Raman Microscopy: Developments and Applications

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Raman Microscopy Developments and Applications Copyright © 1996 Elsevier Ltd. All rights reserved Shortcut URL to this page: http://www.sciencedirect.com/science/book/9780121896904 Edited by: George Turrell and Jacques Corset ISBN: 978-0-12-189690-4

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List of Contributors, Pages xv-xvi

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PDF (70 K) Preface, Pages xvii-xxii, Edgar S. Etz PDF (330 K) Acknowledgements, Pages xxiii-xxviii PDF (272 K) 1 - The Raman Effect, Pages 1-25, George Turrell Abstract | Abstract + References | PDF (1194 K) 2 - Characteristics of Raman Microscopy, Pages 27-49, George Turrell, Michel Delhaye and Paul Dhamelincourt Abstract | Abstract + References | PDF (848 K) 3 - Instrumentation, Pages 51-173, Michel Delhaye, Jacques Barbillat, Jean Aubard, Michel Bridoux and Edouard Da Silva Abstract | Abstract + References | PDF (5081 K) 4 - Raman Imaging, Pages 175-200, Jacques Barbillat Abstract | Abstract + References | PDF (1456 K) 5 - Raman Microscopy and Other Local Analysis Techniques, Pages 201-242, Michel Truchet, Jean-Claude Merlin and George Turrell Abstract | Abstract + References | PDF (1877 K) 6 - Application to Materials Science, Pages 243-287, Paul Dhamelincourt and Shin-ichi Nakashima Abstract | Abstract + References | PDF (1924 K) 7 - Applications in Earth, Planetary and Environmental Sciences, Pages 289-365, Paul F. McMillan, Jean Dubessy and Russell Hemley Abstract | Abstract + References | PDF (4079 K) 8 - Biological Applications, Pages 367-377, Michel Truchet Abstract | Abstract + References | PDF (620 K) 9 - Applications in Medicine, Pages 379-420, Michel Manfait and Igor Nabiev Abstract | Abstract + References | PDF (2303 K) 10 - Applications in Art, Jewelry and Forensic Science, Pages 421-453, Claude Coupry and Didier Brissaud Abstract | Abstract + References | PDF (1413 K) Index, Pages 455-463 PDF (342 K)

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List of Contributors

Jean Aubard, Institut de Topologie et de Dynamique des Systemes (CNRS URA 34), 1, rue Guy de la Brosse, 75005 Paris, France Jacques Barbillat, Laboratoire de Spectrochimie Infrarouge et Raman (CNRS UPR A2631L), Universite des Sciences et Technologic de Lille, 59655 Villeneuve d'Ascq, France Michel Bridoux, Laboratoire de Spectrochimie Infrarouge et Raman (CNRS UPR A2631L), Universite des Sciences et Technologic de Lille, 59655 Villeneuve d'Ascq, France Didier Brissaud, Laboratoire de Police Scientifique, 3, quai de I'Horloge, 75001 Paris, France Claude Coupry, Laboratoire de Spectrochimie Infrarouge et Raman (CNRS UPR A2631T), 2, rue Henri Dunant, 94320 Thiais, France Edouard Da Silva, DILOR, 255 ter, rue des Bois Blancs, 59000 Lille, France Michel Delhaye, DILOR, 255 ter, rue des Bois Blancs, 59000 Lille, France, Emeritus Professor, Universite des Sciencies et Technologic de Lille, 59655 Villeneuve d'Ascq, France Paul Dhamelincourt, Laboratoire de Spectrochimie Infrarouge et Raman (CNRS UPR A2631L), Universite des Sciences et Technologic de Lille, 59655 Villeneuve d'Ascq, France Jean Dubessy, CREGU (GDR CNRS 077). BP-23, 54501 Vandoeuvre-lesNancy, France Edgar S. Etz, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA Russell Hemley, Geophysical Laboratory (CIW), 5251 Broad Branch Road, N.W., Washington, DC 20015, USA Michel Manfait, Laboratoire de Spectroscopic Biomoleculaire, UFR de Pharmacie, Universite de Reims, 51, rue Cognacq-Jay, 51096 Reims, France Paul F. McMillan, Department of Chemistry, Arizona State University, Tempe, AZ 85287, USA

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List of Contributors

Jean-Claude Merlin, Laboratoire de Spectrochimie Infrarouge et Raman (CNRS UPR A2631L), Universite des Sciences et Technologic de Lille, 59655 Villeneuve d'Ascq, France Igor Nabiev, Laboratoire de Spectroscopic Biomoleculaire, UFR de Pharmacie, Universite de Reims, 51, rue Cognacq-Jay, 51096 Reims, France Shin-ichi Nakashima, Department of Applied Physics, Osaka University, Osaka, Japan Michel Truchet, Laboratoire d'Histophisiologie Fondamentale, Universite Pierre et Marie Curie, 12, rue Cuvier, 75005 Paris, France George Turrell, Laboratoire de Spectrochimie Infrarouge et Raman (CNRS UPR A2631L), Universite des Sciences et Technologic de Lille, 59655 Villeneuve d'Ascq, France

