collection efficiencies for different elements. A great amount of chemical and LIBS analysis is needed to establish the optimum conditions (filter, sampling time, LIBS setup, etc.) for each type of sample (concentration level, particle size, etc.).

4.2. Continuous Emission Monitor The amount of toxic metal added to the atmosphere is restricted and controlled by various U.S. Environmental Protection Agency (EPA) rules and permits. The EPA is modifying regulations to further reduce metal emissions. Thus, the measurement of toxic metals is very important for compliance with the existing EPA rules and also for the proposed Maximum Achievable Control Technology (MACT) rule in the future. Several potential techniques that have been evaluated for this application include inductively coupled plasma atomic emission spectrometry (ICP-AES), LIBS and X-ray florescence. Among these methods, LIBS is the only technique which provides real-time, in-situ analysis which is important for a continuous emission monitor (CEM) [29,30]. A CEM system needs to provide immediate warning as the level of the toxic metal in off-gas results in a dangerous level of toxic metal released into the atmosphere. LIBS capability for continuous, real-time analysis makes it an ideal technique for a CEM for thermal treatment plants. The only problem with LIBS is that the sensitivity for some toxic metal might not be enough. A LIBS system developed in the laboratory has been tested in two U.S. Department of Energy (DOE)/EPA CEM tests [29,31]. The CEM test was designed to measure the performance of multi-metal CEMs for regulatory compliance applications. It was conducted at the EPA’s Rotary Kiln Incinerator Simulator (RKIS) facility, which consists of a primary combustion chamber, a transition section, and an afterburner in the secondary combustion chamber [26]. The kiln and secondary combustion chamber were operated with natural gas during the tests. Metals were introduced into the fuel gas by injecting an aqueous metal solution directly into the secondary flame of the incinerator to achieve the target fuel gas concentrations. To simulate actual flue gas conditions, fly ash particles were also injected into the incinerator. The LIBS system used at a port located 5.7 m

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downstream from the air dilution damper. The EPA RM sample port is 1.4 m upstream from the LIBS port. The first test demonstrated LIBS’ rapid sampling rate and potential for metal CEM. It also helped us pinpoint various problems associated with LIBS field measurements. These problems include higher limits of detection (LOD), a need for on-line calibration, degradation of optical components, and the need for simultaneous monitoring of all the RCRA metals. These problems have been extensively studied in the laboratory since the first CEM test. Calibration techniques have been tested in the laboratory. The LOD has been reduced by a factor of eight or more for most of the metals. A method to correct the signal loss due to the degradation of optical components during the field test was developed. The various improvements made to the ICET-LIBS system were evaluated in the second DOE/EPA CEM test [31]. The results of the LIBS calibration study, and the results of LIBS measurements during the second DOE/EPA CEM test, are given here. The CEM test focused on As, Be, Cd, Cr, Pb, and Hg, which are the RCRA metals regulated in the EPA’s MACT rules. The test program consisted of a high- and low-metal test. The target concentrations were 75 g/dscm in high-metal tests and 15 g/dscm in low-metal tests. The EPA’s Reference Method (RM) and CEM measurements were performed concurrently for each test condition. The number of RM measurements performed for each test depended on the target metal concentration. The RM sampling time was one hour for the high-target-metal test and two hours for the low-metal tests. There were in total twenty RM samplings during the entire test, ten for low-metal tests, and ten for high-metal tests. Due to the difficulty in injecting a known amount of sample into a practical gas stream, LIBS calibrations were performed in the laboratory before the field test. The two LIBS detection systems used in the field test were calibrated for all the RCRA metals. The peak area of an analyte line from the calibration LIBS data was used to construct the calibration curves. Linear regression was used to obtain the calibration factor. On-site calibrations for Cr, Pb, Cd, and Be were performed at RKIS during the shakedown test with a calibration setup similar to that shown in Fig. 3a of chapter 5. The on-site calibrations were done by injecting metal aerosol into the RKIS gas stream with a probe. The sample injection probe was mounted across the gas stream on the opposite port. Since the gas flow quickly diluted the injected sample in the gas stream, the metal concentration near the focal volume could not be accurately estimated. Therefore the on-site calibrations were mainly used to check system response. The temperature, flue gas flow rate, and particle loading in the test environment were ∼232 C, 3.4 scm/min, and 25–50 mg/dscm, respectively. The effects of these gas-stream parameters on LIBS calibration had not been systematically studied before. The concentrations of Be, Cd, Cr, and Pb were monitored simultaneously in near real-time during the four-day test. Analyte lines of Cd and Be were monitored in the 220-260-nm spectral region with a 1200-line/mm grating, while analyte lines of Pb 405.8 nm and Cr 425.44 nm were monitored simultaneously in the 400-429-nm spectral region with an 1800-l/mm grating. During the test, it was found that the highquality optics used in the LIBS system degraded quickly, causing the LIBS signal to drop significantly. The dichroic mirrors used in LIBS have high-damage thresholds >GW/cm2  under normal operating conditions. However, the properties of the optical coating changed in the humid and hot test environment, resulting in a lower damage threshold than the specification and damage occurred rapidly in the field test. Fig. 5a

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Fig. 5. (a) LIBS background, (b) Raw CEM data, and (c) Corrected CEM data during RM sampling period. (Reproduced with permission from Ref. [21]).

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shows the LIBS background recorded during one RM sampling period. It clearly shows the LIBS signal falling as the optics gradually degraded. The background normalization technique mentioned above was used to correct LIBS’ raw data. Figs. 5b and 5c show the raw CEM data and background corrected CEM data during the RM sampling period, respectively. A more steady-inferred metal concentration was obtained after correction. This indicates that this technique is effective for the problems of optics damage. This technique was then used to correct all the LIBS data collected during this field test. The effects of fly ash and temperature were taken into account in recalling the laboratory calibration factor for CEM test. This LIBS system was successfully used to simultaneously monitor concentrations of Cd, Be, Cr, and Pb in near real-time during both the high- and low-metal tests. The system response time mainly depends on the sampling rate of the system. In this CEM test, the LIBS system response time was 10–20 seconds. The measured metal concentrations have been compared with the results from EPA’s RM. A comparison of the time-averaged LIBS data (over the RM sampling period) along with the data obtained with RM is shown in Fig. 6. The relative accuracy of LIBS for four elements based on the RM results was found to be in the range 19–78%. The expected accuracy in these measurements was 20 or 50%, which is much higher than expected in an analytical laboratory measurement. The LIBS data taken during the four test days roughly followed the trend of the RM data. It was found that LIBS data was more consistent with RM data for the last test day. This is because the experimental setup was more stable on that test day due to a cooler probe and a new dichroic mirror. During the first three test days, more technical problems were encountered such as optics damage and laser power dropping due to the sensitivity of the frequency doubler affected by the environmental temperature. The rough correction with the background used in this test has shown promising results. However, a more refined correction taking into account the effects of gas-stream parameters should improve the accuracy of LIBS.

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LIBS has shown its capability as a multi-metal CEM for Cd, Be, Cr, and Pb in the above field tests. Background normalization technique has also proved to be a useful method to correct the signal variation due to optics damage during the field test. However, currently LIBS’ sensitivity, precision, and accuracy for certain toxic metals, have still not reached EPA’s requirements to be accepted as a metal CEM. Future development in this area may include improving the detection sensitivity of all the RCRA metals. A calibration routine for automatically compensating plasma-condition changes due to variations of gas-stream conditions or pulse-to-pulse laser fluctuations is also needed.

4.3. Process Control A mobile instrument was used in the advanced analytical instrumentation demonstration (AAID) test at the Science Applications International Corporation (SAIC)’s STAR Center, Idaho Falls, Idaho, to demonstrate LIBS’ capability in process control. The STAR Center’s plasma system consists of the following components: a plasma chamber, a secondary combustion chamber, a HEPA filter, a stack, and instrumentation and system controls. The details of the test facility are given in Reference [32]. Fig. 7 shows the experimental setup of the LIBS system. LIBS measurements were performed continuously at a port between the baghouse and the HEPA filter. The port was purged with nitrogen to keep the window clean and cool and the same port was also used to collect the LIBS signal. A beam dump mounted on the opposite port across the gas stream was used to dump the laser energy. The third port in the direction normal to the laser beam was used to monitor the spark in the gas stream and also to align the spark with the sample injection probe for calibration. The emission from the spark was collected with a UV optical fiber bundle coupled to a spectrograph. An intensified diode array detector (IDAD) was attached to the spectrograph to record the LIBS spectrum. A laptop computer interfaced to the detector controller with a PCMCIA-GPIB card was used for data acquisition and analysis. The EG&G OMA2000 software was used to collect Lens Lens

LIBS probe

Beam block

Window

LIBS data acquisition/ analysis system

Computer monitor

TTL signal receiver/ interface box

Alarm/ indicator

Facility computer

Fig. 7. Schematic of LIBS system and the SAIC’s STAR Center alarm/interface system.

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Hg concentration (mg/scm)

1200000 1000000 800000 600000 400000

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Fig. 8. Variation of Hg concentration with time in SAIC’s STAR Center off-gas during the AAID test.

LIBS data. A user-written macro-program was used to analyze and display the data in near real-time. The real-time elemental concentrations of Pb, Ce, Cr, and Hg were displayed on the computer monitor during data acquisition. TTL signals were sent to the alarm/interface system, and a warning message was also shown at the bottom of the computer screen whenever the concentration of the measured species was above the alarm level during the LIBS measurements. This allows the operator to modify operational parameters of the plasma system to prevent emission that exceeds the pre-established facility. During the test, the metal emission did approach the alarm levels several times (see Fig. 8). The TTL signals were successfully sent to the alarm/interference system when the concentrations exceeded the alarm limit. Details of this measurement can be found in Ref. [32]. The LIBS response was determined mainly by the sampling time of the measurement. In this test, LIBS had a response of 50 seconds, which is sufficient to provide critical information for process control.

4.4. Filter Efficiency The ceramic filters used in waste processing with the plasma torch play an important role for removing toxic metals. LIBS has been used to evaluate metal-removal efficiency. Two LIBS systems were used to record the spectra at the inlet and outlet of a ceramic filter during the Plasma Arc Centrifugal Treatment Pact-6 Slip Stream Test Bed (SSTB), a 100-hour duration demonstration test at Mountain State Energy (MSE) [32]. The elements that appeared in the filter inlet were Fe, Cr, Pb, Ca, Si, Cu, Mn, Mo, C, Mg, K, Na, Sn, Zn, and Cd. The metal identified from the filter outlet spectra were Cr, Pb, Fe, K, and Mn.

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Table 4. The calculated minimum metal removal efficiency in WETO/MSE test [23] Element Cr Zn Cd Sn Mn Pb Fe

Maximum concentration at the Ceramic filter inlet (mg/acm) 245 240 119 200 106 888 400

LOD (g/acm)

Metal removal efficiency (%)

12 570 45 190 75 90 140

9999 997 9996 999 9999 999 9996

During most of the test, the concentrations of the target metals in the filter outlet were below our detection limit. Therefore, only the minimum metal-removal efficiency could be calculated by assuming that the target-metal concentration in the ceramic filter outlet equaled its detection limit. The metal-removal efficiency was calculated by using the ratio of the detection limit of the metal and the highest metal concentration found in the ceramic filter inlet. The results of this calculation are shown in Table 4, which clearly shows that the efficiency of the ceramic filter for most of target metal was better than 99.9%. The estimated efficiencies for Zn, Sn, and Pb were found to be lower than for other metals, and this is probably due to relatively higher LODs for these three metals. Cr, Fe, and Pb were detected occasionally beyond the ceramic filter when the metal concentration levels were momentarily above our detection limit (this might be related to some problems in the ceramic filter operation or torch processing system). The metal partitioning is the ratio of the metal in the off-gas and the metal in the feed. It can be used to monitor the facility operation. The major factors, which can affect the metal partitioning, are the feed rate, the feed composition, and the plasma torch’s operating condition. The metal partitioning was calculated using the time-averaged metal concentrations measured before the ceramic filter during the actual feed and the metal feed rate. The calculated metal partitioning for Cr, Fe, and Pb were found to be 0.17%, 0.066%, and 2.3%, respectively. Since Pb is more volatile, it has a higher partition than Cr and Fe, as expected. To study the plasma torch vitrification process and the performance of various off-gas components, Mn and Cd were selected as tracers during the test, one with a high melting point and the other with a low melting point. The tracers were injected into the plasma torch vessel. The time lag between the metal addition to the plasma torch vessel and the observation of the metal in the gas stream can be defined as the residence, and is an important parameter for evaluating particular waste treatment processes. Mn was found to be the best tracer for the present LIBS system. The concentration spikes of Cd at high concentrations are not as well-defined as Mn although Cd can still be a good tracer when operated at a low injection amounts and for a longer injection interval. Based on the LIBS data obtained in this test, some important facility operation information was obtained such as the minimum metal-removal efficiency of a ceramic filter, suitable tracers for the residence time measurement, and metal partitioning.

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4.5. Combustion Diagnostic LIBS can also be used as a diagnostic tool for combustion systems by measurements of fuel-air ratios, fuel composition, and temperature. In combustion, the hydrocarbon mixture and the equivalence ratio (i.e. fuel-to-air ratio) are two improtant parameters. LIBS can simutaneously detect varous elemental abundance in the combustion environment. It was first applied to the hydrocarbon mixture by Schmieder in 1981 [33]. Phuoc and White have determined the equivalence ratio of a CH4 /Air mixture of a jet diffusion flame with LIBS by measuring the ratio of oxygen-to-carbon, nitrogen-to-carbon and hydrogen-to-oxygen in flame [34]. Sturm and Noll have performed measurements of C, H, O, and N in gas mixtures of air, CO2 , N2 , and C3 H8 to establish the calibration curves for LIBS detection of these elements (i.e. LIBS signal versus the partial pressure) [35]. Stavropoulos et al. have used LIBS in a methane/air premixed flame to demonstrate that the ratio of hydrogen atom and oxygen atom varies linearly with equivalence ratio [36]. Blevins et al. have applied LIBS to high temperature industrial boilers and furnaces [37]. They used a novel LIBS probes designed for these high temperatures and high particle loadings environments. Multi-elements were simultaneously detected with an Echelle spectrometers coupled to intensified CCD cameras. It shows that LIBS can be used as a sensitive, on-line process diagnostic for equivalence ratio monitoring in flame reactors. The feasibility to apply LIBS to practical combustion environment was first evaluated by Singh et al [38]. It has been used to characterize the upstream region of a large magnetohydrodynamic (MHD) coal-fired flow facility (CFFF). The relative concentrations of several species were inferred by fitting the observed CFFF LIBS spectra with computer-simulated spectra. This was the first LIBS experiments in a harsh, turbulent, and highly luminous coal-fired MHD combustion environment. Lee et al. also use LIBS to combustion and other thermal systems for simultaneous measurements of a number of important thermo-chemical parameters, including temperature [39]. They have compared the results of LIBS flame temperature measurements with other methods such as thermocouple and Rayleigh scattering and have found excellent agreement even in sooting flames.

4.6. Rocket Engine Health Monitor Detection and characterization of metallic species in the exhaust plume of hydrocarbon fueled rocket engines can indicate the presence of wear and/or corrosion of metal in the rocket engine. This information on engine wear obtained during engine operation is very useful, allowing the possibility of engine shutdown before any catastrophic failure. It has been observed that a catastrophic engine failure is generally preceded by a bright optical emission, which results from the erosion of metal from the engine parts. This is because of high temperature in the rocket plume ∼2000 K, which partially vaporizes and atomizes the metal species, leading to atomic emission in the near ultraviolet and visible spectral range (300 nm–760 nm). A traditional method for monitoring the engine plume during a test is atomic emission spectroscopy in the near ultraviolet and visible spectral regions. The hydrocarbon-fueled engine contains various species such as atomic carbon, C+ 2 , and other carbon-free radicals, which will increase the background emission comparatively more than the main OH band in the oxygen- and hydrogen-fueled engines.

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Even the scattering from the unburned carbon will also produce a strong background, which has proved to be a disadvantage for the atomic emission spectroscopic technique to detect the presence of metal corrosion and engine wear in a hydrocarbon-fueled rocket engine. LIBS provides an alternative technique to plume diagnostics. It uses a gated detection, which discriminates against strong background emission. It can provide spatially resolved, real-time measurement of several metallic species critical for monitoring the health of a rocket engine. The performance of LIBS was evaluated in detecting the trace of elements in the fuel plume of a hybrid rocket engine simulator at Stennis Space Centre (SSC), Mississippi, USA. The hybrid rocket engine simulator used Plexiglas as its fuel [40]. An adequate flow of oxygen was maintained for proper burning of plexiglass. Initially, spark was started in a small chamber between the electrode and body of the ignition chamber using an electric current from a 12 V battery. This initial spark started the ignition of plexiglass as a main fuel, which generated a high-speed, luminous plume of ∼2 inches from a 3–4-mm-diameter exit nozzle. The laser was focused at various locations of the plume to record the LIBS spectra at different spatial locations with a lens of 10 cm focal length. Copper and stainless steel wires were used as the seeding samples by keeping them axially inside the ignition chamber extending up to the exit nozzle. The sample metals melt and vaporize due to the high temperature of the fuel and then exit with the burnt fuel as a plume. LIBS spectra of the rocket engine simulator plume were recorded when a 316L stainless steel wire of diameter 1.76 mm was inserted into the ignition chamber. The laser was focused 3 inches away from the exit of the nozzle. Fig. 9a shows the strong atomic lines of chromium in the LIBS spectra. The stainless steel 316L contains ∼17% of Cr. LIBS spectra of the plume have shown a significant amount of Cr present in the plume. No Fe lines were found in the spectra at this location, which is probably due to the low concentrations near the exit channel where the plume speed is very high. The presence of chromium lines seems to be due to their high transition probabilities. Fig. 9b shows the LIBS spectrum of the plume ranging from 305 to 350 nm. This spectrum shows the presence of strong OH and NH bands, as well as the two strong lines from atomic copper. OH and NH bands appear as a result of the reaction of hydrogen and oxygen as well as atmospheric nitrogen and hydrogen, respectively. The copper line spectrum is from the sample of copper wire kept inside the ignition chamber. Strong copper lines were recorded in the emission from the plume when copper wire was kept out of contact with the plume near the exit channel. However, it could be detected for a fraction of second as it vaporized and escaped with the high-speed burnt-fuel plume. The LIBS spectra of the plume were recorded- at different spatial locations from the exit nozzle. Fig. 10 shows the LIBS spectra at different spatial locations of the plume when copper wire was used as the seeded sample in the ignition chamber. Spectra were recorded at 13/8", 2", 3", and 5" from the exit nozzle. The presence of copper was detected strongly near the nozzle exit during an initial fraction of a second when the burnt-fuel plume started building up. The Cu signal decreased with time when the plume attained its full length >2", high temperature, and high speed. It was noted that as the measurement point moved away from the plume exit channel (luminous zone), the copper lines appeared throughout the plume. The background

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emission from the plume also decreased in this measurement location. This is likely due to the better mixing of metal vapor in the plume away from the exit channel. In this location, the plume has a lower speed and a lower gas temperature, as compared with the plume near the exit channel. The data from the preliminary test show that

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40000 D = 1-3/8" 30000 20000 10000 0 0

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the measurements made away from the luminous part of the plume can provide more meaningful information about the health of rocket engine. This test demonstrated that LIBS is capable of being used as an engine health monitor to detect a trace amount of metal emerging from any part of a rocket’s engine. The proper calibration and metal seeding techniques are now under investigation.