Preface

It is no surprise to see the micro-Raman Group at Lille come forth with this timely publication to document the present state of Raman microscopy. A quarter century has passed since the early attempts at Raman microsampling when the field began to merge with, and complement, other microprobe techniques. In the late 1960s to the early '70s, it was mainly the electron beam methods that opened up the microscopic domain to instrumental analysis, aside from classical light microscopy. In this realm, the principal goal was to obtain morphological, structural, and compositional information from the analyzed specimen. Scanning electron microscopes (SEMs), electron microprobes for x-ray microanalysis (EPMA), and analytical electron microscopes (AEMs) furnished detailed images of the sample and elemental compositional data from microscopic sampling volumes, for nearly all of the elements in the periodic table. Yet, at that time, one important piece of information was not available from any of these methodologies: the ability to link the compositional data to the atomic or molecular bonding of the elements, their speciation, such as structural coordination and stoichiometry, as well as crystallographic and amorphous structure. This analytical need for spatially resolved information on structure and bonding of the constituent elements brought forth the development of vibrational microspectroscopy. Infrared spectroscopy, of the non-Fourier transform (FT) variety, was widely used at the time, but infrared microspectroscopy was to fully emerge only in the late 1970s with the increasing use of FT-infrared instrumentation. Since the early 1960s, Raman spectroscopy had experienced a renaissance with the advent of the laser as the ideal excitation source. Laser radiation, from the near-ultraviolet across the visible spectrum, could be focused to the optical diffraction limit, for probe spots competitive with electron probing. Thus, it was then recognized that laser excitation utilizing optimally designed fore-optics and coupled to Raman instrumentation employing various types of sensitive detectors, would make possible Raman microspectroscopy and microscopy. This concept, and its earliest implementation, initiated the new frontiers of molecular Raman probing and imaging, to complement the elemental microprobe techniques with their imaging variants.

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The Laboratory for Infrared and Raman Spectroscopy at the University of Lille (Laboratoire de Spectrochimie Infrarouge et Raman, Universite des Sciences et Techniques de Lille Flandres Artois) took the lead at that time in the exploration and development of the promise of Raman microspectroscopy and microscopy. Other research laboratories, principally in the United States, followed suit and virtually in parallel pursued these approaches that would, well before 1975, demonstrate the utility of Raman microprobing through the use of the first generation of prototype Raman microprobes. In this exciting development of the technique, the workers at Lille were widely recognized as the pioneers of this emerging field. During this same time, the microanalytical techniques of ion microprobe/microscopy (based on secondary ion mass spectrometry, SIMS) were developed in France, and the development of laser microprobe mass spectrometry (LAMMS) was undertaken in Germany. These latter techniques, initially furnishing only elemental composition information at high sensitivities, also had the advantage of allowing for isotopic discrimination. It is against this backdrop of the scientific scene, over the past 25 years, that this book sets the stage for a thorough discussion of the important aspects of Raman microspectroscopy and microscopy. The book is laid out in ten chapters addressing the fundamental principles of Raman spectroscopy, their application to the concept of Raman microanalysis, the design and construction of micro-Raman instrumentation, and the application of such instruments to a broad spectrum of problems in materials science. Throughout these discussions, the contributing authors highlight the unique aspects of the technique, emphasizing their analytical strengths and limitations, and placing the material in the wider context of modern methodologies for comprehensive materials characterization. From this perspective, the topics presented should fulfill a variety of needs facing both the newcomer to the field as well as the researcher famihar with the analytical uses of vibrational spectroscopy, be that in an academic environment or in an industrial laboratory setting. As with any edited book, the reader will note differences in style as well as an in-depth coverage of specific topics presented. This in no way distracts from the value of the book but rather underscores the different levels of utility that can be assigned to the treatment of this subject. Each chapter emerges as an excellent guide to the sub-topic that is presented. Where rigorous treatment is required, as in the discussion of fundamental principles, the criteria of careful optical design in instrumentation, and the demands on analytical performance, the respective authors come forth with authoritative insight. The book becomes especially useful through the extensive references to the published literature. Foremost, the book is intended for the analytical microspectroscopist using the vibrational spectrum (and this does not exclude the professed infrared spectroscopist) as the diagnostic fingerprint. Yet, microscopists from other disciplines will find this

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work directly pertinent. This can be asserted with assuredness, as the modern research environment increasingly relies on all available probe techniques for multidisciplinary materials characterization. A selective focus may be given on the main thrusts presented in the book. The authors clearly did not intend for any chapter to be an all-encompassing text but rather emphasize the key issues and their consequences of analytical importance. Chapter 1 provides a concise treatment of the normal or spontaneous Raman effect in the context of classical light scattering, the excitation of molecular vibrations, and the appearance of the Raman spectrum. Discussed are polarization effects that require careful attention in Raman microsampUng, a theme that recurs in subsequent chapters on the design of microRaman fore-optics and its effects on the observed spectrum. Discussed also are non-linear Raman effects, specifically resonance Raman scattering, as they often come into play in actual measurements. Laser-excited fluorescence and luminescence are acknowledged as being among the most troublesome spectroscopic interferences encountered by the analyst. Much attention is given to this aspect also in the appHcations sections, and various strategies are outlined to either minimize or circumvent these effects that are a potential detriment to successful Raman microanalysis and imaging. Chapter 2 underscores the principal characteristics of Raman microspectroscopy and microscopy. Discussed are the requirements for the efficient excitation and collection of the Raman radiation with respect to the spatially resolved microscopic sampling volume. These considerations are out of necessity linked to various possible constraints, such as those presented by the optical properties of the sample, with special emphasis on the comphcations from optical absorption. The essential features of confocal microscopy are discussed since these represent an important recent development in the optical design and performance of various forms of microscopy which now have been embodied in Raman microscopy as well. The most advanced Raman microprobes/microscopes will feature the confocal characteristics to permit optical sectioning of the sample through efficient spatial filtering and improved depth-of-field. Chapter 3 represents the tour deforce on the broad subject of micro-Raman instrumentation. It may well comprise the central treatise of this book as it addresses a diversity of aspects central to micro-Raman methodology. Tied together, in authentic rigor and detail, are the critical design and performance characteristics of all major systems and sub-systems that comprise the functional Raman system, be it for the recording of microprobe spectra or the acquisition of digital Raman images. The treatment of the various topics is effectively aided by numerous illustrations, mainly in the form of optical schematics and diagrams. Special coverage is given various interferometric techniques utilizing Fourier-transform methods of spectral analysis which

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have entered the micro-Raman field, through the experiences from FTinfrared spectroscopy, since the late 1980s. Thus, FT-Raman microspectroscopy is discussed as a novel approach to more successfully deal with the minimization or elimination of sample fluorescence, through excitation at wavelengths in the near-infrared. Discussed in this same context are the astonishing advances made in recent years in Raman instrumentation based on the development of dispersive Raman spectrometers and spectrographs with superb stray light rejection, through revolutionary methods of optical filtering, and highly efficient energy throughput. These instruments are coupled to high-sensitivity photoelectric detectors (common types are the IPDA, CCD, or CID) to allow for efficient Raman excitation and detection beyond the traditional visible wavelengths, now extending into the nearinfrared. A most interesting and useful extension of this chapter is the discussion of digital signal processing. The basic mathematical relationships are presented for the sampling and processing of analog spectral data, the Fourier-transform treatment, and the practical aspects of the digitization of Raman spectra. Chapter 4 concerns the topic of Raman imaging. Current imaging methods are classified as either 'parallel' or 'direct imaging' or 'series imaging' techniques. The early attempts at direct imaging Raman methods were based on the same principles of image generation as are used in x-ray and ion-probe microanalysis. Regardless of the specific approach at Raman imaging, the object is to obtain 2D- or 3D-images that provide information on the spatial compositional distribution (for purposes of compositional mapping) of one or more components of the sample. In all cases, imaging capabilities built into a micro-Raman system require relatively sophisticated techniques and these have been adapted by commercial instruments over the past five years. The various technologies employed for Raman imaging are in a great state of flux and presently experience profound changes with the remarkable development of holographic filters, the introduction of acousto-optic tunable filters, and the continued improvement of two-dimensional detectors. Current advances in this range of technologies now permit true confocal image generation and good image contrast formation for at least major constituents of a sample matrix. The topic of Raman microscopy in combination with other microanalysis techniques is discussed in Chapter 5. The French workers have always toyed with the idea of combining two or more microprobe principles and embodying them in the same instrument. These possibihties have been explored and their implementation worked out in considerable detail for the union of Raman microscopy with: (i) electron microscopy/x-ray microanalysis; (ii) ion microprobe mass spectrometry; and (iii) laser microprobe mass spectrometry.