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5. CONCLUSION Laser-induced breakdown spectroscopy is now a very active field in analytical science. Considerable progress in the area of basic and applied research of LIBS has been made during the last two decades. Many research laboratories all over the world are working in this field. However, the achievements are not always made known or implemented. A better international collaboration is needed for unification and application of current LIBS techniques. This chapter describes the analytical analysis of gaseous samples using LIBS. It demonstrates that LIBS may be utilized to detect trace metals in the off-gas of various industrial plants and also used for combustion diagnostics. The results presented here reveal that glass-fiber filters can be used to collect air samples and are suitable for LIBS analysis. The potential of using LIBS to detect metallic species in the exhaust plumes of rocket engines has also been demonstrated. In the last decade, the LIBS technique has progressed due to improvement in several experimental parameters to detect trace elements in various types of samples. To commercialize the LIBS technique for industrial and environmental applications, its sensitivity and precision need to be further improved. Also more work is required to improve the calibration methods especially an on-line calibration method for CEM.

REFERENCES [1] L. J. Radziemski and D. A. Cremers, Laser Induced Plasma and Applications, Marcel Dekker, New York (1989) Chapter-7. [2] D. C. Smith and R. G. Meyerand, Jr., Principles of Laser Plasma, G. Bekefi, Ed., Wiley, New York (1976) p. 457. [3] R. W. Schmeider, Combustion applications of laser-induced breakdown spectroscopy, 13th Ann. Electro-Opt./Laser Conf. (1981) Anaheim, CA. [4] R. W. Schmeider, and A. Kerstein, Appl. Opt. 19 (1980) 4210. [5] L. J. Radziemski and T. R. Loree, J. Plasma Chem. Plasma Proc. 1 (1981) 281. [6] D. A. Cremers and L. J. Radziemski, Anal. Chem. 55 (1983) 1252. [7] L. J. Radziemski, T. R. Loree, D. A. Cremers and N. M. Hoffman, Anal. Chem. 55 (1983) 1246. [8] D. W. Hahn, Appl. Phys. Lett. 72 (1998) 2960. [9] S. G. Buckley, Environmental Engineering Science, 22 (2005) 195. [10] A. C. Samuels, F. C. Delucia Jr., K. L. McNesby, A. W. Miziolek, Appl. Opt. 42 (2003) 6205. [11] C. A. Munson, F. C. De Lucia, T. Piehler, K. L. McNesby, A. W. Miziolek, Spectrochim. Acta B60 (2005) 1217. [12] J. D. Hybl, G. A. Lithgow, S. G. Buckley, Appl. Spectrosc. 57 (2003) 1207. [13] G. Bekefi, Principles of Laser Plasmas, Wiley, New York (1976) p. 457. [14] R. S. Adrain and J. J. Watson, Phys. D: Appl. Phys. 17 (1984) 1915. [15] D. P. Balwin, D. S. Zamzow and A. P. J. D’Silva, J. Air & Waste Management Association, 45 (1995) 789. [16] J. P. Singh, H. Zhang, F. Y. Yueh and K. P. Carney, Appl. Spectrosc. 50 (1996) 764. [17] W. S. Shepard, et al., “Application of modern diagnostic methods to environmental improvement”, Mississippi State University: Diagnostic Instrumentation and Analysis Laboratory. (1996) 10575 FY 96 Annual. [18] H. Zhang, J. P. Singh, F. Y. Yueh and R. L. Cook, Appl. Spectrosc. 49 (1995) 92. [19] D. K. Ottesen, J. C. F. Wang and L. J. Radziemski, Appl. Spectrosc. 43 (1989) 967.

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[20] M. Corsi, G. Cristoforetti, M. Hidalgo, S. Legnaioli, V. Palleschi, A. Salvetti, E. Tognoni and C. Vallebona, Appl. Opt. 42 (2003) 6133. [21] H. Zhang, F. Y. Yueh and Jagdish P. Singh, J. Air & Waste Manage. Assoc. 51 (2001) 681. [22] R. E. Neuhauser, U. Panne, R. Niessner, G. A. Petrucci, P. Vavalli and N. Omenetto, Anal. Chim. Acta 346 (1997) 37. [23] J. P. Singh, F. Y. Yueh, H. Zhang and R. L. Cook, Process Control and Quality, 10 (1997) 247. [24] F. Ferioli, P. V. Puzinauskas and S. G. Buckley, Appl. Spectrosc. 57 (2003) 1183. [25] A. J. Ball, V. Hohreiter, D. W. Hahn, Appl. Spectrosc. 59 (2005) 348. [26] S. D. Arnold and D. A. Cremers, AIHA Journal, 56 (1995) 1180. [27] K. Y. Yamamoto, D. A. Cremers, M. J. Ferris and L. E. Foster, Appl. Spectrosc. 50 (1996) 22. [28] D. A. Cremers and L. J. Radziemski, Appl. Spectrosc. 39 (1985) 57. [29] H. Zhang, F. Y. Yueh and J. P. Singh, Appl. Opt, 38 (1999) 1459. [30] D. W. Hahn, W. L. Flower, and K. R. Hencken, Appl. Spectrosc. 51 (1997) 1836. [31] J. P. Singh, H. Zhang and F. Y. Yueh, Technique report for continuous emission monitor (CEM) test at the Rotary Kiln Incinerator Simulator (RKIS) at the EPA Environmental Research Center, Research Triangle Park, Raleigh, NC, September (1997). [32] A. L. Kielpinski, J. C. Marra, R. F. Schumacher, J. Congdon, J. Etheridge and R. Kirkland, “Testing of Refractory Materials for Plasma Vitrification of Low-Level Mixed Wastes”, in Proceedings of Waste Management Symposium, Tucson, Arizona, February 26-March 2 (1995). [33] R. W. Schmieder, “Combustion applications of laser-induced breakdown spectroscopy” in Proceedings of the Electro-Optics Laser Conference (Cahners, Chicago, ILL. (1981) p. 17. [34] T. X. Phuoc and F. P. White, Fuel 81 (2002) 1761. [35] V. Sturm, R. Noll, Appl. Opt. 42 (2003) 6221. [36] P. Stavropoulos, A. Michalakou, G. Skevis and S. Couris, Spectrochim. Acta B60 (2005) 1092. [37] L. G. Blevins, C. R. Shaddix, S. M. Sickafoose and P. M. Walsh, Appl. Opt. 42 (2003) 6107. [38] J. P. Singh. H. Zhang, F.-Y. Yueh, and R. L. Cook, Laser-Induced Breakdown Spectroscopy in a Metal-Seeded Flame; 28th Intersociety Energy Conversion Engineering Conference Proceedings (IECEC); August 8–13, Vol. 1 (1993) 995. [39] T.-W. Lee, N. Hegde and I. Han, Laser-Induced Breakdown Spectroscopy for In-Situ Diagnostics of Combustion Parameters Including Temperature, UKC2005: Aerospace Science & Technology Symposium (ASTS), University of California, Irvine (UCI), Irvine, California, USA. August 11–13 (2005). [40] V. N. Rai, J. P. Singh, C. Winstead, F.-Y. Yueh and R. L. Cook, AIAA Journal Vol. 41 (2003) 2192.

Chapter 10

Laser-Induced Breakdown Spectroscopy of Liquid Samples V. N. Raia , F. Y. Yuehb and J. P. Singhb a

Laser Plasma Division, Raja Ramanna Centre for Advanced Technology P.O. CAT, Indore 452 013, INDIA b Institute for Clean Energy Technology, Mississippi State University, 205 Research Boulevard, Starkville MS 39759, USA

1. INTRODUCTION Laser-induced breakdown spectroscopy (LIBS) has been used for qualitative and quantitative analysis of elemental compositions from many different type of samples [1]. LIBS uses a focused high intensity pulsed laser beam to produce laser induced spark on the sample surface. In the resulting high-temperature plasma, the components of sample are basically reduced to atoms and ions. The excited atoms and ions decay to lower energy states by emitting the radiation. Recording the atomic emission spectrum thus enables the identification and quantification of the elemental components in the sample. The main advantage of LIBS technique over conventional methods is the capability of an online and real time analysis of almost all types of materials without any (or with a little) sample preparation [2–6]. LIBS has generally been applied to the analysis of solid samples and comparatively less attention has been paid to LIBS analysis of liquids [4,7–8], suspension in liquids [9–10] and samples submerged in liquids [11]. Production of a viable system for the online LIBS analysis of liquids requires solutions of some general problems encountered with plasmas generated from liquids in addition to a number of technical issues. Frequent cleaning of exposed optical components (focusing lens or window) has to be minimized to remove accumulated matter ejected and splashed from the liquid sample by incident laser pulses. The miniature shock waves associated with vaporization of liquid samples create aerosols above the liquid surface and disrupt both the incident laser beam and the emitted light returning to the spectrometer. Shock waves also tend to induce waves on the liquid surface, which increase shot-to-shot signal variation and lower the precision of spectral measurements. The laser pulses also generate bubbles inside liquids that are transparent at the laser wavelength. These bubbles may reach the liquid surface and change the characteristics of the laser-induced plasma, thereby affecting reproducibility of measurement. When the bubbles created inside the liquid by the laser pulse burst at Laser-Induced Breakdown Spectroscopy Jagdish P Singh, Surya N Thakur (Editors) © 2007 Elsevier B.V. All rights reserved.

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the surface, or the waves induced on the surface by the laser pulse are not dissipated, they change the angle of incidence between the laser beam and the liquid surface. This, in turn, can change the fluence of the laser, and hence the emission intensity. The aerosols created by the laser-liquid interaction also absorb the laser beam, and partially prevent the laser light from reaching the sample surface. This absorption can change the reproducibility of the measurement by affecting the energy delivered to the sample. To overcome these problems a variety of experimental LIBS configurations have been employed for studies of liquid surfaces [12–13], bulk liquids [7] and liquid jets [14]. Main aim of the present article is to discuss various techniques used for recording the LIBS of liquid samples with increased sensitivity.

2. LIBS OF LIQUID SAMPLES 2.1. Elemental Analysis in Liquids The detection and quantification of light and heavy elements in liquid samples are important from application points of view, particularly in industrial processing, environmental monitoring, and the treatment of waste material [1–6]. Golovlyov and Letokhov [15], Esenaliev et al. [16], and Oraevsky et al. [17] have studied the physical mechanisms of the ablation and breakdown on liquid samples. Initially, liquid samples were studied by focusing the laser on the surface of the liquid, which caused heavy splashing as well as shock waves [7–9]. These effects changed the position of the liquid surface with respect to the laser focus and adversely affected the analytical results. Laser induced plasma in the bulk of the liquid prevented splashing, but presented a drawback in terms of decrease in the duration of plasma emission. The duration of light emission from the bulk plasma is extremely short, usually of the order of 1 s or less. Haisch et al. [9] reported a fast plasma decay time of just a few hundred nanoseconds in their bulk liquid experiments. Cremers et al. [7] found that plasma parameters could not be derived for delay times beyond 1.5 s. The major disadvantage of bulk analysis is the severely reduced plasma emission intensity in comparison with that obtained from the liquid’s surface. Watcher and Cremers [18] overcame this problem by using a surface excitation scheme in which the liquid solutions were placed in cylindrical glass vials and the plasma was then bounded on one side by the rigid glass body of the vial. The light emission from the plasma displayed much longer durations of the order of several microseconds with an enhanced emission intensity. Cremers et al. [7] described a method where an initial laser pulse produced a gas bubble within the water bulk, and a time-delayed second laser pulse analyzed the gas present inside the bubble. This approach resulted in an enhancement in the line intensities by a dramatic factor of 50 for oxygen line at 777.44 nm and by a moderate factor of 3 to 4 for the calcium and magnesium resonance lines, increasing the analytical sensitivity and making the bulk analysis a reasonable option. This double-pulse plasma-generation approach has also been used by Pichahchy et al. [11] and, Nyga and Neu [10] in their studies on the metal composition of specimens submerged in water. Despite the evident problems of splashing in the case of surface excitation configuration, some researchers have used this approach. Berman and Wolf [13] and Arca et al. [12] focused the laser

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pulse on the surface of liquid solutions and reported a minimum delay of 1–3 s for detecting trace concentrations of nickel, magnesium, calcium, and chromium. LIBS analysis of liquid samples using the laminar flows of liquid jets have been reported [4,19]. This approach was first used by Ito et al. [19] for the detection of colloidal iron in a turbid solution of FeO in water. The authors observed iron emission lines to about 3.5 s after the laser pulse. The limit of detection (LOD) for iron was estimated as 0.6 ppm. After some time Nakamura et al. [14] used a similar technique along with double-pulse excitation in the presence of purged gas and reported an improved limit of detection (LOD) of 16 ppb. Ng et al. [20] and Ho et al. [21] performed spectroscopic studies of plasma generated from a stable water jet by using two different excitation methods; they derived plasma excitation temperature and electron density for a delay time up to 1 s. The sensitivity of LIBS for quantitative analysis of liquid samples is often poorer than that of other analytical techniques, such as atomic emission spectroscopy ICP-AES and ICP-MS. However, the importance of LIBS comes into prominence if a remote online analysis is required. Remote online analysis is preferred when the measurements are to be carried out under hazardous or difficult environmental conditions, which is not possible by any analysis technique other than LIBS. The work described in the following sections stems from a need in the nuclear industry to conduct real-time, on-line analyses of radioactive waste in liquid specimens. These are encountered during the reprocessing of nuclear fuel and in monitoring of nuclear waste storage tanks. Particularly, technetium (Tc) is a radioactive element and a product of the nuclear power cycle. The most stable Tc isotope has a half-life of 21 × 105 years and decays via beta emission. Due to the long half-life and the relatively high yield from uranium decay, it is desirable to separate Tc from non-radioactive and short-life components found in the tank waste. It is important to isolate it with other long-life radionuclides in geologically stable waste for long-term safe storage. Similar problems are also encountered in other industries where toxic liquid effluent and/or waste are present, and a real need exists for proper regulation of these materials. This requires a real-time, remote, on-line LIBS analysis system. Remote LIBS analysis was conducted with a fiber-optic probe that focused a laser beam onto the fiber, whereas the output of the optical fiber was focused on the surface of the sample. This technique proved suitable for remote LIBS analysis of solid samples [22–23]. A modified version of the optical fiber system, along with other telescopic techniques was used for remote analysis of liquid samples [4]. The laboratory based experiments for recording the LIBS of liquid samples generally use a simple system of convex lenses for focusing the beam, either on the liquid surface or on the jet. For quick analysis of a liquid sample (typical laser pulse repetition rates 10 to 20 Hz), the laser beam is focused on the smooth vertical surface of a laminar jet stream of the liquid, which produces plasma by surface excitation. The analyte in the liquid jet sample is vaporized into ambient air above the liquid surface, as in the LIBS of solid surfaces. In this case, the problem of splashing is largely minimized because only very minute amounts of solution are vaporized. Furthermore, the vaporized material is extremely well defined (the volume roughly equals the laser spot size multiplied by the jet thickness). As a consequence of the normally unchanged laser spot size and water jet thickness, the experimental repeatability is greatly enhanced (due to low scattering of data). In this case, plasma largely evolves in air with minimal interaction with the rest of the liquid,

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the luminous phase of the plasma is prolonged, and many spectral lines can be observed well beyond 10 s. Low pulse repetition rates (less than 1 Hz) are required to avoid the splashing and oscillations on the liquid surface due to the generation of resonant shock waves. A reliable and quantitative LIBS analysis requires knowledge of some plasma parameters like electron density, excitation temperature, line shapes, and the time-evolution of the spectral line emissions. These parameters are then utilized for generating operating conditions optimal for trace element detection. It is important to identify and optimize the parameters in order to guarantee a reliable analysis. Since LIBS is a relative measurement technique, calibration curves of observed signal response versus element concentration must be available. Such calibration curves have been generated for the elements of interest, which were selected as dictated by some of the intended applications. Finally, enhancing the sensitivity of LIBS using other techniques is required for decreasing the LOD of elements.

2.2. LIBS of Molten Metal The metal-producing industry faces the major challenge of increasing productivity to reduce cost and maximize the benefits from existing equipment. During refining, it is critical that operating parameters be adjusted and controlled so that the chemistry of the melt is within predetermined limits. The current analytical approaches to the determination of the chemical composition of the melt by spark optical emission spectroscopy, atomic absorption spectroscopy (AAS), X-ray fluorescence (XRF), inductively coupled plasma (ICP) spectroscopy, and ICP mass spectrometry (MS) are limited in practice by their off-line character. Furthermore, these methods are either based on analysis of the cold materials, or on laborious manual sampling from the melt at elevated temperatures between 500–1600  C, which results in insufficient turn-around time, and increased process and personnel costs. Motivated by potential savings in time, energy, and materials, as well as improved quality assurance, several LIBS groups are investigating the real time analysis of molten metals [22–29]. However, LIBS analysis of high temperature molten metals in processing vessels often presents major difficulties and analytical challenges. For a reliable and accurate LIBS sensor, many requirements should be met such as: (1) The vaporized volume should be truly representative of the liquid bulk. This forbids interrogating the same surface for an extended period of time since a hot liquid metal surface can quickly get enriched with elements having higher affinity for oxygen or nitrogen, or become poorer in elements with a lower vaporization threshold. (2) Perturbations from aerosols and ejected particles should be eliminated since their plasma emission is not representative of the melt, and they cause variations in the laser power reaching the liquid surface and available for ablation. (3) The sensor should be sufficiently rugged for use in the harsh environment of the industrial plant.

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The determination of the composition of molten phase samples by LIBS in a furnace has been the subject of numerous studies in laboratory and several trials in industry [22–29]. St. Onge et al. [29–30] employed a patented approach based on the use of a lance without optical components in which gas under pressure is introduced, thereby producing bubbles inside the molten metal. In this approach, a new surface truly representative of the melt is continuously exposed to the laser beam. The problem of analyzing a non stationary surface and insuring high quality data representative of the bulk was solved by selectively processing all acquired data in the presence of bubble motion and classifying spectra from molten or solid phases. The probe has been successfully tested in many industrial facilities for the production and processing of molten materials (zinc, zinc alloys, copper, magnesium, copper matte, electrolyte bath, etc.). For zinc bath analysis the above technique permitted rapid identification and treatment of data from multiple species and/or phases [30]. The probe was also subjected to the harsh conditions of the copper smelting industry at 1200  C where it was introduced through a tuyere into a thousand-ton molten matte vessel to monitor Fe, Bi, and Ag content [29–30]. Probe robustness was established over many days of intense experimentation. The use of a similar probe also successfully demonstrated in-situ analysis of molten electrolyte used for magnesium production at 700  C. Measurements were performed in both a pilot plant and also on-line under hostile conditions in an operating plant. In all these conditions, the patented probe overcame problems related to non-representative melt surfaces due to oxidation, contamination, and surface migration or depletion. Consequently, excellent measurement reproducibility and accuracy were obtained compared to conventional LIBS measurements on stable and stationary liquid surfaces. To the best of our knowledge, this probe has for the first time demonstrated LIBS measurement reproducibility of 1% for molten metal [30].