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The conceptualization of these conjugated microanalytical techniques presents no great intellectual challenge. However, their realization and implementation into a practical analytical tool is quite another. ReaUstic design considerations and performance attributes are set forth for each of these dual-use microprobe systems, the aim being not to compromise the performance of either the Raman probe or the second probe function of the coupled instrument. The concept has been fully realized in the construction of a prototype coupled micro-Raman/electron probe instrument. The other two variants of a dual-use microprobe system have so far remained on the drawing board, though there are no outright technical obstacles that would prevent the construction of prototypes to demonstrate feasibiUty. The remaining five chapters of this volume cover the appUcations of Raman microscopy in various fields of chemistry and physics, the geological and environmental sciences, biology and medicine, and closing with a chapter outside the typical realm of either the natural or life sciences. This last chapter gives selected examples from the investigation of art objects, the characterization of gems, and cases from forensic science. Micro-Raman researchers, from the early days, have attempted to explore the full range of analytical applications, in part to define the limitations of the technique. These efforts, from laboratories world-wide, have resulted in an extensive pubUshed Uterature. In their task to review and focus on specific fields of application, the authors had to be selective in their choice of discussion topics and in the extent and depth of coverage. In great measure, this goal has been achieved, so that the broad spectrum of applications fully documents the wide-ranging analytical utility of Raman microscopy. It is not possible here to even provide a narrow, Umited focus on several of the areas chosen. The appUcations delve into formidable research problems from the realm of high-technology materials, such as high-Tc superconductors, to probing the molecular make-up of single living cells. Even the reader who brings only a limited understanding to one or more of these areas of application, will find much useful, and often tantalizing, information in the coverage of these chapters. In closing this overview of the book, some concluding comments may be passed on. The period of development of Raman microscopy represents an interesting time for anyone associated with modern methods of microanalysis applied to materials science. The Lille pioneers of this field were greatly helped, along these avenues of progress, by many other researchers world-wide. In summary, then, as is reflected by this book, the present state of Raman microscopy is the outcome of an exciting and intensive team effort, marked by plenty of cross-fertilization. In view of this, one can conclude that the future of this field will hold no lesser accomplishments and discoveries. I thank the editors of this book, and the contributing authors, for allowing

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me an early look at this work and giving me the opportunity to comment, in this Preface, on the content of this volume. Edgar S. Etz Chemical Science and Technology Laboratory National Institute of Standards and Technology Gaithersburg, Maryland 20899, USA April 1996

Acknowledgements

The Editors wish to thank all of the contributors to this volume. Their efforts have made it possible for us to cover the field of Raman microscopy as widely as we are able. We are honored that Dr Edgar Etz of the National Institute of Standards and Technology, one of the founders of this spectroscopic technique, has accepted to write the Preface. The aid in the production of this work which was provided by many members of the research and technical staff of our laboratory is greatly appreciated. We are especially indebted to Professor Paul Dhamelincourt for his contributions to this book and for his invaluable editorial help. Thanks are also extended to Mme Irene Lepreux, who typed many of the contributions - some from handwritten texts. The publisher and the authors wish to thank the following copyright holders who have kindly granted permission to reprint or adapt the illustrations cited. Chapter 2 Figures 1-5 are reproduced by permission of Springer-Verlag GmbH & Co. KG, from G. Turrell (1989). In: Practical Raman Spectroscopy, D. J. Gardiner and P. R. Graves (eds), chapter 2. Chapter 3 Figures 51-53 are reproduced by permission of DILOR from Technical Documentation (1991). Chapter 4 Figures 1, 2, 7 and 9 are reproduced by permission of John Wiley & Sons, Ltd from Barbillat, J., Dhamelincourt, P., Delhaye, M. and Da Silva, E. (1994). /. Raman Spectrosc. 25, 3. Figure 3 is reproduced by permission of Hiithig & Wepf Verlag, Basel, Switzerland, from Batchelder, D. N., Cheng, C , Miiller, W. and Smith, B. J. E. (1991). Makromolekulare Chemie-Makromolecular Symposia 46, 171.

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Figures 4 and 13 are reproduced by permission of the Society for Applied Spectroscopy from Puppels, G. J., Grond, M. and Greve, J. (1993). Appl Spectrosc. 47, 1256. Figures 5 and 17 are reproduced by permission of the Society for AppHed Spectroscopy from Treado, P. J., Levin, I. W. and Lewis, E. N. (1992). Appl. Spectrosc. 46, 1211. Figure 6 is reproduced by permission of the Society for AppUed Spectroscopy from Battey, D. E., Slater, J. B., Wludyka, R., Owen, H., Pallister, D. M. and Morris, M. D. (1993). Appl. Spectrosc. 47, 1913. Figures 8 and 12 are reproduced by permission of John Wiley & Sons, Ltd from Bowden, M., Gardiner, D. J., Rice, G. and Gerrard, D. L. (1990). /. Raman Spectrosc. 21, 37. Figure 10 is reproduced by permission of Elsevier Science Ltd from Treado, P. J. and Morris, M. D. (1990). Spectrochim. Acta 13, 355. Figure 11 is reproduced by permission of S. Hirzel Verlag GmbH & Co. from Dhamelincourt, P. and Bisson, P. (1977). Microscop. Acta 79, 267. Figure 14 is reproduced by permission of the Society for AppUed Spectroscopy from Batchelder, D. N. and Cheng, C. (1993). Appl. Spectrosc. 47, 922. Figure 16 is reproduced by permission of the Society for AppUed Spectroscopy from Treado, P. J., Govil, A., Morris, M. D., Sternitzke, K. D. and McCreery, R. L. (1990). Appl. Spectrosc. 44, 1270. Chapter 5 Figures 18-22 are reproduced by permission of Springer-Verlag GmbH & Co. KG, from Turrell, G. (1989). In: Practical Raman Spectroscopy, D. J. Gardiner and P. R. Graves (eds), chapter 2. Chapter 6 Figures 9 and 10 are reproduced by permission of the publication board of the Japanese Journal of Applied Physics, from Mizoguchi K. Nakashima S., Fujn, A., Mitsuishi, A., Miromoto, H., Onada, H. and Kato, T. (1987) Jpn J. Appl. Phys. 26, 903. Figure 11 is reproduced by permission of the American Institute of Physics, from Mizoguchi, K., Harima, H., Nakashimi, S. I. (1995). J. Appl. Phys. 11, 3388. Figures 12 and 13 are reproduced by permission of the American Institute of Physics, from Nakashima, S., Inoue, Y. and Mitsuishi, A. (1984). / . Appl. Phys. 56, 2989. Figure 14 is reproduced by permission of the publication board of Oyo-Buturi from Mizoguchi, K., Nakashima, S., Inoue, Y., Miyauchi, M. and Mitsuishi, A. (1986). Oyo-Buturi 55, 73. Figures 18 and 19 are reproduced by permission of the American Institute of Physics from Nakashima, S., Yugami, H., Fujii, A., Hangyo, M. and Yamanaka, H. (1988). /. Appl. Phys. 64, 3067.

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Chapter 7 Figure 1 is reproduced by permission of Springer-Verlag GmbH & Co. KG, from Mernagh, T. P. and Liu, L. G. (1991). Phys. Chem. Minerals 18, 126. Figure 2 is reproduced by permission of Macmillan Magazines, Ltd, from Smith, D. C. (1984). Nature 310, 641. Figure 3 is reproduced by permission of Springer-Verlag GmbH & Co. KG, from McMillan, P. F., Wolf, G. H. and Lambert, P. (1992). Phys. Chem. Minerals 19, 71. Figure 4a,b is reproduced by permission of Springer-Verlag GmbH & Co. KG, from Velde, B., Syono, Y., Kikuchi, M. and Boyer, H. (1989). Phys. Chem. Minerals 16, 436. Figure 4c is reproduced by permission of the American Geophysical Union from Velde, B. and Boyer, H. (1985). /. Geophys. Res. 90, 3675. Figure 5 is reproduced by permission of Pergamon Press, Ltd, from Virag, A., Wopenka, B., Amari, S., Zinner, E., Anders, E. and Lewis, R. L. (1992). Geochim. Cosmochim. Acta 56, 1715. Figure 6 is reproduced by permission of Plenum Publishing Co. from Etz, E. S., Rosasco, G. J. and Blaha, J. J. (1978). In: Environmental Pollutants, T. Y. Toribara and J. R. Coleman (eds), p. 413. Figures 7a and 8 are reproduced by permission of the Americal Geophysical Union from Hemly, R. J. (1987). In: High-pressure Research in Mineral Physics, M. H. Manghnani and Y. Syono (eds), pp. 347 and 355. Figure 7b is reproduced by permission of the Mineralogical Society of America from McMillan, P. and Akaogi, M. (1987). Am. Mineral. 72, 361. Figure 7c is reproduced by permission of Springer-Verlag GmbH & Co. KG, from McMillan, P. and Ross, N. L. (1987). Phys. Chem. Minerals 14, 225. Figure 7d is reproduced by permission of Springer-Verlag GmbH & Co. KG, from McMillan, P., Akaogi, M., Ohtani, E., Wilhams, Q., Nieman, R. and Sato, R. (1989). Phys. Chem. Minerals 16, 428. Figure 7e is reproduced by permission of the Americal Geophysical Union from Hemley, R. J., Cohen, R. E., Yeganeh-Haeri, A., Mao, H. K., Weidner, D. J. and Ito, E. (1989). In: Perovskite: A Structure of Great Interest to Geophysics and Materials Science, A. Navrotsky and D. J. Weidner (eds), p. 35. Figure 9 is reproduced by permission of the American Chemical Society from Sato, R. K. and McMillan, P (1987). /. Phys. Chem. 91, 3494. Figure 10 is reproduced by permission of the American Geophysical Union from Gillet, P., Richet, P., Guyot, F. and Fiquet, G. (1991). /. Geophys. Res. 96, 11 805. Figure 11a is reproduced by permission of the Mineralogical Society of America from McMillan, P. (1985). In: Microscopic to Macroscopic.