3. INSTRUMENTATION FOR LIQUID SAMPLES 3.1. Experimental Setup for Surface Excitation Standard LIBS analysis systems consist of three typical major blocks, namely (a) the laser source, (b) the laser light delivery and plasma emission collection system, and (c) the system for spectral analysis. The choice of light transfer arrangement depends mainly on target exposure procedures, which may be either a direct surface excitation from the liquid surface, a laminar jet stream of the liquid or inside the bulk liquid or a sample submerged in liquid. The schematic diagram of the experimental setup for recording the laser-induced breakdown emission on the bulk liquid surface as well as in the case of a laminar jet (see Fig.1, Chapter 5) has been reported by Rai et al. [31]. They used a Q-switched frequency doubled Nd: YAG laser (Continuum Surelite III) that delivers energy of 400 mJ in a 5-ns pulse duration. The laser was operated at 10 Hz during this experiment and was focused on the target (in the center of the liquid jet or on the surface of the bulk liquid, depending on the experiment) using an UV grade quartz lens with a focal length of 20 cm. The same focusing lens was used to collect the optical emission from the laser-induced plasma. Two UV-grade quartz lenses with focal lengths of 100 mm and 50 mm were used to couple the LIBS signal to an optical fiber bundle. The fiber bundle consisted of a collection of 80 single fibers with a core

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diameter of 0.01 mm. The rectangular exit end of the optical fiber was coupled with an optical spectrograph (Model HR 460, Instrument SA Inc., Edison, NJ.) as an entrance slit. The spectrograph was equipped with an 1800 and 3600 l/mm diffraction grating, with dimensions of 75 mm × 75 mm. A 1024 × 256 element intensified charge coupled device (ICCD) (Princeton Instrument Corporation, Princeton, NJ) with a pixel width of 0,022 mm was attached to the exit focal plane of the spectrograph and used to detect the dispersed light from the laser-produced plasma. The detector was operated in gated mode with the control of a high-voltage pulse generator (PG-10, Princeton Instruments Corporation, Princeton, NJ) and was synchronized to the laser output. Data acquisition and analysis were performed using a personal computer. The gate delay time and gate width were adjusted to maximize the signal-to-background (S/B) and signal-to-noise (S/N) ratios, which are dependent on the emission characteristics of the elements as well as the target matrix. In order to increase the sensitivity of the system, around 100 spectra were accumulated to obtain one averaged spectrum. For liquid jet experiments a Teflon nozzle of diameter ∼1 mm was used with a Peristaltic pump (Cole-Parmer Instrument Co.) to form laminar liquid jet. The laser was focused on the jet such that the direction of laser propagation was perpendicular to the direction of the liquid jet. The laser was focused ∼15 mm below the jet exit, where the liquid flow was laminar. However, the extent of laminar flow was found dependent on the speed of the pump. The liquid jet was aligned in a vertically downward direction.

3.2. Experimental Set up for Bulk/Molten Liquid Liquids have also been analysed by generating plasma in the bulk of liquid [32]. This setup has some advantages as well as disadvantages but this technique is of great importance for qualitative chemical analysis in marine environment or in the case of molten metals. Normally optical fiber probes are suitable for this type of experiments [22–23,32]. Beddows et al. [32] have used a single large core optical fiber, both for delivering the laser radiation to the target and collecting the plasma emission for subsequent analysis. The fiber end-faces were prepared by a cleaving process, which provided fault free optical surfaces. The fiber was guided in a flexible tube and held at the end of the tube in a short glass capillary. A suitable buffer gas (ordinary air, dry N2 or argon) was bled through this tube/capillary assembly. However the input energy of laser was optimized on the basis of threshold for the damage of optical fiber and the energy loss during its transmission through a long distance. The irradiance on target was kept well above the threshold for plasma generation. The fiber end was held within the capillary tube at a suitable distance away from the sample surface to create the luminous plasma without damaging the fiber during the process. The buffer gas was blown down through the annular passage between the fiber cladding and the inside of the capillary tube, resulting in a bleed stream of gas displacing the water at the position of plasma generation. Due to close proximity of the fiber end (D ∼ 1.5–2 mm) to the sample surface sufficient light could be collected from the plasma by optical fiber end without a lens, which made alignment issue quite easy. The plasma radiation was finally delivered to the spectrometer with the help of a reflecting mirror. Similar arrangements of fiber probes have been used by many researchers for the study of molten metal [22–30]. A ceramic guiding tube is generally used in the case of

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molten materials so that it could sustain the high temperature of the bath. Some of the researchers have used, lens system to focus the laser, whereas others have used the fiber directly to produce plasma.

3.3. Liquid Configuration for Plasma Formation Trace elements in different types of liquid matrices without any sample preparation are likely to be detected using LIBS. Matrices such as colloids, turbid, liquids, sludge, oils, etc., require the production of plasma at the liquid surface [33]. Bulk-liquid analysis is also possible by optical fiber probe in the liquid samples having a turbid nature, which would prevent the laser beam from reaching the bulk liquid. As discussed above the general configuration used for LIBS of liquid surfaces consists of a laser beam perpendicular to the surface. This arrangement, however, leads to splashing, because the plasma expansion at atmospheric pressure is directed perpendicular to the liquid surface. A tilted laser beam configuration with respect to the liquid surface can minimize this phenomenon [33]. The use of low laser repetition rate of ∼1 Hz can minimize the perturbation that takes place at the liquid surface following the laser pulse. It was shown that measurements with a l-Hz laser repetition rate were more reproducible. Although the droplet and jet configurations of the liquid sample demand a little sample preparation, the use of a pump-backed jet has several advantages: (1) The volume evaporated in the plasma formation process is extremely well defined, being equal to the laser spot diameter multiplied by the thickness of the jet; little or no interaction with the residual material takes place, since nearly all of the sample volume is vaporized. (2) A suitable flexible tube system on the entrance side of the pump can be used, in arbitrary location within a large volume of a liquid, and can be probed, both in lateral direction and in depth; this, therefore, provides the possibility of probing in real time the spatial distribution of concentrations in a liquid tank specimen. (3) One can add known amounts of elements to the flow of the sample in order to get a standard element for normalization, which will help in providing a better calibration curve. St. Onge et al. [34] have evaluated three different configurations for the analysis of liquid formulations using LIBS: analysis in closed (transparent) bottles, on the surfaces of a horizontally flowing liquid stream and in open containers (on the non flowing liquid surfaces). They used sodium chloride in solution form as a model compound. It was found that analysis of a non-flowing surface provided the best compromise in terms of ease of implementation and precision. This approach is also the one most easily adapted to the configuration of an existing commercial LIBS instrument. The choice of a given configuration may, however, be dependent on other practical issues. However a simple comparison of bulk and liquid-jet experiment from the spectroscopic point of view is discussed in the following section. On the basis of previous discussion for minimizing the splashing and surface distortion as well as considering its application for most of the liquid samples a liquid-jet system was found more suitable. Most of the data presented here have been obtained using this method.

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3.4. Optimization of Experimental Parameters The most sensitive emission lines from the possible trace elements are used to study the effects of various experimental parameters on the sensitivity of the system. The experimental parameters that can most affect the limits of detection are the laser energy, lens-to-surface distance (LTSD), gate delay time, gate width and the physical properties of the sample. The effects of these parameters on the emission characteristics were carefully studied using targets, as bulk liquid and as liquid jet.

3.4.1. Effects of laser energy and lens-to-sample distance (LTSD) A high intensity laser beam was used to produce the plasma, where the required excited elements present in the liquid sample emitted the characteristic radiation. The LIBS spectra of all the elements were recorded at different laser energies. The emission intensity (signal) was found proportional to the laser energy, when the laser-produced plasma was in the optically thin region. Initially this increase was the result of more ablation from the sample. The plasma temperature remains highest near the critical density surface, where continuum emission is dominant. The atomic emission from the plasma decreased as the laser energy was increased to still higher values, which may be either due to a decrease in the coupling of laser energy to the plasma as a result of shielding by critical density, or due to the generation of instability in the plasma. The optimized laser-pulse energy for the jet and bulk-liquid targets was found to lie between 150 and 250 mJ. Any change of a few millimeters in the lens-to-sample distance (LTSD) affects the intensity of the atomic lines from the trace elements [35]. Therefore, keeping the LTSD constant during the measurements was very important for accuracy and precision of the system. The LTSD was more critical in the case of liquid-jet experiments due to the smaller surface area. It was noted that a shorter focal length lens produced a small beam waist (tight focusing) and, therefore, a stronger breakdown. A smaller depth of focus, made it more sensitive to any change in the LTSD. It was also noticed that the movement of the target location by 1 mm away from the focal distance of the lens caused the LIBS signal to drop by ∼25%. To improve the LIBS precision with a liquid-jet system, a longer focal length lens was preferred in order to increase the depth of focus.

3.4.2. Effects of gate delay The lifetime of the laser-induced breakdown plasma plume has been found to be about two times shorter in liquid than in air [1]. Since water has high, ionization potential (12.6 eV) and relatively high electro-negativity −09 eV, it produces fewer charged particles during laser-induced breakdown [4] leading to a much weaker laser-induced plasma in water than in air. In our experiments, the variation in the intensity of chromium atomic lines as well as that of the background emission, with the gate delay indicated that the continuum background emission was dominant in the first several microseconds but decayed much faster than the atomic line signal. The background emission, (Bremsstrahlung) is mainly dependent on the plasma temperature which decays faster as a result of plasma expansion. Atomic line emission dominates as a result of radiative recombination of the charged particles in plasma, which becomes prominent only at

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lower temperatures after expansion of plasma. Since the background continuum and atomic emission decay at different rates, it was possible to obtain an optimum LIBS signal with a properly selected detection window. In the liquid-jet target with Mn, the S/B ratio reached its peak between 5 and 10 s duration while background decayed to its lowest value. The bulk-liquid data also indicated a similar trend. It was found that LIBS data with an optimal signal-to-background ratio could be obtained by adjusting the gate delay time. The LIBS spectra of magnesium (Mg), recorded at different delay times in bulk-liquid and liquid-jet targets were compared [35] and it was found that the background emission decreased and line emission increased in the case of the liquidjet experiment in comparison to the bulk liquid experiment. Ultimately, the liquid-jet experiment provided a better S/B ratio in comparison to the bulk experiment. A similar trend was found for the decay of background and line emission intensity in the case of the chromium-seeded liquid-jet experiment also.

3.4.3. Analytical measurements For quantitative measurements, the recorded emission intensities should be related to the absolute or relative elemental concentration in liquid. To obtain best sensitivity, LIBS signals were optimized for different atomic and ionic lines by adjusting the gate delay time and gate width of the detector, as well as the laser energy. The LIBS signals of various trace elements (Cr, Mg, Mn, and Re) were recorded for different concentrations to obtain a calibration curve under optimized experimental conditions for estimating the limit of detection. The linear calibration curves for rhenium (Re) obtained from liquid-jet measurements [35] at a delay time of 8 s and a gate width of 15 s at two different laser energies showed that an increase in excitation laser energy increases the LIBS sensitivity for each concentration. The detection limits for Cr, Mg, Mn, and Re were calculated based on the calibration curves and were reported as 0.4, 0.1, 0.87 and 10 g/ml respectively [35]. The LOD of elements Pb, Si, Ca, Na, Zn, Sn, Al, Cu, Ni, Fe, Mg, and Cr obtained from bulk water and oil matrices (Table 1) has been reported Table 1. Limit of detection of elements obtained from bulk liquid experiments Element Pb Si Ca Na Zn Sn Al Cu Ni Fe Mg Cr

Wavelength (nm)

Detection Limit in water (ppm)

Detection limit in oil (ppm)

40587 28815 39336 58899 33450 28399 30927 32475 34147 37199 28521 42543

100 25 03 05 120 100 10 7 20 35 1 10

90 20 03 07 130 80 10 5 35 20 1 20

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V. N. Rai et al. Table 2. Limit of detection of elements recorded in liquid-jet experiment Elements Al Ca Cr Cu K Li Mg Mn Na Pb Tc U Re

Wavelength (nm) 39615 42267 52045 42543 32475 76649 67077 28521 40308 58899 40578 42971 40902 34605

Limit of detection (ppm){Ref# }

Limit of detection

18 06 200 04 5 4 0009 3 10 008 40 25 450 10

01 087

10

in the literature [33]. Similarly, the LOD of elements Al, Ca, Cr, Cu, K. Li, Mg, Mn, Na, Tc, and U obtained using the liquid-jet system by Samek et al. [4] is reported in Table 2. The LOD of Cr, Mn, Mg, and Re reported by Rai et al. [35] were found to be better in comparison to the results described in the literature. Various experimental parameters such as matrices, wavelength of emission, gate delay, and the process of obtaining a calibration curve affect the estimation of the limit of detection, which is why, an exact comparison of the LOD data from the two research teams is difficult. The limit of detection reported for various elements in this experiment as well as in the literature has proved the LIBS technique suitable for finding pollutant trace elements at high and moderate concentrations. However, it is not possible to detect them at very low concentrations. A serious effort is needed to make the system versatile for very low concentration measurements. Recently, efforts were made by various research groups as well as by DIAL (now ICET, MSU, USA) to enhance the sensitivity of the LIBS system for different types of sample configuration. The technique includes use of external magnetic field and double laser pulse excitation, which will be discussed in the following sections.

4. ENHANCEMENT IN THE SENSITIVITY OF LIBS 4.1. Effects of a Magnetic Field on Plasma Magnetic fields were utilized in the last decade for enhancing the analytical characteristics of various low-energy density plasma sources used for elemental analysis [36–46]. The magnetic field ranged from a few hundred gauss to a few tens of kilogauss and mainly operated in the pulsed mode. Pulsed magnetic field was generated by discharging a capacitor through the pair of coils that produced a mirror-like structure of magnetic

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field lines, which helped in confining the plasma. Various types of plasma sources such as dc arc [36–37], low-pressure glow discharge [38–39], microwave plasma [40], spark discharge [41], and exploding conductor plasma [42–43] were used with these magnetic fields. The magnetic field used to confine the plasma may also have an effect on its atomization, ionization and lifetime. The particle confinement time remained much longer in the presence of a magnetic field than the electron temperature decay time. The plasma cooled mainly through the radiation losses. Thus energy given to the plasma may be useful if its expansion is limited by applying the magnetic field. As reported previously [43,47], magnetic confinement of the laser-produced plasma enhanced the emission of radiation in the wavelength range from X-rays [47] to the visible region [43–46]. The confinement of a laser-produced plasma in a ∼100 kG pulsed magnetic field was found useful in increasing the gain of the medium for the X-ray laser [48–49]. In another experiment, the characterization of laser plasma with ∼80 kG pulsed magnetic field enhanced the visible emission and broadened the line spectra. Enhancement in the UV and visible emissions from beryllium plasma in the presence of a magnetic field has also been reported. Instead of a pulsed high-intensity magnetic field, the application of a low (0.6 T) but steady magnetic field in laser-produced plasma caused a two to three-times enhancement in X-ray emission [47]. However, the above magnetic field confinement experiments were performed mainly with solid targets. The complication involved in generation of a high-intensity pulsed magnetic field as well as its synchronization with the laser and the detection (measurement) system made the whole system difficult as well as very critical. The effects of a low and steady magnetic field on the optical emission characteristics of a laser-produced plasma from the trace elements present in the liquid solution were studied systematically.

4.1.1. Emission from magnetically confined plasma The emission from the laser-produced plasma under the effect of magnetic confinement can be better understood by a simple analysis reported by Rai et al. [50]. It is well known that various types of radiations are emitted from plasmas, the nature of which depends mainly on the density, temperature, and opacity of the plasma [1,51]. If the plasma is optically thick and has a high temperature, there will be a continuum black body radiation from it, whereas optically thin high-temperature plasma emits Bremsstrahlung radiation due to electron-ion collision (free-free transition), which also provides continuum spectra. When a free electron recombines with the ion at a comparatively low temperature, it provides combination of continuum and line emission spectra. There will be a three-body recombination process between electrons and ions resulting in the line emission. The line emission can occur either from excited ions or atoms present in the plasma. The emission from the plasma ranges from X-rays to the visible spectral region, depending on the plasma parameters, which decide the dominant process of its emission. Emission can vary from Bremsstrahlung to line emission as the plasma temperature and density decrease. Laser-induced plasma in the present experiment yielded all types of emission, ranging from continuum (background) to line emission (signal) because spatially integrated emission from the plasma plume was recorded in the direction opposite the laser propagation. Generally LIBS plasma was created at atmospheric pressure, thus X-rays will be absorbed in the air. For the simple analysis of the plasma emission condition, one can consider that for the same incident laser energy, the plasma plume

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expanding in the absence and in the presence of a magnetic field will have nearly the same plasma parameters, such as plasma density, temperature, and charge state Z. In this situation all the emissions such as Bremsstrahlung, recombination, and line emission are dependent on the electron and ion density in the plasma. Therefore, the total amount of plasma emission will be proportional to the square of the plasma density (∝ ne ni where ne = ni ) as well as the volume of the emitting plasma plume. The laser-produced plasma is decelerated in the presence of the magnetic field, as no charged particle can cross the magnetic field lines [52]. However, cross-field diffusion of plasma particles is possible only when the plasma is either collisional or turbulent. This situation prevails either at a low plasma temperature (collisional plasma) or in the presence of instability in the plasma (turbulent). For the analysis of plasma emission in the presence of a magnetic field, we have assumed v1  t1 and v2  t2 as the asymptotic expansion velocity of plasma and emission time in the absence and the presence of the magnetic field, respectively. Generally, the plasma plume expands in a hemispherical fashion, so the extent of plasma expansion after its formation can be given as v1 tl and v2 t2 and can be considered as the radius of the hemispherical coronal plasma in the absence and presence of the magnetic field, respectively. The mass ablated from the sample during the time duration of laser irradiation L  can be given as M=

dm  r 2 L dt

(1)

is the mass ablation rate, and r is the focal spot radius. In this case, the density of Here, dm dt the plasma can be calculated as the ratio of the total mass ablated (M) and the volume of the hemispherical-shaped plasma plume 23 v1 t1 3  for the case in which the magnetic field is not present. Similarly, the density of plasma can be obtained in the presence of the magnetic field. Considering optical emission from the plasma proportional to its density square and its volume, one can write the ratio of the plasma emission in the presence I2  and in the absence I1  of the magnetic field as [47]   v 1 t1 3 I2 = I1 v 2 t2

(2)

This indicates that plasma emission intensity will be inversely proportional to the cube of the size (product of expansion velocity and the emission time) of the plasma plume. Eq. (2) has already been verified experimentally in the case of X-ray emission by recording the two-dimensional time-integrated image of the plasma plume in the absence as well as in the presence of the magnetic field using an X-ray pinhole camera [53]. The measured plume dimension provided more than two times enhancement in the X-ray emission, which was found in agreement with the observation of ∼2–3 times enhancement in X-ray emission measured using X-ray vacuum photodiodes. The ratio of the plasma expansion velocities, explaining the plasma deceleration in the magnetic field, can be expressed in terms of the plasma [54] as 1  v2 1 2 = 1− v1

(3)

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where = 8 nkTe /B2 is defined as the ratio of plasma pressure nkTe  and magnetic pressure B2 /8 . Finally, Eq. 2 for enhancement in the plasma emission can be written as  3   1 − 2 t1 3 I2 = 1− t2 I1

(4)

Eq. (4) indicates that the enhancement in plasma emission in the presence of the magnetic field is correlated with deceleration in plasma expansion velocity. Finally enhancement in the emission was mainly found dependent on the plasma , which is the function of plasma density and temperature. It was dependent on the ratio of the emission time duration as well. v1 and v2 will be nearly equal (Eq. 2 & 3) for higher values of , when the plasma is hot. The expansion velocity of the plasma v2  in the presence of the magnetic field decreases (plasma confines) as the value of goes down due to a decrease in plasma density and temperature after breakdown. This clearly indicates that plasma confinement will be effective only when the plasma is low; that is, either the plasma temperature and the density are low or the intensity of the magnetic field becomes high. Similarly, Eq. (4) indicates that no enhancement is possible if the plasma is hot and has a high value of . The variation of v2 /vl and I2 /I1 with plasma is shown in Figure 1, which clearly indicates the role of in the enhancement of emission from plasma as well as on change in the expansion velocity of plasma. I1 and I2 remains same for higher values of , which is possible in the case of Bremsstrahlung radiation only, which is dominant particularly in the high temperature plasma regime. Ultimately, enhancement seems possible mainly during the low >  ∼ 053 m. Further analysis of laser-plasma interaction considering all aspect of plasma formation and its dynamics will provide a quantitative understanding of the experimental results.