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Atomic Environments to Mineral Thermodynamics, S. W. Kieffer and A. Navrotsky (eds), P. H. Ribbe, Reviews in Mineralogy 14, 9. Figure llb,c is reproduced by permission of the American Physical Society from Shapiro, S. M., O'Shea, D. C. and Cummins, H. Z. (1967). Phys. Rev. Lett. 19, 361. Figure 12 is reproduced by permission of Scanning Microscopy International from Beny-Bassez, C. and Rouzaud, J. N. (1985). Scanning Elec. Micros. 1, 119. Figure 13 is reproduced by permission of the American Physical Society from Hemley, R. J. and Mao, H. K. (1988). Phys. Rev. Lett. 61, 857. Figures 14 and 21 are reproduced by permission of E. Schweizerbart'sche Verlagsbuchhandlung from Dubessy, J., Boiron, M. C , Moissette, A., Monnin, C. and Sretenskaya, N. (1992). Eur. J. Mineral. 5, 885. Figure 15 is reproduced by permission of Pergamon Press, Ltd, from Schiffries, C. M. (1990). Geochim. Cosmochim. Acta 55, 721. Figure 16 is produced by permission of the Mineralogical Society (UK) from Guilhaumou, N., Jouaffre, D., Velde, D. and Beny, C. (1990). Bull. Mineral. I l l , 517. Figure 17 is reproduced by permission of Pergamon Press, Ltd, from Pironon, J., Sawatzki, J. and Dubessy, J. (1991). Geochim. Cosmochim. Acta 55, 3885. Figure 18 is reproduced by permission of Elsevier Science Pubhshers from Zhang, Y. G. and Frantz, J. D. (1992). Chem. Geol. 100, 51. Figure 20 is reproduced by permission of San Francisco Press, Inc., from Pasteris, J. D., Seitz, J. C , Wopenka, B. and Chou, I.-M. (1990). In: Microbeam Analysis, R. Geiss (ed.), p. 228. Figure 22 is reproduced by permission of E. Schweizerbart'sche Verlagsbuchhandlung from Dubessy, J., Poty, B. and Ramboz, C. (1989). Eur. J. Mineral. 1, 517. Figure 23a is reproduced by permission of Pergamon Press, Ltd, from McMillan, P., Piriou, B. and Navrotsky, A. (1982). Geochim. Cosmochim. Acta 46, 2021. Figure 23b is reproduced by permission of the American Institute of Physics from Furakawa, T., Fox, K. E. and White, W. B. (1981). /. Chem. Phys. IS, 3226. Figure 23c is reproduced by permission of the Societe Frangaise de Mineralogie et Cristallographie from McMillan, P. and Piriou, B. (1983). Bull. Mineral. 106, 57. Figure 24 is reproduced by permission of the American Physical Society from Hemley, R. J., Mao, H. K., Bell, P. M. and Mysen, B. O. (1986). Phys. Rev. Lett. SI, lAl. Figure 25 is reproduced by permission of the American Institute of Physics from Wolf, G. H., Durben, D. J. and McMillan, P. (1990). /. Chem. Phys. 93, 2280.

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Figure 26 is reproduced by permission of Elsevier Science Publishers from Mysen, B. O. and Frantz, J. D. (1992). Chem. GeoL 96, 321. Chapter 9 Figures l b and 4 are reproduced by permission of Macmillan Press, Ltd, from Puppels, G. J., de Mul, F. F. M., Otto, C , Greve, J., Robert-Nicoud, M., Arndt-Jovin, D. J. and Jovin, T. M. (1990). Nature 347, 301. Figures 2 and 3 are reproduced by permission of Springer-Verlag GmbH & Co. KG, from Puppels, G. J., Olminkhof, J. H. F., Sergers-Nolten, G. M. J., Otto, C., de Mul, F. F. M. and Greve, J. (1991). Exp. Cell Res. 195, 361. Figure 5 is reproduced by permission of the National Academy of Sciences (USA) from Yu, N.-T., Cai, M.-Z., Ho, D. J.-Y. and Kuck, J. F. R., Jr. (1988). Proc. Natl Acad. Sci. USA 85, 103. Figure 6 is reproduced by permission of Academic Press, Inc., from Bot, A. C., Ashkin, A. and Dziedzic, J. M. (1987). Science 235, 1517. Figure 17 is reproduced by permission of the Royal Society of Chemistry from Manfait, M., Morjani, H., Efremov, R., Angiboust, J.-F., Polissiou, M. and Nabiev, I. (1991) In: Spectroscopy of Biological Molecules, R. E. Hester and R. B. Girling (eds), p. 303. Figure 18 is reproduced by permission of the Royal Society of Chemistry from Millot, J.-M., Morjani, H., Aubard, J., Pantigny, J., Nabiev, I. and Manfait, M. (1991). In: Spectroscopy of Biological Molecules, R. E. Hester and R. B. Girling (eds), p. 305. Figures 19 and 20 are reproduced by permission of Springer-Verlag GmbH & Co. KG, from Nabiev, I., Morjani, H. and Manfait, M. (1991). Eur. Biophys. J. 19, 311. Chapter 10 Figures 1, 3, 5, 6, 7 and 9 are reproduced by permission of Palais de la Decouverte (Paris) from Coupry, C. (1992). Revue du Palais de la Decouverte 20(196), 15. Figure 2 is reproduced by permission of Centre d'Etude des Manuscrits, Bibliotheque Royale (Bruxelles) from Guineau, B., Coupry, C , Gousset, M. T., Forgerit, J. P. and Vezin, J. (1986). Scriptorium XL, 157. Figure 8 is reproduced by permission of John Wiley & Sons from Coupry, C , Lautie, A., Revault, M. and Dufilho, J. (1994). / . Raman Spectrosc. 25, 92. Figure 10 is reproduced by permission of Association frangaise de Gemmologie from Dele-Dubois, M. L., Poirot, J. P. and Schubnel, H. J. (1986). Rev. Gemmologie 88, 15. Figure 11 is reproduced from Dubois-Fournier, M. L. (1989). Diplome de Gemmologie, Universite of Nantes, France. Figure 12 is reproduced by permission of Elsevier Science Publishers from

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Dele-Dubois, M. L., Dhamelincourt, P., Poirot, J. P. and Schubnel, H. J. (1986). /. Mol Struct. 143, 135. Figure 13 is reproduced by permission of Association frangaise de Gemmologie from Dele-Dubois, M. L., Dhamelincourt, P. and Schubnel, H. J. (1981). Rev. Gemmologie 63, 11.

1 The Raman Effect George Turrell

I. INTRODUCTION

The subject of this book is the Raman effect, a phenomenon which results from the interaction of hght and matter. In particular, the book is concerned with the small world of microcrystals, microorganisms, microelectronics, etc., some of those areas of application in which Raman microspectroscopy enjoys wide acclaim. This introduction to the various apphcations and methods of this technique therefore includes an analysis of the interaction of electromagnetic radiation with molecular systems. The scattering of light, as a result of its interaction with matter, can be classified as elastic (Rayleigh or Mie-Tyndall scattering) or inelastic (Raman or Brillouin scattering). In the former case the scattered light is observed at the same frequency as the incident Ught. On the other hand, inelastically scattered light, which is detected at different frequencies, constitutes the Raman or Brillouin spectrum of the sample.

II. HISTORY OF THE RAMAN EFFECT

Before addressing the scientific problem at hand, it would seem appropriate to recall its historical background. The inelastic scattering of hght by matter was predicted on theoretical grounds by Brillouin (1922) and by Smekal (1923). It was included in the Kramers-Heisenberg theory of second-order optical phenomena (1925). The first experimental observation of the inelastic scattering of light was made by Raman and Krishnan (1928). The experimental setup consisted of a source - a focused, filtered beam of sunUght; a sample - a large volume of a neat Hquid; and a detector - the human eye! The basic experimental arrangement has not varied significantly since that time.

2

G. Turrell

The first qualitative observations were very rapidly confirmed and placed on a quantitative basis by Cabanes (1928), Landsberg and Mandelstram (1928), Rocard (1928) and by Raman and Krishnan (1929). A complete semi-classical theory of the Raman effect was published a few years later (Placzek, 1934). In spite of the initial excitement over its discovery, in the period between the experiments of Raman and Krishnan and the first use of laser excitation (Porto and Wood, 1962; Stoicheff, 1963), the spectroscopic application of the Raman effect made relatively little progress. Its role was limited to a somewhat esoteric complement to infrared spectroscopy. The renaissance of Raman spectroscopy was inspired by the invention of the laser (Schawlow and Townes, 1958; Maiman, 1960), the ideal source for Raman spectroscopy; through the following years, a considerable number of instrumental developments were made. Among them should be mentioned the fabrication of high-quality holographic gratings, improved detectors including multielement arrays - and efficient computer treatment of experimental data. More recently, the application of Fourier transform methods to Raman spectroscopy has shown considerable promise, particularly in the suppression of interference due to sample fluorescence. The birth of Raman microspectroscopy dates from 1966, when Delhaye and Migeon (1966) published two often overlooked papers in which they pointed out that the intensity of Raman scattered light should not decrease with decreasing sample volume, as might be intuitively expected. In fact, these authors showed that the intensity remains constant with decreasing sample size, down to dimensions determined by the diffraction Hmit, and hence the wavelength of the laser excitation. Within a few years the basic principles of Raman microspectroscopic instrumentation were defined (Hirschfeld, 1973) and, soon after, two different Raman microspectrometer systems were described (Delhaye and DhameUncourt, 1974; Rosasco et al., 1974). The former instrument, which was subsequently commercialized, provides for Raman imaging (mapping), as well as single-point analysis.