4.2.7. Analytical measurements with double laser pulses The variation in the emission intensity with concentration was recorded for Mg, Cr and Re solutions in the single and double laser pulse excitation in order to find the limit of detection (LOD) of these elements [66]. The calibration curve for magnesium ion emission (279.55 nm) was obtained using single and double laser pulse excitation. The emission intensity from ions showed a linear variation in the concentration range of 0.1 to 5 ppm in single pulse excitation mode, whereas increase of emission intensity with concentration was nonlinear in the double laser pulse excitation mode. Two slopes were observed in double pulse excitation mode,the first slope covered the 0 to 1 ppm concentration range whereas the second slope (1–5 ppm) seemed to be due to saturation of emission as a result of self-absorption. The limit of detection was defined here as the ratio of three times standard deviation with the slope of calibration curve. The limit of detection was calculated as 69 ppb in double laser pulse excitation in 0–1 ppm concentration range, whereas, it was 230 ppb for the single pulse excitation mode. The calibration curve for neutral magnesium emission was recorded under single and double pulse excitation, which showed only one slope between 0.1 to 5 ppm concentration range for both excitation modes. The limit of detection for neutral emission was estimated as 370 ppb in double pulse in comparison to 970 ppb in the single pulse excitation. This shows that limit of detection obtained for magnesium ion as well as for the neutral atom improved (decreased) in double laser pulse excitation mode. The limit of detection for chromium was also obtained as 120 ppb in double pulse mode in comparison to 1300 ppb in single pulse excitation mode. This shows that the double laser pulse excitation can improve the limit of detection of Cr by an order of magnitude. Similar observations were noted in the case of Re. Table 5 shows the LOD obtained for Mg, Cr and Re using different spectral lines.

Table 5. Comparison of Limit of Detection (LOD) for different elements in single and double laser pulse excitation Elements

Wavelength (nm)

Limit of Detection (LOD) (ppm) Single Pulse

Mg Cr

Re

27955 28520 42544 42748 42897 34604 34647

023 097 130 216 184 2221 1442

Double Pulse 006 057 012 016 018 855 885

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5. CONCLUSION In summary, it has been demonstrated that LIBS is a useful technique for the analysis of trace element present in the liquid matrices. Different techniques were used for the plasma formation depending on the type of the sample. Excitation of plasma on the liquid surface in the bulk as well as in the molten state of metal was discussed in the light of their different requirements. A simple comparison indicated that normally liquid jet system could be useful for most of the samples present in the liquid form. Optical fiber probe was found to be more suitable for the measurements of molten metal and the samples in bulk of liquid (under sea). It has also been realized that inspite of its many advantages over the conventional techniques, LIBS is still lacking the sensitivity in the measurement of very low concentration of trace elements in samples. It was demonstrated that application of external magnetic field and double laser pulse excitation can be used for increasing the sensitivity of LIBS by a factor of two and six respectively. Analytical measurements also confirm a significant change in limit of detection. It has been found that confinement of plasma in the presence of magnetic field was the main reason for an increase in intensity of emission from plasma. Our analysis shows that enhancement in intensity can be increased even more by keeping plasma close to one.During double laser pulse excitation, it was found that the first pulse created an expanding plasma, which absorbed second laser pulse more efficiently and excited more number of plasma particles. A simple analysis shows that for optimum increase in the plasma emission the plasma scale length must be larger than the laser wavelength.

ACKNOWLEDGMENT This work was supported by Savannah River Technology Center through Education, Research & Development Association of Georgia Universities, grant no. GA0046 and Department of Energy contract no. DE-FG02-93CH-10575.

REFERENCES [1] L. J. Radziemski and D. A. Cremers, Laser Induced Plasma and Application Marcel Dekker, New York (1989). [2] T. L. Thiem, Y. I. Lee and J. Sneddon, Appl. Spectrosc. Rev. 32 (1997) 183. [3] K. Song, Y. I. Lee and J. Sneddon, Microchem. J. 45 (1992) 1. [4] O. Samek, D. C. S. Beddows, J. Kaiser, S. V. Kukhlevsky, M. Liska, H. H. Telle and J. Young, Opt. Engg. 38 (2000) 2248. [5] F. Y. Yueh, J. P. Singh and H. Zhang, Encyclopedia of Analytical Chemistry, John Wiley & Sons Ltd. 3 (2000) 2066. [6] A. K. Rai, V. N. Rai, F. Y. Yueh and J. P. Singh, Trends in Appl. Spectrosc. 4 (2002) 165. [7] D. A. Cremers, L. J. Radziemski and T. R. Loree, Appl. Spectrosc. 38 (1984) 721. [8] R. Knopp, F. J. Scherbaum and J. I. Kim, J. Anal. Chem. 355 (1996) 16. [9] C. Haisch, J. Lierman, U. Panne, R. Niessner, Anal. Chim. Acta. 346 (1997) 23. [10] R. Nyga, W. Neu, Opt. Lett. 18 (1993) 747. [11] A. E. Pichahchy, D. A. Cremers, M. J. Ferris, Spectrochim Acta B 52 (1997) 25.

LIBS of Liquid Samples [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54]

253

C. Arca, A. Ciucci, V. Palleschi, S. Rastelli and E. Tognoni, Appl. Spectrosc. 51 (1997) 1102. L. M. Berman, P. J. Wolf, Appl. Spectrosc. 52 (1998) 438. S. Nakamura, Y. Ito, K. Sone, H. Hiraga, K. I. Kanedo, Anal. Chem. 68 (1996) 2961. V. V. Golovlyov and V. S. Letokhov Appl. Phys. B 57 (1993) 417. R. O. Esenaliev, A. A. Karabutov, N. B. Podymova and V. S. Letokhov Appl. Phys. B59 (1995) 73. A. A. Oraevsky, S. L. Jacques and F. K. Tittel, J. Appl. Phys. 78 (1995) 1281. J. R. Watcher and D. A. Cremers Appl. Spectrosc. 41 (1987) 1042. Y. Ito, O. Ueki and S. Nakamura, Anal. Chim. Acta 299 (1995) 401. C. W. Ng, W. F. Ho and N. H. Cheung Appl. Spectrosc. 51 (1997) 976. F. W. Ho, C. W. Ng and N. H. Cheung Appl. Spectrosc. 51 (1997) 87. A. K. Rai, F. Y. Yueh and J. P. Singh, Rev. Sci. Instrum. 73 (2000) 3589. A. K. Rai, F. Y. Yueh and J. P. Singh, Appl. Opt. 42 (2003) 2078. C. Aragon, J. A. Aguilera, J. Campos, Appl. Spectrosc. 47 (1993) 606. L. Pasky, B. Nemet, A. Lengyel and L. Kozma, Spectrochim. Acta B 51 (1996) 27. J. Gruber, J. Heitz, H. Strasser and N. Ramasedar, Spectrochim. Acta B 56 (1981) 685. J. Gruber, J. Heitz, N. Arnold, N. Ramsedar, W. Meyer, F. Koch, Appl. Spectrosc. 58 (2004) 457. L. Peter, V. Sturn and R. Noll, Appl. Opt. 42 (2003) 6199. M. Sabsabi, R. Heon and J. Lucas US Pat. No. 700660 Issued Sept. (2003). E. Baril, L. St. Onge, M. Sabsabi and J. Lucas (Galvatech 2004) Chicago (IL) April 4–7 (2004). Conf. Proceeding (Association for Iron & Steel Technology 2004) pp 1095. V. N. Rai, H. Zhang, F. Y. Yueh and J. P. Singh, Appl. Opt. 42 (2003) 3662. D. C. S. Beddows, O. Samek, M. Liska and H. Telle, Spectrochim. Acta B 57 (2002) 1461. P. Fichet, P. Mauchien, J. F. Wagner and C. Maulin, Anal. Chemica 429 (2001) 269. L. St. Onge, E. Kwong, M. Sabsabi and E. B. Vadas, J. Pharm. And Biomed. Anal. 36 (2004) 277. F. Y. Yueh, V. N. Rai, J. P. Singh and H. Zhang, AIAA-2001-2933, 32nd AIAA Plasmadynamics and Laser Conference, 11–14 June (2001) Anaheim, CA. A. A. Fakhry, M. A. Eid and M. S. Hasem Appl. Spectrosc. 32 (1978) 272. K. Trivedi, D. Coll and R. Sacks, Appl. Spectrosc. 42 (1988) 1025. L. McCaig, R. Sacks and D. Lubman, Appl. Spectrosc. 43 (1989) 912. S. Tanguay and R. Sacks, Appl. Spectrosc. 43 (1989) 918. S. R. Goode and D. T. Pipes, Spectrochim. Acta B 36 (1981) 925. V. Majidi and D. M. Colemen, Appl. Spectrosc. 41 (1987) 200. D. Albers, M. Tisak and R. Sacks, Appl. Spectrosc. 41 (1987) 131. E. T. Johnson and R. D. Sacks, Appl. Spectrosc. 42 (1988) 77. K. J. Mason and J. M. Goldberg, Anal. Chem. 59 (1987) 1250. K. J. Mason and J. M. Goldberg, Appl. Spectrosc. 45 (1991) 370. K. J. Mason and J. M. Goldberg, Appl. Spectrosc. 45 (1991) 1444. V. N. Rai, M. Shukla and H. C. Pant, Laser and Particle Beam 16 (1998) 431. S. Suckwer and H. Fishman, J. Appl. Phys. 51 (1980) 1922. S. Suckwer, H. Skinner, H. Milchberg, C. Keane and D. Noorhees, Phys. Rev. Lett. 55 (1985) 1753 V. N. Rai, A. K. Rai, F. Y. Yueh and J. P. Singh, Appl. Opt. 42 (2003) 2085. R. H. Huddlestone and S. L. Leonard, Plasma Diagnostic Techniques, Academic Press, New York (1965). F. F. Chen, Introduction to Plasma Physics and Controlled Fusion, Plenum Press, New York (1974). V. N. Rai, M. Shukla and H. C. Pant, Pramana, J. Phys. 52 (1999) 49. S. Yu. Guskov, T. Pisarczyk and V. B. Rozanov, Laser and Particle Beam 12 (1994) 371.

254

V. N. Rai et al.

[55] R. D. Bleach and J. D. Nagel, J. Appl. Phys. 49 (1978) 3832. [56] V. N. Rai, M. Shukla and H. C. Pant, Proc. National Laser Symposium IRDE, Dehradun 10–14th Feb. (1995) pp 269. [57] W. J. Trestle, K. W. March, J. B. Orenberg, P. W. Carr, D. Miller and D. Glick, Anal.Chem. 43 (1971) 1452. [58] V. N. Rai, J. P. Singh, F. Y. Yueh and R. L. Cook, AIAA-2001–2812, 32nd AIAA Plasmadynamics and Laser Conference, 11–14th June (2001) Anaheim CA. [59] H. R. Griem, “Plasma Spectroscopy” Mc Graw Hill, New York (1964). [60] V. N. Rai, H. Zhang, F. Y. Yueh and J. P. Singh, Bulletin of Am. Phys, Soc. 46 (2001) 120. [61] J. Uebbing, J. Brust, W. Sdorra, F. Leis and K. Niemax, Appl. Spectrosc. 45 (1991) 1419. [62] R. Sattmann, V. Sturn and R. Noll, J. Phys. D 28 (1995) 2181. [63] D. N. Stratis, K. L. Eland and S. M. Angel, Appl. Spectrosc. 5 (2001) 1297. [64] L. St. Onge, M. Sabsabi and P. Cielo, Spectrochim Acta B 53 (1998) 407. [65] L. St. Onge, V. Detalles and M. Sabsabi, Spectrochim Acta B 57 (2000) 121. [66] V. N. Rai, F. Y. Yueh and J. P. Singh, Appl. Opt. 42 (2003) 2094. [67] A. Kumar, F. Y. Yueh, T. Millar and J. P. Singh, Appl. Opt. 42 (2003) 6047. [68] A. Kuwako, Y. Uchida and K. Maedo, Appl. Opt. 42 (2003) 6052. [69] W. Pearman, J. Scaffidi and S. Michael Angel, Appl. Opt. 42 (2003) 6085. [70] J. Scaffidi, J. Pender, W. Pearman, S. R. Goode, B. W. Colston Jr., J. C. Carter and S. M. Angel, Appl. Opt. 42 (2003) 6099. [71] A. De. Giacomo, M. Dell’Aglio, F. Colao and R. Fantoni, Spectrochim Acta B 59 (2004) 1431. [72] A. De. Giacomo, M. Dell”Anglio, F. Colao, R. Fantoni and V. Lazic, Appl. Surf. Science (2005) in Press. [73] W. A. Spencer, F. M. Pennebaker, N. M. Hassan, J. E. McCarty and C. W. Jenkins, Rep. BNF-003-98-0199 Rev. (Savannah River Technology Center, Aiken SC. 2000). [74] F. Y. Yueh, R. C. Sharma, H. Zhang, J. P. Singh and W. A. Spenser, J. Air Waste Manage. Assoc. 52 (2002) 747. [75] C. E. Max, “Physics of the coronal plasma in laser fusion targets” Ed. R. Balian and J. C Adams Laser Plasma Interaction, North Holland Publishing Co. (1982). [76] H. Nakano, T. Nishikawa, H. Ahn and N. Uesugi, Appl. Phys. B 63 (1996) 107. [77] H. M. Milchberg, R. R. Freeman, J. Opt. Soc. Am. B 6 (1996) 1351. [78] R. M. Moore, K. H. Warren, D. A. Young, G. B. Zimermann, Phys. Fluid 31 (1988) 3059.

Chapter 11

Laser-Induced Breakdown Spectroscopy of Solid and Molten Material A. K. Raia , F. Y. Yuehb , J. P. Singhb and D. K. Raic a

Department of Physics, University of Allahabad, Allahabad-211002, INDIA Institute for Clean Energy Technology, Mississippi State University, 205 Research Boulevard, Starkville, MS 39759, USA. c Department of Physics, Banaras Hindu University, Varanasi-210005, INDIA b

1. INTRODUCTION The current analytical techniques used for both qualitative and quantitative analysis of toxic elements (e.g. chromium, lead, nickel, cadmium, copper, mercury etc) have significant limitations as regards their practical application. It involves sample collection, transportation, sample preparation and laboratory analysis which are labour intensive, costly, and require a considerable amount of time (a few days to a few weeks) for the results to be available. Thus it is desirable to develop an analytical technique which is quick, sensitive, and is also capable of analyzing the material in-situ, especially in situations involving hazardous materials. Laser Induced Breakdown Spectroscopy (LIBS), a powerful spectro-analytical technique rapidly making a transition from laboratories to field use, has many advantages in this regard. It requires no special sample preparation, any type of material (solid, liquid, gas, slag) may be analyzed in situ and it is capable of detecting and analyzing several elements at the same time. The present chapter aims at summarizing in some detail the diverse analytical methods employing LIBS that have been developed during the past two decades. The LIBS technique makes use of a simple plasma spectrochemical approach. A high peak power laser pulse is focused on the sample (solid, liquid, or gas) to produce a spark whose emission contains characteristic spectral signatures from excited atoms, radicals, and ions in the plasma plume. The emitted radiation is collected by using optical fibers or lenses and passed through a monochromator where the spectrally resolved light is detected by a CCD/ICCD detector. The light intensity as a function of wavelength is recorded in a computer, and this digitalized data is analyzed using appropriate software. The end product of the analysis provides identification as well as concentration information about the various elements present in the sample. Thus, LIBS is an advanced diagnostic tool for rapid and remote analysis of target-composition [1–5]. LIBS can also provide on-line elemental analysis of compounds at the preparation stage so that quality assurance and quality control decisions can be made during Laser-Induced Breakdown Spectroscopy Jagdish P Singh, Surya N Thakur (Editors) © 2007 Elsevier B.V. All rights reserved.

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processing [6,7]. This facility may lead to enhanced control of product quality, save time and improve the efficiency of processes involved in glass, metal and pharmaceutical industries. Currently, no real-time measurement of melt constituents is available for the glass, aluminum, and steel industries. Melt composition is determined by collecting molten samples and taking them to a laboratory for analysis, making it a time- and energy-consuming process. Moreover, the melt composition itself can change due to the vaporization of the more volatile components during its transportation and hence compositional fluctuations cannot be effectively monitored using the currently available methods. To improve production efficiency, such industries require a technique that can provide rapid, on-site melt composition measurement. This technique should also allow chemical additions to be made (if necessary) to the melt, so that an acceptable product composition is achieved prior to draining a melter/furnace. On-site, real-time measurements are expected to be more cost effective than separate sampling and off-site analyses. In the following sections we describe the experimental arrangements based on fiber optic (FO) LIBS sensor to measure on-line, in-situ elemental composition of solid and molten samples.

2. FO LIBS SENSOR FOR DETERMINATION OF ELEMENTAL COMPOSITION OF SOLID ALUMINUM ALLOYS The laser breakdown threshold is known to be lower in solids than in a gas, so lower optical energy is needed for measurements on a solid sample. Analytical results of LIBS studies on solids are more frequent in the literature than on liquids or gaseous samples. Several publications [8], describe the determination of elemental composition in steel, Al alloys, soil, and paints, using the LIBS technique. LIBS has also been used for on-line quality control of rubber mixing and in the analysis of mining ores. A number of review articles on these topics have also appeared in scientific literature in recent years [4,9, and 10]. Gomba et al. [11] have determined the very low concentration of Li in an aluminumlithium alloy by recording its LIBS spectra in a vacuum chamber in a controlled xenon atmosphere. Hemmerlin et al. [12] have demonstrated that LIBS is comparable to the spark technique for the quantitative determination of trace elements in steel. Femtosecond laser pulses have been used by Drogoff et al. [13] to obtain detection limits in the range of a few ppm in Al alloys. Rosenwasser et al. [14] have used LIBS to identify the metallic elements in ores, while Samek et al. [15] utilized it to measure trace element concentration in hard biological tissue (e.g. teeth and bone). LIBS has also been employed to analyze wood [16], glass [17], concrete [18], limestone [19], and paint [20], soil and sand [21]. The early LIBS systems consisting of a number of lenses required elaborate alignment for recording the spectra [22,23]. Such an experimental set up is not well suited for industrial/field use where minimum of on-site alignment is a great advantage. Recent advances in fiber optic materials have opened up new areas of applications for the LIBS technique. A beam delivery system is used to send the laser beam to the desired location and the signal collected through optical fibers greatly facilitates remote measurement. One of the most difficult tasks in designing a FO-LIBS probe is to couple a high-energy laser beam into an optical fiber without damaging the fiber [24–26]. In the initial stages fiber bundle replaced the lenses for collecting the emission from the laser-spark but in

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later experiments, two optical fibers; one for delivering the laser pulse and the other for collecting the emission from the spark were used [27,28]. In harsh and hazardous environments such as in aluminum, glass and steel industries the adjustment of two separate optical fibers is a delicate and difficult task, and therefore it is desirable to use the same fiber for delivery as well as for collecting the optical energy [29–31]. The schematic diagram of the FO-LIBS probe is shown in Fig. 3 of chapter 5. The second harmonic (532 nm) of a pulsed Nd: YAG laser (Big Sky, Model CFR 400) operating at 10 Hz, pulse duration 29 ns, beam diameter 7 mm and the full angle divergence 1.0 mrad, is directed towards the optical fiber by a 532/1064 nm beam splitter and a 532 nm dichroic mirror. A 45 dichroic mirror (DM), with special coating that reflects at 532 nm and transmits 180–510 nm and 550–1000 nm, is used for delivery of the laser energy and collection of the optical signal from the laser-spark. This simple design protects the detector from the potential damage by the reflected laser light. To transmit sufficient laser energy through the fiber optic cable while keeping it below the damage threshold of the fiber, laser beam was focused at a spot ∼3 mm in front of the fiber tip using a 10 cm focal length lens. A cap with a 0.8 mm pinhole was placed at the fiber input end to avoid the possibility of any damage to the core and cladding of the fiber. The laser beam transmitted through the optical fiber is collimated with a 10 cm focal length lens and then focused on the sample by a 5 cm focal-length lens. The emission, from the laser produced plasma, is collected by the same lenses and the optical fiber. The collimated radiation passes through the dichroic mirror and is focused onto an optical fiber bundle with a 20-cm focal length lens. The fiber bundle consists of 78 fibers each of 100 m diameter and 0.16 numerical aperture (NA). The slit type output end of this fiber bundle delivers the emitted light to the entrance slit of a 0.5 m focal length spectrometer (Model HR 460 JOBIN YVON-SPEX) equipped with a 2400 lines/mm grating blazed at 300 nm. An intensified charge couple detector (ICCD, Model ITE/CCD Princeton Instruments) with its controller (Model ST 133, Princeton Instruments) was used as the detector. A programmable pulse delay generator (MODEL PG-200, Princeton Instruments) was used to gate the ICCD. The entire experimental apparatus was controlled by a (Dell Dimension M 200a) computer running the WinSpec/32 (Princeton Instruments) software. Multiple (100) laser shots were recorded and the resulting spectrum was stored in “accumulations” mode. Fifty spectra were stored in one file for analysis to obtain average area/intensity value for the spectral line of interest.

2.1. Parametric Studies To obtain optimum signal for the quantitative analysis of minor elements in the aluminum alloys, LIBS signals were recorded by changing the various experimental parameters (laser energy, sample surface, detector gain, gate delay and width etc).

2.1.1. Transmission of Laser energy through Optical Fiber The fiber used in our experiment [31] was a silica core/silica cladding multimode fiber (FG-1.0-UAT from ThorLabs Inc.). The stability of silica cladding allows for high powerhandling capability and correcting any laser mis-alignment. The silica cladding design

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also provides superior UV transmission required to transfer the LIBS signal. The length of the fiber was 3 m and SMA 905 stainless steel fiber connectors (ThorLabs Inc.) were used at both ends. The fiber was polished with 0.3-mm size aluminum oxide particles in the final step. The core diameter is 1.0 mm, the cladding diameter 1.25 mm, the numerical aperture 0.16, and manufacturer’s suggested maximum power capability is 5 GW/cm2 . The low numerical aperture provides for low beam divergence and a uniform spot size that facilitates focusing the beam after transmission through the fiber. The Nd: YAG laser (Big Sky Inc. CFR400) was operated at 10 Hz and its second harmonic ( = 532 nm) radiation has a pulse width (FWHM) 8 ns and maximum pulse energy 180 mJ. The laser had a Gaussian beam profile and beam diameter was 6.5 mm. A spherical plano-convex fused silica lens of 10-cm focal length was used to couple the laser beam into the fiber. A 30-mJ-laser beam after passing through this lens can create breakdown in air, and hence this value (30 mJ) is the maximum laser energy that might be transferred through the fiber. A metal cover with a 0.8-mm pinhole at the center was placed just in front of the fiber end to avoid any damage to the core-cladding boundary during alignment. The fiber was placed about 5 mm behind the focal point and it is estimated that only about 0.6–0.7 mm of the core diameter was illuminated by the diverging laser beam. A simple calculation indicates that a 30-mJ-pulse energy with a spot size of 0.5-mm diameter will produce an energy density of 2 GW/cm2 in the fiber, which is lower than its damage threshold. However, even at this energy level it is still possible that damage may occur on the input surface of the fiber due to randomly occurring hot spots in the laser profile. Fig. 1 shows that the energy transmission efficiency with our coupling setup is about 88%, which is fairly high.

2.1.2. Influence of Laser Power on Fiber Damage In order to improve signal-to-background (S/B) ratio, effects of various experimental parameters were tested and during this process, the optical fiber was damaged several times. In most of the cases damage occurred inside the fiber when the laser energy input exceeded 20 mJ. It was soon realized that as long as the laser energy was kept

Laser energy after fiber (mJ)

20

y = 0.8884x – 0.3631

18

R2 = 0.9972

16 14 12 10 8 6 4 2 0 0

2

4

6

8

10

12

14

16

18

20

Laser energy before fiber (mJ)

Fig. 1. Laser transmission from the optical fiber. (Reproduced with permission from Ref. [31]).

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below 20 mJ at the fiber input end, no damage occurred and for all the experiments laser energy was kept below this threshold. On several occasions the damage caused due to core-cladding breakdown took place not at the input end but at a point 2–5 cm inside from the input face. This kind of damage is most likely at the location where the first reflection of the laser beam inside the fiber takes place. In our first experiments, the fiber was clamped about 10 cm behind the input end face and several times the fiber damage occurred just behind the clamped position, due to additional stress caused by clamping. In later experiments the fiber was kept straight and was clamped around 30 cm behind the input end and no such damage occurred. We recorded the LIBS spectra by varying the laser power up to the damage threshold, and it was observed that the signalto-background ratio is best at laser-pulse energy of 13.6 mJ. All subsequent experiments were performed at this laser power.

2.1.3. Effect of Laser Radiation on the Surface of the Sample If the focused pulsed laser beam is directed at the same spot on the sample surface, the LIBS signal decreases with time. This decrease is believed to be due to the formation of an oxide layer and a crater, which modifies the optical properties of the target. If the laser is focused continuously at the same location, the crater size changes and this results in a time varying LIBS signal. Therefore, to obtain reproducible signals, measurements were made by slowly translating the sample with a stepping motor to ensure that the laser strikes a fresh spot for each new measurement.

2.1.4. Influence of Gain of the Detector In our initial experiments, the gain of the ICCD was kept high, but the S/B ratio was found to be very poor. These experiments were performed with a short gate delay and atomic lines were found to be buried in a strong background that caused saturation of the detector. In order to improve the S/B ratio, a longer time delay was used which not only reduced the background but also caused the disappearance of some of the weak spectral lines. To reduce the scattered laser light, a notch filter was placed in front of the input end of the receiver fiber, but no significant improvement in the S/B ratio was observed. This shows that scattered laser light is not the main cause of the strong background. Finally it was found that by keeping the detector gain at a moderately low level, discrete spectral lines in Al alloys could be recorded with better S/B ratios. Fig. 2 shows LIBS spectra recorded at detector gains of 1 and 2 respectively and S/B ratio is higher for the lower gain setting (see upper spectra of Fig. 2). At lower gain, one can record the LIBS spectra with good S/B ratio even by setting a shorter delay time and without losing the weak lines (for example, the 404.136 nm line of Mn). We thus conclude that the gain-setting of the detector is an important parameter in the present experimental setup. To avoid saturation of some strong lines at short delay time and at low detector gain, the spectra were recorded by using neutral density filters.

2.1.5. Effect of Detection-time Window The spectral line emission signal is always accompanied by a strong continuum from the laser-produced plasma. The continuum background dominates during the first several

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40000

Pr 900.69

Hq 909.09

Hq 909.29

60000

Ha 900.67

Intensity

80000

Pr 975.02

Pr 971.95

100000

Pr 902.04

(a)

20000 0 370

375

380 Wavelength (nm)

385

390

375

380 Wavelength (nm)

385

390

(b)

Intensity

400000 300000 200000 100000 0 370

Fig. 2. LIBS Spectra of solid Al alloy at two different gain of the ICCD detector; upper spectra (a) recorded at low gain and lower one (b) at high gain. (Reproduced with permission from Ref. [31]).

microseconds after the laser pulse but decays faster than atomic emission. Therefore, one can use a time-resolved technique to discriminate against the continuum radiation. Fig. 3 shows S/B ratio for a spectrum recorded with a FO-LIBS system at various gate delay times with gate width fixed at 2 s. The best S/B is obtained with delay times of 2–3 s and hence in the present work, LIBS spectra for parametric studies were recorded at 2 s gate delay using 2 s gate-width. The plasma temperature estimated

140 120

S/B ratio

100 80 60 40 20 0 0

2

4

6

8

10

12

14

16

Delay time (μs)

Fig. 3. LIBS spectra of solid Al alloy recorded at different gate delay. (Reproduced with permission from A. K. Rai et al. [31]).

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from the spectra recorded in this experimental condition was found to be 5570 K. The critical electron density for local thermal equilibrium (LTE) is found to be 5 × 1015 cm−3 , evaluated on the basis of the Griem equation [32]. Since the measured electron density 2 × 1016 cm−3  is higher than the calculated value, this indicates that LTE exists under these conditions.

2.2. Effect of Angle of Incidence on LIBS Signal We visited an aluminum factory at Syracuse, NY, USA to explore the possibility of using a LIBS probe inside their furnace. From an inspection of their facilities we came to the conclusion that it was not possible to insert the probe from the top of furnace so that it is perpendicular to the melt surface but the probe insertion was possible only at an angle with the melt surface. In order to evaluate the performance of the FO-LIBS probe in a factory environment, laboratory studies were performed to assess the effect of angle of incidence of the laser beam on the intensities of the analyte emission. The LIBS signals using fiber optic probe as well as without such a probe were recorded for various angles of incidence (0  15  30  45 and 60  where 0 corresponds to normal incidence. Great care was taken to maintain the constancy of the lens-to-sample distance at each angle of incidence. For this, the axis of rotation of the sample was made coincident with the axis of the incident beam. Lenses of different focal lengths were used to focus the laser radiation on the sample and the results are summarized in the following sections.

2.2.1. Fiber with Lenses of Focal length 5 and 10 cm We have recorded the LIBS spectra from neutral (Fe, Cr, Mg, Mn etc.), and ionic species at gate delays of 0.3, 0.5, 1, 2 and 3 s and at various angles of incidence ranging from 0 to 60 . Our results show that intensities of both line and continuum emission decrease as the angle of incidence changes from 0 (normal incidence) to 60 . In the case of lines from neutral atoms, the decrease in intensity is steeper at higher time delay, but in the case of the background continuum the trend is opposite (Fig. 4 (a) and 4 (b)). This observation is in accordance with the fact that in the first microsecond after the laser pulse the continuum emission is strong whereas the line emission appears strong only after several microseconds. A similar experiment has been performed by Multari et al. [33] who noticed that emission intensities were the largest for incidence at 0 and decreased as the incidence angle was increased upto 40 . An increase of intensity for angles of incidence beyond 40 was also noted. This increase was greatest for the neutral emission, which became almost as intense as at normal incidence for angle of incidence of 60 . In contrast the intensity for ionized species and the background continuum continued to decrease beyond 40 and become a minimum at 60 . In our experiments the intensity in all three types of emissions (neutral, ion and background continuum) is largest at 0 and is smaller for all other angles. As the sample is rotated with respect to the incident laser beam the mass of the ablated material as well as the temperature of the atomic material ejected from the surface may change, which may lead to changes in emission intensities. Multari et al. [33]

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(a)

1.2

1.2 Mn2

1 Cr2

0.8

Fe3

0.6

Backgr

0.4

0 –50

0.6 Mn2 Cr2 Fe3 Backgr

0.2

–100

0.8

Intensity

Intensity

1

0

Angle

50

100

–60

0.4 0.2 0 –10

40

Angle

Fig. 4. Variation of line intensity and background continuum with angle of incident of laser beam on the sample surface in the LIBS spectra of solid Al alloy recorded using 5 cm focal length lens with fiber and at gate delay; (a) 05 s (b) 2 s. (Reproduced with permission from A. K. Rai et al. [6]).

have reported that there is no variation in the mass of the ablated material as the angle of incidence is changed from 0 to 60 , thereby eliminating changes in total ablated mass as the cause of the observed changes in the emission intensity. Thus, change in temperature of the ejected material may be the cause in the decrease of the LIBS signal at higher incidence angle. Our measurements showed a monotonic decrease in the plasma temperature as the sample was rotated from 0 to 60 , which would decrease the intensity of emission from the neutral as well as the ionized species. Another cause of decrease in the measured intensity is probably the fact that the symmetric central axis of emissions (which is perpendicular to the surface for all sample orientation) no longer remains aligned to the collecting optics. The third reason for the decrease in the LIBS signal may be the difference in the amount of laser light reflected from the sample surface at different angles of incidence. In our experiments, we noticed an increase in reflection with increase in the angle of incidence which causes a reduction in the laser energy available for producing the spark. In fiber optic experimental setup also, the LIBS signal decreases with an increase in the rotation angle of the sample, but there are differences in the trend of decrease in the atomic emission. The decrease in the intensity of atomic lines is steeper at lower delay time (Fig. 5a) but for higher delay time the effect of rotation on the intensity of the atomic lines is small (Fig. 5b).

2.3. Calibration Curve It is clear from the above parametric studies that the FO-LIBS probe is just like a flash light with analytical capability, so that if you shine this flashlight at any material you are directly able to see the various elements in that material. This probe/sensor is very suitable for qualitative analysis or even for semi quantitative elemental analysis of the sample material. For quantitative analysis, however, some shortcomings must be overcome before one can use this technique. If one wants to perform the quantitative

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(a)

(b) 1.2

1.2

Mn2 Cr2 Fe3 Bkgr

1

1 0.8

Intensity

Intensity

0.8 0.6

0.6 0.4

0.4 Mn2 Cr2 Fe3 Bkgr

0.2

0.2 0

0 –70

30

–20

80

–100

50

0

–50

Angle

100

Angle

Hundreds

Fig. 5. Variation of line intensity and background continuum with angle of incident of laser beam on the sample surface in the LIBS spectra of solid Al alloy recorded using 10 cm focal length lens with fiber and at gate delay; (a) 05 s, (b) 2 s. (Reproduced with permission from A. K. Rai et al. [6]).

180 160 140

Intensity

120 100 80 60 40 20 0 0

1

2

3

4

Weight %

Fig. 6. Ideal Calibration curve for Quantitative Analysis of minor elements in Alloys.

analysis of minor elements (i.e. Mn, Mg, Cr, Cu etc.) in an Al alloy, one should first prepare calibration curve between the concentration of the element in the alloy and the LIBS signal intensity of a spectral line of this element. A perfect calibration curve is one, which passes through the origin and also has a small standard deviation. (Fig. 6). In actual practice it is difficult to get such an ideal calibration curve. To obtain calibration curves for Mn, Cr, Mg and Cu etc, which are most important minor elements in the Al alloys, we have obtained commercially available Al alloys, whose component concentrations are given in Table 1 and their LIBS spectra in different spectral regions have been recorded using the experimental set-up shown in Fig. 3 of chapter 5.

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A. K. Rai et al. Table 1. Concentration of different elements in weight % present in Al alloy Sample

Si

Fe ∗

7075 2017 6061 6262 2026 2011 6063 ∗ +

006 013+ 033∗ 030+ 065∗ 035+ 035∗ 037+ 006∗ 006+ 015∗ 013+ 023∗ 036+

Cu ∗

0018 015+ 028∗ 026+ 029∗ 033+ 048∗ 050+ 010∗ 007+ 0039∗ 039+ 018∗ 015+

Mn ∗

137 135+ 397∗ 380+ 027∗ 031+ 031∗ 035+ 433∗ 429+ 538∗ 565+ 006∗ 000+

∗

002 002+ 045∗ 056+ 0073∗ 009+ 001∗ 000+ 048∗ 059+ 002∗ 000+ 000∗ 000+

Cr

Ni ∗

020 019+ 018∗ 020+ 0073∗ 007+ 007∗ 006+ 001∗ 000+ 001∗ 000+ 000∗ 000+

Zn ∗

000 000+ 001∗ 000+ 0073∗ 000+ 000∗ 000+ 000∗ 000+ 000∗ 000+ 000∗ 000+

Mg ∗

578 569+ 010∗ 009+ 0053∗ 006+ 001∗ 000+ 005∗ 006+ 003∗ 002+ 000∗ 000+

∗

246 262+ 073∗ 057+ 085∗ 086+ 100∗ 106+ 139∗ 145+ 009∗ 000+ 160∗ 048+

Al 8986∗ 8893+ 9646∗ 9385+ 9795∗ 9757+ 9766∗ 9732+ 936∗ 9307+ 9400∗ 9375+ 9903∗ 9868+

Analysis based on MSU chemical lab (atomic absorption). Analysis based on ICP.

In the atomic spectrum of Mn there is a group of four lines in the wavelength region of ≈400 nm (Fig. 7) of which three lines (403.448, 403.306, 403.075 nm) are very strong and one line (404.135 nm) is weak. A calibration curve using one of the strong lines at 403.075 nm is shown in Fig. 8a. One can easily see that for this particular line of Mn the calibration curve is not a straight line. It seems that the LIBS signal for this particular line gets saturated due to self absorption in the case of samples with higher concentration of Mn. The three lines (403.448, 403.306, 403.075 nm) are the resonant lines which means that the lower state of these lines is the ground state of the atom. Therefore, it is more likely that these lines would suffer from self absorption. The line at 404.135 nm is not a resonant line. The lower state for this line is an excited state of the atom. Therefore, self absorption is not likely for this line. The calibration curve using this line is shown in Fig. 8b and is a straight line. To reduce the influence of experimental parameters like laser power, sample to lens distance and the nature of the matrix elements on the LIBS signal from different samples, one can use a ratio calibration curve. In other words, one uses the ratio of the intensity of the analyte atomic line and the intensity of a reference atomic line. Since Fe has atomic lines in almost every spectral region, we have divided the intensity of the analyte atomic line with the intensity of a Fe reference line. Fig. 9a shows the ratio calibration curve for the strong Mn line (403.075 nm) and once again the calibration curve is nonlinear. For the nonresonant weak Mn line, however, the calibration curve is a straight line (Fig. 9b). The next minor element in the Al alloy tested for calibration is Cr. As shown in Fig. 10, there are two groups of Cr lines; one in the wavelength region ≈360 nm and the other in the wavelength region ≈425 nm which may be utilized for drawing the calibration curves. Fig. 11a shows the nonlinear calibration curve using Cr line at 359.35 nm whereas the calibration curve using Cr line at 428.97 nm is a straight line (Fig. 11b). This is again because the Cr line at 359.35 nm is nearly ten times more intense than the Cr

LIBS of Solid and Molten Material

265 Mn 41789.48 cm–1 404.135 nm

24802.25 cm–1

24788.05 cm–1 403.448 nm 403.306 nm 403.075 nm

24779.32 cm–1 17052.29 cm–1

Ground level

403.075 120000

403.306

100000 403.448 80000 60000 40000 404.135

20000 0 400

405

410

415

Wavelength [nm]

Fig. 7. Atomic energy level diagram of Mn and its spectrum.

(b) 1.4

Intensity of Mn line Thousands

Millions

(a) ♦ Mn(403.075) nm

1.2

Intensity of Mn line

1 0.8 0.6 0.4 0.2

300

♦ Mn(404.135) nm

250 200 150 100 50 0

0 0

0.2

0.4

0.6

Weight % of Mn

0.8

1

0

0.2

0.4

0.6

0.8

1

Weight % of Mn

Fig. 8. (a) Calibration curve using absolute intensity of Mn (403.075 nm) resonant line; (b) Calibration curve using absolute intensity of Mn (404.135) non-resonant line.

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A. K. Rai et al. (b)

(a)

6

12

♦ Mn(404.135)/Fe(406.39) nm

5

10

Intensity of Mn/Fe

Intensity of Mn/Fe

♦ Mn(403.075)/Fe(406.39) nm

8 6 4

4 3 2 1

2

0

0 0

0.5

1

1.5

0

1

2

3

4

Weight % of Mn/Fe

Weight % of Mn/Fe

Fig. 9. (a) Calibration curve using ratio of Mn (403.075 nm) line with Fe (406.39 nm) line; (b) Calibration curve using ratio of Mn (404.135 nm) line with Fe (406.39 nm) line.

27935.26 cm–1

Cr

27820.23 cm–1 27728.87 cm–1

23498.84 cm–1 23386.35 cm–1 23305.01 cm–1

360.53 nm 359.35 nm

425.43 nm

357.87 nm

427.48 nm 428.97 nm

Ground level

357.87 12000

359.35 360.53

30000

427.48

425.43 Intensity

Intensity

80000

40000

428.97

20000

10000

0

0 350

355

360

Wavelength (nm)

365

420

425

Wavelength (nm)

Fig. 10. Atomic energy level diagram of Cr and its spectrum.

430

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267 (b)

1.4

Intensity of Cr line Millions

Millions

(a) Cr(359.35 nm) 1.2

Intensity of Cr line

1 0.8 0.6 0.4 0.2 0

1.4 Cr(428.97 nm)

1.2 1 0.8 0.6 0.4 0.2 0

0

0.1

0.2

0.3

0.4

0

0.5

0.1

0.2

Weight % of Cr

0.3

0.4

0.5

Weight % of Cr

Fig. 11. (a) Calibration Curve using absolute intensity of Cr (359.35 nm) Line, (b) Calibration Curve using absolute intensity of Cr (428.97 nm) Line.

(a)

(b) 8 Cr(428.97 nm)/Fe(432.71 nm)

Intensity of Cr/Fe

Intensity of Cr/Fe

7 6 5 4 3 2 1 0 0

0.2

0.4

0.6

Weight % of Cr/Fe

0.8

10 9 8

Cr(359.35 nm)/Fe(364.98 nm)

7 6 5 4 3 2 1 0 0

0.2

0.4

0.6

0.8

Weight % of Cr/Fe

Fig. 12. (a) Calibration curve using ratio of Cr (428.97 nm) line with Fe (432.71 nm) line, (b) Calibration curve using ratio of Cr (359.35 nm) line with Fe (364.98 nm) line.

line at 428.97 nm and the atomic line at 359.35 nm saturates the detector for higher Cr concentrations in the sample. The calibration curve based on intensity ratio at 428.97 nm is a straight line whereas the similar curve for the 359.35 nm emission is not linear (Figs. 12a, 12b). Since saturation of the detector may be avoided by reducing the incident laser power, we have shown in Fig. 13 the calibration curves for the 359.35 nm at two-laser powers (13.6 mJ and 10.2 mJ) and it is clear that the calibration curve at lower laser power is nearly linear. In the spectrum of Mg there are three close lying atomic lines (382.93, 383.22, 383.82 nm) having a common upper level. The intensity of 383.82 nm line is the largest whereas intensity of 382.93 nm line is the smallest (Fig. 14). The calibration curve corresponding to 383.82 nm line is not linear because of the saturation of the detector whereas the calibration curve corresponding to 382.93 nm line is a straight line (Figs. 15a, 15b). Similar behavior is seen in the ratio calibration curves for these two lines of Mg. (Figs. 16a, 16b).

A. K. Rai et al.

Line intensity (area) Millions

268

0.6

Cr(359.349 nm) at laser power 13.6 mJ Cr(359.349 nm) at laser power 10.2 mJ

0.5 0.4 0.3 0.2 0.1 0 0

0.05

0.1

0.15

0.2

0.25

Weight %

Fig. 13. Calibration curve using absolute line intensity of Cr 359.35 at two different laser powers.

Mg

47957.06 cm–1

383.82 nm 383.22 nm 382.93 nm 21911.18 cm–1 21870.46 cm–1 21850.41 cm–1

383.82 Mg

383.22 Mg

382.93 Mg

Ground level

Intensity

300000

200000

100000

0 380

385

390

Wavelength (nm)

Fig. 14. Atomic energy level diagram of Mg and its spectrum.

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269 (b)

Thousands

700

Mg(383.82 nm) 600 500

Intensity of Mg line

Intensity of Mg line

Thousands

(a)

400 300 200 100 0 0

0.1

0.2

0.3

0.4

0.5

450

Mg(382.93 nm)

400 350 300 250 200 150 100 50 0 0

0.6

0.1

0.2

0.3

0.4

0.5

0.6

Weight % of Mg

Weight % of Mg

Fig. 15. (a) Calibration curve using absolute intensity of Mg (383.82 nm) line, (b) Calibration Curve using the absolute intensity of Mg (382.93 nm) line.

(a)

(b) 4.5

3 Mg(382.93 nm)/Fe(382.04 nm)

Intensity of Mg/Fe line

Intensity of Mg/Fe line

Mg(383.82 nm)/Fe(382.04 nm) 3.75 3 2.25 1.5 0.75

2.5 2 1.5 1 0.5 0

0 0

0.2

0.4

0.6

Weight % of Mg/Fe

0.8

0

0.2

0.4

0.6

0.8

Weight % of Mg/Fe

Fig. 16. (a) Calibration curve using the ratio of Mg (383.82 nm) line with Fe (382.04 nm) line, (b) Calibration curve using the ratio of Mg (382.93 nm) line with Fe (382.04 nm) line.

It is thus clear that by selecting the proper atomic line, one can get a linear calibration curve but the fluctuations about the straight line as measured by the standard deviation, have still to be tackled. For the calibration curves shown in the Figs. 8b, 12b, 15b, the standard deviation is quite large ∼20%. The standard deviation may be reduced by excluding certain data points which differ from the average value by more than 0.5  ( is the original standard deviation) i.e. we have to exclude all points which deviate beyond A ± 05, where A is the average value of the intensity.

2.4. Effect of Sample-Lens Distance and Focal Length It has been found that a change in the sample to lens distance causes a change in the intensity of the LIBS signal from one laser shot to another which ultimately increases the standard deviation and hence the error bar for the measurements. Since the confocal

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parameter is short for a shorter focal length lens, we expect that the fluctuation in intensity of the LIBS signal would be larger for short focal length lens than for a large focal length lens. We have performed experiments to see the effect of sample-to-lens distance on LIBS signal by using lenses of focal lengths 20 cm and 5 cm (with and without fiber). The experimental results clearly demonstrate that even a slight (0.5 mm) change in the sample-to-lens distance may considerably reduce (by as much as 30%) the LIBS signal for a lens having f = 5 cm (Fig. 17a). For a lens having f = 20 cm, similar reduction in signal strength takes place for a 2 mm change in the sample to lens distance (Fig. 17b). Therefore, it is advisable to use a longer focal length lens to focus the incident radiation on the surface of the sample. Further, we noticed that the rate of decrease in LIBS signal is slow when the focal point is situated in front of the sample surface and it decreases more rapidly when the focal point is beyond the sample (Fig. 17a). This observation is seen more clearly in the case of a longer focal length lens (Fig. 17b). If the sample to lens distance is not kept constant during the translatory motion of the sample, the LIBS signal will fluctuate in intensity from one laser shot to another thereby affecting the analysis. Our experimental observations demonstrate that to reduce the standard deviation in calibration curve, one should use a longer focal length lens. To confirm this, we have calculated the percentages of standard deviation (Table 2) for intensity of the Cr line for different Al alloy samples by recording the LIBS spectra using lenses of focal length 5 cm and 20 cm. It is seen from Table 2 that the standard deviations for the intensity of the analyte lines obtained from the LIBS spectra using a lens of focal length of 5 cm are larger in comparison to those from the LIBS spectra using a lens of focal length of 20 cm. One can also notice that the standard deviation is large for the sample, which has lower Cr concentration (Table 2). We can now conclude that in calibration curve for quantitative analysis of an element, non-resonant spectral lines should be preferred and focal length of the lens collecting emission from the laser spark should have a large value.

(a)

(b) 1

7

Si Mg2 Mn Bkg

0.8

Millions

Millions

Mn2 Cr2 Fe3

6 5

Intensity

Intensity

0.6

0.4

4 3 2

0.2 1 0

0 –4

–2

0

2

Position from focal point (cm)

4

–6

–4

–2

0

2

4

6

Position from focal point (cm)

Fig. 17. Variation of LIBS signal intensity with sample-to-lens distance using; (a) focusing lens of 5 cm focal length and gate delay of 07 s (b) focusing lens of 20 cm focal length and gate delay of 2 s.

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Table 2. Variation of the STD of the intensity of analyte line Cr  = 357869 nm Sample

6262 1258 1259 7075 2017 2011 6061 2024

Cr concentration

0.07 0.01 0.17 0.20 0018/002 0.01 0.073 0.01

F = 20 cm

F = 5 cm

% STD

% STD

57 1088 1159 573 935 1439 862 1201

2111 2221 1524 1063 279 2387 944 1973

3. LIBS SPECTRA OF MOLTEN ALUMINUM ALLOY IN A LABORATORY FURNACE The use of fiber optic (FO) LIBS technique for in-situ and on-line compositional analysis/studies of the molten alloy inside a furnace is not yet a practical proposition and even laboratory studies are rare. Recently, Paksy et al. [34] performed quantitative analysis of metals in the molten phase. They focused the laser light on the surface of the molten alloy with the aid of a fixed optical system while the emission from the laser induced plasma was collected using optical fiber in a direction perpendicular to the laser beam. Gruber et al. [35] have used the LIBS technique for monitoring of Cr, Cu, Mn and Ni in steel by focusing the laser beam on the surface of the molten sample in the furnace. Noll et al. [36] analyzed the top gas composition for monitoring the elemental composition of molten steel in the blast furnace. It is to be noted that the surface of the molten alloy as well as the top gas may not contain the actual elemental composition due to the formation of slag-like/oxide material on the surface of the molten alloy. Also measurements on the surface will not provide any information about the uniformity of mixing in the Al melt inside the furnace. Therefore, it is desirable to analyze the molten alloys by recording the LIBS signal by probes inserted fairly inside the melt surface. To achieve this goal, we have modified the FO-LIBS probe, which was developed for the compositional analysis of solid Al alloy [31]. The laser beam is coupled with the optical fiber in the same way as for the experiment on solid Al alloy. The main modification in the FO-LIBS probe is after the exit point of the optical fiber. The laser beam at the exit of the optical fiber is collimated by a plano-convex lens f ≈ 15 cm and focused with the help of another plano-convex lens f ≈ 5 cm which is kept at a distance of 75 cm from the collimating lens. Both these lenses are kept in a stainless steel (s. s.) holder with an internal diameter ≈2.2 cm and outer diameter ≈3.0 cm (Fig. 18). At the bottom of the holder a cave is cut to hold the focusing lens, which sits on an iron ring. The s.s. holder below the collimating lens contains an inlet designed for purging an inert gas that cools the lens and applies pressure to the aluminum melt surface. The purging gas comes out through eight holes near the focusing lens and does not allow the Al melt to reach the lens surface. This s.s. holder is then inserted into a ceramic pipe with the

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Optical fiber connector

Collimating lens

Swagelok Purge gas

Focusing lens

Iron ring N2 gas out

Fig. 18. Stainless steel holder for collimating and focussing lens.

help of an s.s. flange and a thermocouple is also placed in the s.s. holder to monitor the temperature near the optics. The Al alloy melt was produced in a laboratory furnace (L-83102-56622, GS, LINDBERG) placed in a crucible 813 cm ID×915 cm OD×165 cm high, AC 36265 Al2 O3 crucible, Ozark Technical Ceramics, Inc.). To avoid breakage and thermal shock, this crucible was placed in another crucible 1524 cm OD × 14 cm ID × 76 cm high and the temperature of the furnace was increased in steps of 60 every half hour until it reached 800 C. The schematic diagram of the experimental set up of FO-LIBS probe for measuring the elemental composition of molten Al alloy in laboratory is shown in Fig. 19. Initially, the focal point was nearly 2.5 cm inside the ceramic pipe but in later experiments we found it necessary to insert the probe more than 2.5 cm below the melt surface. The focusing lens was damaged due to splashing of the melt at low flow rates of the purging nitrogen gas. To avoid damage to the focusing lens the design of the probe was changed by focusing the laser beam at the circumference of the ceramic pipe. The LIBS spectra of seven molten alloys were recorded without any damage to the focusing lens by adjusting the inlet flow rate of the purging gas between 1.5 and 3 l/min, and the outlet flow rate between 100 and 600 ml/min [7]. At times, the LIBS signal strength decreased, but recovered once the flow rate of the purging gas was adjusted. We also recorded the LIBS spectra by inserting the probe at different depths inside the melt and it was found that at greater depths a higher inlet-flow rate was necessary for sufficient LIBS signal. These experiments demonstrated the success of the probe to record LIBS signals from inside the melt. The LIBS spectra of the melt could be

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273 H

Nd: YAG laser

B

2X

F FO

L

L

D

L Collimating optics

Pulse generator Computer

Al melt

Controller Spectrograph

Data acquisition /Analysis system

ICC

Furnace BD – Beam Dump DM – Dichroic Mirror FO – Fiber Optics HS – Hormonic Separator L – Lens ICCD – Intensified Charge Coupled Device 2X – KDP Doubler

Fig. 19. Schematic diagram of the experimental set up of FO LIBS probe. (Reproduced with permission from A. K. Rai et al. [7]).

recorded with laser pulse energy of 9.5 mJ whereas the minimum laser-pulse energy needed for recording the spectra of solid Al alloy is 13.2 mJ [31].

3.1. Effect of the Surrounding Atmosphere on LIBS Signal In order to obtain the optimum LIBS signal in the molten alloy, the effects of the gaseous atmosphere surrounding the sample on the emission characteristics of the laser-induced plasma were also studied. The intensities of the different atomic lines, the continuum emission and noise were measured in nitrogen, argon and helium atmosphere in two spectral regions (  360 nm and 300 nm). As seen from Fig. 20 the most intense emission signal is obtained in argon atmosphere. Kuzuya et al. [37] have performed a similar study for solid samples and they also observed that maximum emission intensity is obtained in the argon atmosphere. Paksy et al. [34] also performed experiments to study the effect of air and argon atmospheres on the plasma emission from both solid and molten samples. These authors noted that the background intensity is larger in argon atmosphere if the plasma is generated from a solid sample, whereas it is smaller, if the plasma is generated from a molten sample. Our results are not directly comparable to others because while they had focussed the laser on the surface of the molten aluminum alloy, we have measured the emission intensity after inserting the probe more than 2.5 cm below the melt surface. Further, they measured the emission intensity in a direction perpendicular to the laser beam, while our measurements are of the emission in the backward direction. On the basis of the present experiments we conclude that for the same experimental condition the background (BKG) continuum in the presence of Ar is almost two times more intense than in the N2 atmosphere. The BKG intensity in helium atmosphere is lower than both, but the intensity of the plasma emission in helium atmosphere is too low to be detected when one uses a 2 s gate delay for which N2 and Ar measurements have been carried out. Therefore, for the case of helium the intensity data has been recorded at 1 s gate delay keeping the other experimental parameters the

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250000

Ar Purging gas

Intensity

200000

150000

Si

100000

Cr Fe

50000

0

287

289

291

293

295

297

299

301

303

305

307

303

305

307

Wavelength (nm) 140000 120000

N2 Purging gas

Intensity

100000 80000 60000 40000

Cr

Fe Si

20000 0 287

289

291

293

295

297

299

301

Wavelength (nm)

Fig. 20. LIBS spectra of molten Al alloy in laboratory furnace taken with different purging gases. (Reproduced with permission from A. K. Rai et al. [7]).

same as in argon and nitrogen atmosphere. The shorter delay is necessitated since the breakdown threshold of helium is higher than for the other two gases [37]. From the viewpoint of analytical performance, it is useful to evaluate the lineto-background ratio (LBR) of the spectrum. The values of LBR for the Si 288.158 nm, Fe 297.334 nm and Cr 301.757 nm lines were derived from the intensity data for different atmospheres and are presented in Table 3 where LBR are found to be higher in argon atmosphere than in nitrogen atmosphere. LBR value for Si is nearly 2.1 times larger in Table 3. Calculated S/B and S/N of different elements from the LIBS spectra of Al molten alloy in the presence of various atmospheric gases Gas

Elements Cr

N2a Ar a Heb a

Fe

Si

S/N

S/B

S/N

S/B

S/N

S/B

1232 1487 42

049 065 085

4205 4944 749

178 196 157

4682 10382 1668

197 413 334

2-s gate delay; b 1-s gate delay.

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Ar as compared to N2 whereas for Cr and Fe the increase is by a factor of 1.32 and 1.10 respectively. For helium atmosphere the LBR value for Cr is larger than both for argon and nitrogen atmospheres but for Fe helium yields a lower LBR than argon and nitrogen atmosphere. For Si, the LBR in He is lower than in argon but is larger than in nitrogen. The expansion of the laser-induced plasma is dependent on the pressure of the surrounding gas and it is related to the mass density of the gas. Since the density of argon is larger in comparison to helium and nitrogen, hence at the same pressure, the confining effect on the plasma is stronger in the case of argon atmosphere, which results in an increase in the emission intensity for both BKG and line emission. We have also calculated the line-to-noise ratio (LNR) for Si 288.158 nm, Fe 297.334 nm and Cr 301.7569 nm (See Table 3). As in the case of the LBR value, it is seen from Table 3 that LNR value for Si is also 2.2 times larger in case of argon atmosphere than in the nitrogen atmosphere, whereas, for Cr and Fe the increase in LNR value in argon is only 1.20 and 1.17 times respectively. In contrast to LBR value, the value of LNR for Si, Fe and Cr for helium atmosphere is lower in comparison to argon and nitrogen atmosphere. In summary, we can state that for the analysis of the molten phase an argon atmosphere is more appropriate, because: 1. it ensures higher LBR value, 2. it ensures higher LNR value (favorable detection limit), 3. and it helps avoid surface oxidation

3.2. Calibration Curves for Molten Aluminum Alloy Calibration based on line intensity is a very straightforward method for elemental analysis. However, calibration curves based on absolute intensity are only applicable for the samples where data are taken under the same experimental conditions (laser power, detection duration and delay, sample to lens distance and for samples of similar material/matrix). To obtain the spectrum from a molten-phase sample, one has to heat the sample slowly up to 800 C which takes several hours (nearly 4 to 5). Therefore, one is able to record the LIBS spectra in desired spectral range for only one sample a day, and it takes a whole week to obtain the LIBS data for seven samples. Since, it is difficult to keep all the experimental parameters the same for such an extended (in time) experiment, calibration curves using ratios of the intensity of the analyte line to the intensity of a reference line of another element are considered more reliable. To obtain reliable calibration data, the reference element should have reasonably high concentration in each standard sample. The selected reference line should be interference-free and its upper energy level should be close in energy to that for the analyte line. Although Al is the major species in all Al alloys, the Al lines are present only in two spectral regions in our experiments and most of them suffer from spectral interferences. Since iron lines are abundant in the wavelength range covered in the present experiment (300–420 nm) an interference free Fe line from each spectral region was selected as the reference line for ratio calibration. Calibration curves were obtained for seven different aluminum alloy samples. Figs. 21 (a)–(e) show some typical calibration curves using this method. The calibration curves

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(b) 6

Cu 327.396 /Fe 344.06

8

5 2017

4

6 7075

4 2

2011 626

6061

0

Mn 404.136 /Fe 406.39

202

Intensity ratio (Cu /Fe)

Intensity ratio (Cu/Fe)

10

8 4

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0

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2 0

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201

20

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606 3

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Weight ratio (Cu /Fe)

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2017

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Weight ratio (Mn/Fe) 2

Weight ratio (Cu/Fe)

3

4

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6

7

Weight ratio (Mn/Fe)

(c) Intensity ratio

80 70 60 50 40 30 20 10 0

(Mg 383.82 nm /Fe 383.63 nm) y = 2.9245x + 4.3719 R2 = 0.9475

0

2

4

6

8

10

12

14

16

18

20

Weight ratio Mg/Fe

(d)

(e)

0.7 0.5 0.4 0.3 y = 79.56x + 0.0862

0.2

2

R = 0.9062

0.003

0.004

Weight ratio Fe/Al

0.005

0.006

2017

0.1 0

6262 6063

0.15 0.05

0.002

y = 66.892x – 0.0121 R2 = 0.9227

0.2

0

0.001

6061

0.25

0.1 0

(Si 72 /Al 901)

0.3

Intensity ratio

0.6

Intensity ratio

0.35

(Fe 297.344 nm /Al 305.468 nm)

2024

0

0.001

7075 2011

0.002

0.003

0.004

Weight ratio Si/Al

Fig. 21. (a)–(e): Calibration curves of the molten Al alloys. (Reproduced with permission from A. K. Rai et al. [7]).

of the different atomic lines of the same element are reproducible. The calibration curve for the Cu 327.396 nm line is linear up to a concentration of 3.8 wt% (Fig. 21a) but for larger concentrations exhibits curvature. The nonlinear behavior in the calibration curve of Cu is believed to be due to self absorption as was already noticed in our previous work [31] on solid samples. In contrast to the solid sample [31], the calibration curve for Mn 404.36 nm is also showing curvature after 0.54 wt% (Fig. 21b). Calibration curve for Mg 383.82 nm is, however, a straight-line (Fig. 21c). To obtain the absolute concentration of analyte elements from the intensity ratio-based calibration we need to know the concentration of the reference element (i.e. Fe in the present case). The concentration of Fe may be obtained from the ratio calibration curve of Fe and the major element Al. The large intensity and interference-free location of Al line at 305.468 nm makes it possible to obtain the calibration curve for Fe 297.334 nm/Al 305.468 nm as a straight-line (Fig. 21d). Since Al is the major constituent, by using this curve one can get the concentration of Fe and the concentrations of other elements may

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be calculated from their ratio calibration curves with Fe. As Si (288.158 nm) line is also available in the region of Al 305.468 nm line, we have obtained the calibration curve for Si 288.158 nm/Al 305.468 nm, which again shows a linear variation with concentration (Fig. 21e). Calibration curves obtained in the present work clearly demonstrate that the experimental set up with FO LIBS probe is quite suitable to monitor the concentration of minor elements in the molten Al alloy in situ (furnace).

3.3. Comparison of LIBS Spectra of Molten and Solid Alloy Samples To compare the LIBS spectra of molten alloy with that of the of solid sample, LIBS spectra of solid sample were also recorded with the same FO LIBS probe. By comparing the melting points of Al, Cu, Cr, Mg, Mn, Si, Fe, and Zn, one can broadly divide these elements into two categories: one having a melting point between 500 to 1000 C and the other having a melting point between 1000 to 2000 C (Table 4). Al, Cu, Zn and Mg come in the first category, whereas the rest of the elements fall in the second category. Comparing the LIBS signal of these elements in molten and solid phase, we have the following observations: (i) Ratio of the line intensity of Al/Fe, Cu/Fe (Fig. 22a), and Mg/Fe (Fig. 22b) in molten phase was found quite small in comparison to its value in solid phase (Table 5). These observations may be understood by considering the fact that the concentration of elements having a lower melting point is higher above the melt surface. Therefore, the intensity of the line of an element, having lower melting point and having higher concentration, would often decrease in the molten phase due to self-absorption. The observation already noted earlier strengthens the above explanation that the spectra in melt show that the Cu, Al, and Mg lines have suffered self-absorption. Fig. 23 shows LIBS spectra of the solid and of the melt in the spectral region of 380 nm where Mg line in the melt is found to be broader than in the solid. This observation indicates that the concentration of Mg is higher above the melt surface due to its lower melting temperature.

Table 4. Melting points of analyte elements Sample Al Cu Cr Mg Mn Ni Si Fe

Temperature  C for vapor pressure of 1 Torr 1557 1617 1737 605 1217 1907 2057 1857

Melting point  C 660 1084 1857 649 1244 1453 1410 1535

278 (b)

20 18

Intensity ratio

16 14

500 With Fiber Mg 688/Fe 938

Solid with Fiber Cu (327.40 nm)/ Fe (344.06 nm) Molten Cu (327.40 nm)/ Fe (344.06 nm)

Intensity ratio (Mg2/Fe4)

(a)

A. K. Rai et al.

12 10 8 6 4 2

Molten Mg 690/Fe 939

400 300 200 100 0

0 0

5

10

15

20

0

5

Weight %

10

15

20

25

Weight %

Fig. 22. (a) Comparison of intensity ratio (Cu/Fe) vs concentration ratio in LIBS spectra of solid and melt. (Reproduced with permission from A. K. Rai et al. [7]; (b) Comparison of intensity ratio (Mg/Fe) vs concentration ratio in LIBS spectra of solid and melt. (Reproduced with permission from Ref. [6]).

Table 5. Intensity ratio of analyte lines of solid and molten aluminum alloy in the LIBS spectra Ratio Sample 6063

Mn/Fe – 01199 01331 ∗ 01846 32945 ∗ 33769 54129 ∗ 40906 00763 ∗ 01614 03623 ∗ 03749 – ∗ 05006 ∗

2011 2017 2024 6262 7075 6061 ∗

Cu/Fe

Mg1/Fe

Mn2/Fe

∗ 254198 – – ∗ 09951 165333 – 123104 00535 01087 ∗ 46166 ∗ 53482 ∗ 71848 155868 200832 366633 ∗ 62498 ∗ 68436 ∗ 100633 768489 1863045 3239095 ∗ 72346 ∗ 338356 ∗ 531594 37796 161861 263214 ∗ 13413 ∗ 98259 ∗ 151845 188995 1754791 2836476 ∗ 38857 ∗ 324723 ∗ 537989 – – – ∗ 11343 ∗ 48786 ∗ 73426 ∗

Fe/Al 01471 01722 03010 ∗ 03623 02026 ∗ 03068 00879 ∗ 01629 03862 ∗ 04929 01035 ∗ 02335 026079 ∗ 04184 ∗

Cr/Al

Si/Fe

00244 00585 00275 ∗ 00310 00523 ∗ 02420 00146 ∗ 02099 01414 ∗ 02821 03152 ∗ 04872 01495 ∗ 05094

22398 1460426 03042 ∗ 025966 1118637 ∗ 0589848 0515252 ∗ 0179353 1343422 ∗ 0839196 0813926 ∗ 0428902 2015179 ∗ 0640486

∗

∗

Cr/Fe 0122045 0422136 007999 ∗ 0172954 0304452 ∗ 0805809 0163362 ∗ 012970 0369332 ∗ 0816889 302612 ∗ 2557745 0584828 ∗ 1143162 ∗

molten phase

(ii) In contrast to the above observation, the ratio of Mg/Fe (Fig. 22b) in the molten phase was found to be larger than in the solid phase for one of the samples 2011 (Table 5), whereas the concentration of Mg in this sample is very small (0.09%) in comparison to other samples (Table 1). The actual concentration of Mg at the surface of the melt becomes larger than in the melt due to the lower melting point of Mg. Since the effect of self-absorption is small for the low Mg concentrations in sample 2011, the intensity ratio of Mg/Fe in this sample is larger in the molten phase than in the solid phase.

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279

1.2 Solid Melt

1

Intensity

0.8 0.6 0.4 0.2 0 380

381

382

383

384

385

386

Wavelength (nm)

Fig. 23. Comparison of the LIBS spectra of an Al alloy recorded in solid and molten phases. (Reproduced with permission from A. K. Rai et al. [7]).

Intensity ratio (Cr/Al)

0.6 Solid with Fiber Cr (301.756 nm)/Al (305.468 nm) Molten Cr (301.756 nm)/Al (305.468 nm)

0.5 0.4 0.3 0.2 0.1 0 0

0.0005

0.001

0.0015

0.002

0.0025

Weight %

Fig. 24. Comparison of intensity ratio (Cr/Al) vs concentration ratio in LIBS spectra of solid and melt. (Reproduced with permission from A. K. Rai et al. [7]).

(iii) Ratio of Cr/Fe and Cr/Al (Fig. 24 and Table 5) for molten phase were found to be larger than in the solid phase. This is due to the fact that Al and Fe have a smaller melting point in comparison to Cr, therefore, the line intensity of Al and Fe is reduced in molten phase due to self-absorption. (iv) The assumption that the intensity of the element having a lower melting point decreases in molten phase gets further support from our experimental ratio for Si/Fe. Ratio Si/Fe in the molten phase is quite small in comparison to its value in the solid phase. Melting point of Si is small in comparison to that for Fe and the line intensity for Si in the molten phase is reduced by self absorption yielding a small intensity ratio for Si/Fe. (v) The comparisons of LIBS spectra from melt and solid samples show that the intensity ratios from melt data strongly depend on the concentration levels and melting temperature of the analyte elements. The selective vaporization in melts

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causes the elemental concentration in laser plasma to be different from the true elemental composition in the melt. Therefore, the intensity of the analyte lines in the molten phase is quite different from the intensity in the solid phase. In conclusion, these results reveal that the FO LIBS probe is quite suitable for realtime in-situ monitoring of minor elements (Cr, Cu, Mg, Mn, Si, Zn etc) present in molten Al alloys in a laboratory furnace. These results also show that Ar atmosphere yields a higher line-to-background ratio and a higher line-to-noise ratio, and hence it is more suitable for molten alloy measurements. Experimental observations clearly demonstrate that the melting points of elements can also affect the calibration curve for the molten alloy. So, we cannot use the calibration curve obtained from studies on solid alloy to the case of the molten alloy. To obtain accurate elemental concentrations in melt, one needs calibration data, properly obtained for the melt. The calibration curve for an element having a lower melting temperature should cover a wider concentration range.

4. FO-LIBS PROBE FOR ALUMINUM ALLOY IN INDUSTRIAL PILOT FURNACE The results obtained in the laboratory furnace demonstrated the suitability of the present FO LIBS probe to monitor the concentration of minor elements in the molten Al alloy even in an industrial setting. The distance between the collimating lens (for the laser beam coming from the fiber) and the focusing lens (to focus the laser beam in the molten Al alloy) in the LIBS probe used in the laboratory furnace was nearly 75 cm. Use of the same FO LIBS probe for the measurement of elemental composition of the Al alloy in an industrial pilot furnace, would require a part of the fiber to be inside the furnace making the fiber, especially the fiber connector vulnerable to damage due to the high 800 C temperature. To ensure that the fiber remains wholly outside the furnace, a small modification in the design of the FO LIBS probe was effected. Keeping in view the large volume of the industrial furnace, the distance between the collimating lens and the focusing lens should be nearly 200 cm. Stainless steel (s.s.) holders were constructed which can house the collimating and the focusing lens without disturbing the optical alignment even at high temperature of about 800 C [38]. This holder protects the fiber and the collimating and focusing lenses from damage when the probe is inserted inside the furnace into molten material. This part of the probe is shown in Fig. 25. It is constructed from six different pieces of stainless steel tubes each having an internal diameter .2.2 cm, an outer diameter of 3.0 cm, and a length of 30 cm. These holders are connected to one another with the help of fine male and female threads as shown in Fig. 25. At the top of the holder a provision for swagelok connection is made for the inlet flow of the purge N2  gas. As shown in Fig. 25 an aluminum flange of outer diameter 15 cm and internal diameter 3.1 cm is connected to the s.s. holder with lock screw and Teflon system. With the help of this flange the s.s. holder is tightened in the s.s. pipe which is described in Ref. [38]. A provision for the insertion of a thermocouple, which measures the temperature at the bottom end of this holder, is made in the aluminum flange. A flow meter is connected which controls the gas flow to cool the whole lens holder. At the top of stainless steel holder an aluminum holder having the same internal and outer diameters is connected using male and female threads. This aluminum holder

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281

Optical fiber

Optical fiber connector

Aluminum holder

Collimating lens

Swagelok Purge gas Teflon

Thermocouple

O ring

Lockscrew

Lock screw

Aluminum flange O ring

N2 gas out O ring

Male thread Female thread Male thread

Focusing lens

Snap ring N2 gas out

Fig. 25. Stainless steel holder for collimating and the focusing lens in the pilot furnace. (Reproduced with permission from A. K. Rai et al. [6]).

houses the collimating lens with the help of a spiral lock ring. At the top of the aluminum holder a provision is made to connect the optical fiber through SMA 905 stainless steel fiber connectors. The aluminum holder also has a provision for a rotating ring, which permits fine adjustment of the distance between output end of the fiber and the collimating lens. With the help of this adjustment procedure the laser beam passing through the stainless steel holder is collimated without any change of the circular spot of the laser beam. The bottom part of the holder houses the focusing lens. A circular cave is cut in the internal wall at the bottom end, to hold the internal snap ring (SAE 1060–1090 steel). This snap ring prevents the movement of the focusing lens during the experiment. Provision is made by the side of the lens and the snap ring for the out flow of the purge (nitrogen) gas, which enters through the upper portion of this holder. This flow of purge gas helps to keep the lens and snap ring cool and also prevents the aluminum melt to reach the lens surface. In the present sensor we are able to adjust

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the distance between the output end of the fiber and the collimating lens so that the collimating beam illuminates the focusing lens without any loss of intensity of the laser beam. Finally the stainless steel holder was kept inside a stainless steel pipe to prevent its direct contact with the Al melt.

4.1. Testing the Long Stainless Steel Probe First, the alignment of the laser beam was tested by checking the image of the laser beam outside the stainless steel probe. By properly adjusting the lens holder with respect to the 180 cm pipe, one is able to get the proper circular spot of the laser beam, outside the probe. A special cap, which closes the bottom end of the probe, was designed to ensure that the focal point of the laser beam remains at the periphery of the probe. An Al rod attached to the cap was kept in the center of the probe, and its tips remained at the periphery of the probe. By adjusting the length of the spacer, we focused the laser on the tip of the Al rod. LIBS spectra of this Al rod were recorded to check the performance of the probe and after the successful test performance, we inserted the probe in the furnace melt. After properly adjusting the inlet and outlet gas flow, we were able to record the LIBS spectra of molten Al alloy.

4.2. LIBS Measurements inside the Industrial Pilot Furnace The LIBS assembly (which includes the FO LIBS probe, spectrometer, laser, computer, etc.) was packed and taken for field measurement. After testing the optical alignment, proper connections for water cooling and N2 gas flow were made and the probe was slowly inserted into the pilot furnace containing the molten alloy. By adjusting the depth of insertion of the probe and the flow of gas in the inlet, one is able to get LIBS spectra of the molten alloy with good S/N ratio. During the experiment, it was observed that the

8

Intensity ratio

7

Cr 359.35 nm /Fe 364.78 nm

6 5 4 3 2 1 0 0

0.2

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Fig. 26. Variation of the line intensity ratio vs. the concentration ratio in the LIBS spectra of the molten Al alloy during metal feed tests in a pilot furnace. (Reproduced with permission from A. K. Rai et al. [6]).

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100000 50000 0 326

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Fig. 27. LIBS spectra recorded with different gate delay time. (Reproduced with permission from A. K. Rai et al. [6]).

signals are quite sensitive to the depth of this probe in molten alloy, as well as to the flow rate of the purging gas. The data were taken by varying the experimental parameters for the best signalto-noise ratio. Fig. 26 shows the variation in the intensity of Cr emission line for the three different tests which shows a significant increase in the line intensity of the seeded metal. Fig. 27 shows the LIBS spectra recorded in the spectral region ≈336 nm with three different gate delay times. It is clearly seen that the intensities of Fe and Zn lines are enhanced at shorter gate-delay times, as compared to that of Cu 327.4 nm line which

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may suffer self-absorption at the shorter delay times. After successfully recording the spectra of the original melt in all spectral regions of interest, known amounts of Cr, Mn, Mg, and Cu metals were added into the melt. After waiting for one hour to let the metals get mixed in the Al alloy, the LIBS probe was again inserted into the melt to detect the change in the melt concentration. These observations demonstrate that the present FO LIBS sensor would be useful for on-line, in-situ monitoring of minor metal concentrations in pilot furnaces.

5. CONCLUSIONS In conclusion, this chapter describes a complete optical fiber (OF) LIBS system that was developed for on-line, in-situ elemental composition measurements of solid and molten samples. The critical issues associated with the coupling of the pulsed laser beam with an optical fiber are described. The parametric study was performed to optimize the performance of the OF-LIBS system. Application of the OF LIBS system to solid and molten aluminum measurements has been demonstrated and the test results show that OF-LIBS system can be used for on-line process monitoring and control of industrial furnaces.

ACKNOWLEDGMENTS This work was supported by the U. S. Department of Energy, Office of Industrial Technology (OIT) grant number DE-SC02-99 CH-10974, through a subcontract from the energy Research Company, and DOE Cooperative Agreement DE-FC 26–98 FT-40395. We are also thankful to Shiwani Pandhija, Junior Research Fellow for help in the preparation of the manuscript. During preparation of the manuscript, the financial assistance from DRDO project (No ERIP/ER/04303481/M/01/787) is fully acknowledged.

REFERENCES [1] M. Sabsabi and P. Cielo, Appl. Spectrosc., 49 (1995) 499. [2] D. E. Kim, K. J. Joo, H. K. Park, K. J. Oh, and D. W. Kim, Appl. Spectrosc. 51 (1997) 22. [3] R. Q. Auccolio, B. C. Castle, B. W. Smith and J. D. Winefordner, Appl. Spectrosc. 54 (2000) 832. [4] F. Y. Yueh, J. P. Singh and H. Zhang, Encyclopedia of Analytical Chemistry, Wiley, New York (2000). [5] C. F. Su, S. Fang, J. P. Singh, F. Y. Yueh, J. T. Rigsby III, D. L. Monts, and R. L. Cook, Glass Technol. 41 (2000) 16. [6] A. K. Rai, F. Y. Yueh and J. P. Singh, Rev. Sci. Instrum., 73 (2002) 3589. [7] A. K. Rai, F. Y. Yueh and J. P. Singh, Appl. Opt., 42 (2003) 2078. [8] V. Sturm, L. Peter and R. Noll, Appl. Spectrosc, 54 (2000) 1275. [9] D. A. Rusak, B. C. Castle, B. W. Smith and J. D. Winefordner, Anal. Chem., 27 (1997) 257. [10] A. K. Rai, V. N. Rai, F. Y. Yueh and J. P. Singh, Trends in Appl. Spectrosc., 4 (2002) 165. [11] J. M. Gumba, C. D. Angelo, D. Bertuccelli, and G. Bertuccelli, Spectrochim. Acta B56 (2001) 695.

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[12] M. Hemmerlin, R. Meil, R. Hazlett, J. Martin, T. Pearee, and A. Zigler, Spectrochim. Acta B56 (2001) 707. [13] B. L. Drogoff, J. Margot, M. Chaker, M. Sabsabi, O. Barthelemy, T. W. Johnston, S. Laville, F. Vidal, and Y. V. Kaenel, Spectrochim. Acta B56 (2001) 987. [14] S. Rosenwasser, G. Asimellis, B. Bromley, J. D. Caceres, and A. Gonzales Urena, Spectrochim. Acta B56 (2001) 865. [15] O. Samek, D. C. S. Beddows, H. H. Tella, J. Kaiser, M. Liska land, H. Falk, P. Wintjens, and L. Paulard, Spectrochim. Acta B56 (2001) 661. [16] A. Uhl, K. Loebe, and L. Kreuchwig, Spectrochim. Acta B56 (2001) 795. [17] G. Zikratov, R. Vasudev, F. Y. Yueh, J. P. Singh, and J. C. Mara Glass Technol, 40 (1999) 84. [18] A. V. Pakhomov, W. Nichols & J. Borysow Appl Spectrosc, 50(1996) 880. [19] I. Gobernado-Mitre, A. C. Prietro, V. Zafiropoulos, Y. Apetsidou, and C. Futakis, Appl. Spectrosc., 51 (1997) 1125. [20] R. Salimbeni, R. Pini, S. Siano, Spectrochim. Acta B56 (2001) 877. [21] R. Krasniker, V. Bulatov, and I. T. R. Lorce, Appl. Spectrosc., 38 (1984) 721. [22] V. Majidi and M. R. Joseph, Spectroscopic Sehechter, Spectrochim. Acta B56 (2001) 609. [23] D. A. Cremers, L. J. Radziemski, Applications of Laser Induce Plasma, Critical Reviews in Analytical Chemistry, 23 (1992) 143. [24] K. Y. Yamamoto, D. A. Cremers, M. J. Ferris and L. E. Foster, Appl. Spectrosc, 50 (1996) 222. [25] R. Barbini, F. Colao, R. Fantoni, A. Palucci, S. Ribezzo, H. J. L. Van der Steen and M. Angelone, Appl. Phys., 65 (1997) 101. [26] B. J. Marquardt, D. N. Stratis, D. A. Cremers and S. M. Angel, Appl. Spectrosc., 52 (1998)1148. [27] R. E. Neuhauser, U. Panne and R. Niessner, Analytical Chemica Acta 392 (1999) 47. [28] R. E Neuhauser, U. Panne and R. Niessner, Appl Spectrosc, 54 (2000) 923. [29] D. A. Cremers, J. E. Barefield II and A. C. Koskelo, Appl. Spectrosc. 49 (1995) 857. [30] A. I. Whitehouse, J. Young, I. M. Botheroyd, S. Lawson, C. P. Evans and J. Wright, Spectrochim. Acta B56 (2001) 821. [31] A. K. Rai, H. Zhang, F. Y. Yueh, J. P. Singh and A. Wiseburg, Spectrochim. Acta B56 (2001) 2371. [32] H. R. Griem, Plasma Spectroscopy, McGraw Hill, New York (1964). [33] R. A. Multari, L. E. Foster, D. H. Cremers and M. J. Ferris, Appl. Spectrosc. 50 (1996) 1483. [34] L. Paksy, B. Nemet, A. Lengyel, L. Kozma and J. Czevkel, Spectrochim. Acta B51 (1996) 279. [35] J. Gruber, J. Heitz, H. Strasse, D. Bauerle and N. Ramaseder, Spectrochim. Acta B56 (2001) 685. [36] R. Noll, H. Bette, A. Brysh, M. Kraushaar, I. Monch, L. Peter and V. Sturm, Spectrochim. Acta B56 (2001) 637. [37] M. Kuzuya, H. Matsumoto, H. Takechi and O. Milkami, Appl. Spectrosc, 47 (1993) 1659. [38] H. Zhang, A. K. Rai, J. P. Singh and F. Y. Yueh, Fiber optic laser-induced breakdown spectroscopy probe for molten material analysis. Patent No. 6762835 (2004).

Chapter 12

LIBS Technique for Powder Materials Bansi Lala , L. St-Ongeb , Fang-Yu Yuehc and Jagdish P. Singhc a

Centre for Laser Technology, Indian Institute of Technology Kanpur, Kanpur 208016, INDIA National Research Council Canada, Industrial Materials Institute, 75 de Mortagne Blvd, Boucherville, Québec J4B 6Y4, CANADA c Institute for Clean Energy Technology, Mississippi State University, Starkville, Mississippi 39759, USA b

1. INTRODUCTION Powder materials both granular as well as fine powder represent the most common form of raw material in the industry world-wide. Industries like chemical, pharmaceutical, glass, ceramic, food, mining, metallurgy, construction and many others use the powder material continuously in their applications and processes. Most of the time the powder material used in an industrial application is a mixture of various pure chemicals and the quality of the end-product invariably depends on the composition of the mixture being used necessitating the on-line/in-situ monitoring of the elemental composition of the powder material before it is fed into a process. This on-line/in-situ monitoring of elemental composition can be also helpful in resolving the environmental issues by identifying the pollutants before starting of the process through which the powder material has to undergo. A large number of analytical techniques like wet chemistry, infrared/visible/ultraviolet absorption/fluorescence spectrometry, light scattering, chromatography, continuous/pulsed NMR, mass spectrometry and X-ray diffraction/fluorescence can be used to monitor the elemental composition of the powder material. State of the art instruments based on these techniques are available commercially with enough speed and sophistication of data collection required to meet the ever increasing demand for higher sensitivity, selectivity, precision, accuracy and number of samples to be processed. However, almost all these techniques need sample preparation and most of the operating costs and work activity are spent in sample preparation for injection into a measurement device. The operating costs are further escalated due to waste storage, segregation and disposal of the chemicals/solvents used for sample preparation. Hence, there is need for a better on- line/in-situ analytical technique which does not need any sample preparation; products from the various stages of an assembly line can be directly checked. Laser induced breakdown spectroscopy (LIBS) is almost an ideal technique for such applications as it needs minimal sample preparation, results are Laser-Induced Breakdown Spectroscopy Jagdish P Singh, Surya N Thakur (Editors) © 2007 Elsevier B.V. All rights reserved.

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obtained in a few seconds (results in principle are available with a single laser pulse) and several elements can be monitored simultaneously. Also, the data collection techniques of LIBS are sophisticated enough to be used for the control of the process in which powder material is used. No doubt the speed and the data collection sophistication of LIBS is comparable/even better than the techniques listed above, yet the precision of LIBS in general and more-so with powder materials, is less than the other available analytical techniques. This chapter summarizes the optimization of various experimental conditions of the LIBS technique so as to obtain the best possible reproducible data in case of powder materials. Also, application of LIBS to pharmaceutical and glass industry are discussed as both these industries use raw material in the form of powders extensively.

2. LIBS TECHNIQUE FOR POWDER MATERIALS In LIBS technique a high energy laser pulse when focused on the sample of interest results in the formation of micro-plasma, characteristic of the sample composition. The emission from this micro-plasma is analyzed for the quantitative determination of the elemental composition of the sample. This micro-plasma formation is accompanied by the generation of a high-pressure absorption wave (shock wave) whose propagation is a function of the incident laser energy [1]. This shock wave is the cause of highly inaccurate data in the case of powder samples because: 1. the surface of the powder sample is disturbed so that the focal spot of the laser is not same for the subsequent laser pulses and 2. powder is ejected out due to shock wave (aerosol production) so that a varying part of laser pulse is absorbed in front of the sample. Both these factors cause pulse-to-pulse fluctuations in the irradiance level resulting in the poor reproducibility of the LIBS data. Wisburn et al [2] investigated the quantity of heavy metals in soils, sand and sewage sludge using powder samples. The LIBS data they obtained has a relative standard deviation (RSTD = 100×standard deviation/mean) of about 25% which they explained in terms of persistent aerosols and relatively bigger crater formation. Pascal et al. [3] used an echelle spectrometer based portable LIBS instrument for the analysis of powder soil samples but could not quantitatively interpret their results. Similar observations have been reported by Lal et al. [4] while applying LIBS for the determination of the elemental composition of glass batch. On the other hand, dramatic change in the reproducibility of the LIBS data by using pellets instead of powder samples has been reported by several workers. Martin et al. [5] employed LIBS to determine the concentration of carbon and nitrogen in a variety of soil samples in pellet form. Rosenwasser et al. [6] used LIBS for quantitative analysis of phosphate ores and they reported RSTD of about 3.79% with pellet samples. Krasniker et al. [7] used polyvinyl alcohol as binder to prepare soil and sand pellets for the investigation of matrix effects in LIBS. Lal et al. [8] have investigated the effect of various parameters on the accuracy of the LIBS data recorded with pellet samples. The various experimental parameters which are to be optimized so as to obtain reproducible data for powder material are discussed in the following sections.

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2.1. Preparation of the Pellets of the Powder Samples The dramatic improvement, as discussed earlier, in the RSTD of LIBS data when the pellets of the powder materials are used has prompted the investigation of the effects of various factors involved in pellet making on the reproducibility of LIBS data. Typically finely ground powder material after thorough mixing with a binder is pressed into a pellet by a die-hydraulic press combination. The degree of roughness of the powder material, nature and amount of the binder, pressure used to pelletize the powder and heat-treatment of the pellets are the experimental parameters affecting the RSTD of the LIBS data. The conclusions of the various studies are summarized as follows:

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(i) The RSTD of LIBS data improves by finer grounding of the powder material [6]. (ii) Improvement in the RSTD of the LIBS data on drying the pellet for about 15 minutes at about 90  C has been reported [8] in the literature. LIBS data collected from one side of the freshly prepared pellet (0.8ml of 2 wt% PVA with 5g of lime) has RSTD of about 17%. After drying the pellet the RSTD reduces to about 5% when data is collected from the other side of the pellet. Similar observations have been reported for mineral ores [6]. (iii) Variation of the RSTD of the LIBS data with the amount and nature of the binder has been investigated in detail in Ref. 8. In this study three types of binders (polyvinyl alcohol, sucrose and starch) have been investigated using industrially important powder materials namely, silica, alumina and lime. The variation in the intensity of the Ca 395.7 nm spectral line as a function of the amount of the polyvinyl alcohol (PVA) is shown in Fig. 1. The relative standard deviation (RSTD) of the same emission spectral line is also plotted as a function of the PVA amount added to the powder as binder. As shown in Fig. 1, the emission intensity observed in case of pellet with 0.8ml binder is more than 1.5 times of that of

0 0

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Fig. 1. The intensity of Ca 395.7 nm line and the relative standard deviation of the data as a function of the amount of polyvinyl alcohol (PVA) added as a binder to 5g of lime. 24 MPa has been used to make the pellets.

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the pellet with no binder. On the other hand, there is decrease in RSTD with the increase in the amount of binder used to make the pellets. The RSTD of the Ca 395.7 nm emission line is about 21% when pellets of the lime prepared with no binder are used to record LIBS spectra. It is about 5% for the same emission line when the pellets are made from 5g of lime mixed thoroughly with 0.8ml of PVA. Any further increase in the amount of binder used to make a pellet does not decrease RSTD in any significant manner. Similar results have been obtained with silica and alumina powders with PVA (2 wt%) as binder. However, alumina and silica, unlike lime, cannot be pressed into pellets without binder. For alumina the minimum amount of PVA binder required to press 5g of powder into a pellet is 0.2ml and the LIBS data recorded with such a pellet has RSTD of about 47% in the emission line of Al 394.4 nm. The RSTD for the same line decreases to about 5% when LIBS data is recorded from a pellet made from 5g of alumina powder mixed with 0.8ml of PVA. As is the case with lime, no significant decrease in RSTD is observed on further increasing the amount of PVA. On the other hand, the minimum amount of PVA needed to press 5g of silica in a pellet is 0.4ml. The RSTD of Si 390.5 nm emission line recorded from this pellet is as high as 67% which reduces to about 6.5% when a pellet made with 0.8ml PVA is used. Again no significant decrease in RSTD is observed when pellets with PVA more than 0.8ml are used to record LIBS spectrum. In this study 2% by weight of PVA (CAS#9002-89-5, Alfa Aesar) dissolved in distilled water has been used. No significant decrease in RSTD or increase in emission intensity has been observed by further increasing >2% the concentration of PVA. The general trend of increase in the intensity of emission lines and decrease in the RSTD of the data with the increase in the amount of binder up to certain limit beyond which there is no significant increase (decrease) in intensity (RSTD) has been observed for sucrose (CAS# 57-50-1, Alfa Aesar) as well as starch (CAS # 9005-84-9, Alfa Aesar). However, in the case of sucrose (2% wt concentration) the minimum RSTD of Ca 395.7nm emission line observed with 0.8ml added to 5g of lime powder is about 6.5% which does not change significantly by further increase in the amount of sucrose added to make pellets. In the case of alumina the minimum RSTD observed in Al 394.4nm spectral line is about 7% while for Si 390.5nm (from silica) it is about 9%. Both these figures do not change significantly by further increasing the concentration of sucrose from 2 wt%. The general observations about the role of the amount/nature of binder used in pellet making are: (a) The LIBS spectra of the pellets made with 2 wt% solution of PVA in distill water has the lowest RSTD. (b) The lowest RSTD is observed when an optimum amount of PVA is added as binder. This optimum amount depends on the nature of the powder material and has to be found-out experimentally. The reduction in RSTD is mainly because the pellets are more rigid than the powder so that the position of the focal spot is almost unchanged for all the laser pulses arriving at the pellet surface during a data acquisition cycle By increasing the amount of binder, pellets with higher rigidity are obtained. The LIBS data acquired from such pellets is more precise. This dependence of LIBS data accuracy on pellet rigidity is

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10 9 8 7 6 5 4 3 2 1 0 1250

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Fig. 2. Variation of the RSTD of the intensity of Ca 395.7nm with the amount of pressure used to make pellets.

further illustrated in Fig. 2 where RSTD of the acquired data has been plotted as a function of the pressure used to make a pellet. The RSTD changes from about 8.5% to about 5% when the pressure used for the pellet-making is changed from 10MPa to about 24MPa.

2.2. Apparatus The experimental setup used for recording the LIBS spectra of pellet samples is similar to the one used for solid material. A typical setup is shown in Fig. 3. Laser pulses Harmonic separator

Beam dump

Nd: YAG LASER

Optical fiber

L

L

Dichroic mirror

Prism FL (300 mm) Rotary-translation stage mounted sample holder

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Controller

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Pulse generator

Fig. 3. Schematic diagram of the apparatus for recording the LIBS spectra.

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generally from a frequency doubled Nd: YAG laser are focused on the sample using a fused silica convex lens of proper f-number. This focusing lens can also be used to collect the plasma emission which is fed to a spectrometer, generally through a UV grade fiber. The spectrograph could be a Czerny-Turner 0.5m spectrometer fitted with a gated intensified diode array detector (IDAD) or an echelle spectrometer having a gated intensified charge coupled device (ICCD) as optical detector. Both gate delay and gate width are controlled by a pulse generator which is synchronized with the laser. A number of PC software are available for data acquisition and processing. With Czerny-Turner spectrometer, a spectral region covering about 20nm is recorded in a single run while with echelle spectrometer LIBS spectra in a broader region (typically 200–800nm) can be recorded simultaneously. The pellet samples are mounted on a rotating platform.

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Studies have shown [8] that the RSTD of the LIBS data depends on the position of the focal spot on the pellet. The dependence of the intensity of 395.7 nm emission spectral line of Ca from a lime pellet, on the position of the focal spot on the pellet is shown in Fig. 4 alongwith the variation of the RSTD of the same data. The “0” position on the x-axis of both these figures corresponds to the position of focal spot on the surface of the pellet, +ve value to a focal spot above the surface while −ve value corresponds to a focal spot inside the surface of the pellet. These figures show that the intensity of the emission line is maximum for the “0” position of the focal spot while RSTD of the

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Fig. 4. Variation of the intensity and RSTD of Ca 395.7nm emission spectral line with the change in the position of the focal spot. The “0” position corresponds to the focal spot on the surface of the pellet, +ve value to the position of focal spot above the surface while −ve value to the position of the focal spot inside the surface of the sample.

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LIBS data is minimum