III. MECHANISM OF THE RAMAN EFFECT

The Raman effect results from the interaction of vibrational and/or rotational motions of molecules with the electromagnetic radiation, while Brillouin scattering involves the translational motion of molecules in liquids and solids. The latter effect, which produces only very small frequency shifts, and which has not as yet yielded important appHcations, will not be considered in this volume. A simple classical picture of the Raman effect can be obtained by analogy with the ampHtude modulation of a radiofrequency carrier wave by an audio

The Raman Effect

3

signal. The resulting sidebands are similar to the Raman spectrum produced by the combination of the frequencies of molecular vibrations with the frequency of the laser excitation. However, for most purposes a quantum mechanical model is more useful. According to quantum theory, a molecular motion can have only certain discrete energy states. A change in state is thus accompanied by the gain or loss of one or more quanta of energy. A quantum of energy is defined by A£ = hv}^, where h in Planck's constant and v^ is the classical frequency of the molecular motion. The interaction of a molecule with electromagnetic radiation can thus be analyzed in terms of an energy-transfer mechanism. For example, the simplest absorption process involves the gain of a quantum of energy by the molecule, accompanied by the annihilation of a quantum of light or photon. Similarly, spontaneous emission can be described as the creation of one or more photons due to the corresponding loss in molecular energy. Scattering processes involve at least two quanta acting simultaneously in the light-matter system. Simple elastic scattering occurs when a quantum of electromagnetic energy is created at the same time that an identical one is annihilated. Thus, the molecule is unchanged by the event. In the case of an inelastic process such as the Raman effect, the two photons are not identical and there is a net change in the state of the molecule. If, for example, the created photon is less energetic than the annihilated one, the scattered light is observed at a frequency that is lower than that of the incident light. This case is referred to as Stokes Raman scattering. On the other hand, if the created photon is the more energetic of the two, the Raman frequency will be higher than that of the laser and the anti-Stokes spectrum will be produced. The scattering processes described above are illustrated in Fig. 1. The laser excitation at frequency VQ reappears as the relatively strong Rayleigh line. The much weaker Raman 'sidebands' are the result of inelastic scattering by, say, a molecular vibration of frequency v^. It should be emphasized that the efficiencies of these scattering processes are very low. Typically, the intensity of the Rayleigh line is about 10""^ with respect to the incident excitation, while the Raman features are at least another factor of 10"^ weaker. It should be obvious from Fig. 1 that the Raman frequencies can be measured relative to that of the excitation. Thus, the origin of the abscissa scale in Fig. 1 can just as well be placed at the position of the excitation frequency and the Raman frequencies will then appear at ±Vy. In practice, as a vibrational frequency has a value of the order of 10^^ s~^, the frequency values are usually divided by the velocity of fight expressed in cms~^. The resulting quantity is then a wavenumber in units of cm~^, which is defined by P v = Vy/C=

1/Av,

where Ay is the corresponding wavelength.

(1)

4

G. Turrell

> 0)

z

RAYLEIGH

iU

o


f^ FREQUENCY,V

Figure 1 Raman and Rayleigh scattering of excitation at a frequency VQ. A molecular vibration in the sample is of frequency v^.

Again referring to Fig. 1, a single Raman band is shown on each side of the Rayleigh line. This spectrum is representative of the spectrum of a diatomic molecule, which has but one vibrational frequency, in the liquid state, where rotational motion of the molecule is usually suppressed by the intermolecular forces. The spectrum in more conventional form is shown in Fig. 2a. Note that the direction of the abscissa scale has been reversed so that the Stokes spectrum appears on the right-hand (positive) side. Diatomic molecules in the gas phase at moderate pressures rotate freely and the resulting modulation of the Raman bands due to this quantized motion is represented in Fig. 2b. Note here the 'pure rotational' structures on each side of the Rayleigh line, as well as the rotational branches on each side of the vibrational Raman bands. In the solid state the rotational motion of piolecules is usually restricted to oscillatory-type 'librational' modes which

The Raman Effect

5

RAYLEIGH (a) STOKES ANTI-STOKES

A

5^:=^ vo

Vv

-Vv

RAYLEIGH (b) STOKES

ANTI-STOKES

JM Vv

m.

•//AAI>LW

-Vv

Vo WAVENUMBER SHIFT

Av (cm~^)

Figure 2 Raman and Rayleigh spectra typical of a diatomic molecule, (a) In the liquid phase, (b) In a gas, where rotational structure becomes apparent. Note that the abscissa scale is expressed in wavenumbers with respect to the excitation frequency. may appear as weak satellite feaatures. Furthermore, in crystals, splitting of vibrational bands may occur, depending on the structure of the unit cell (Turrell, 1972).

IV. ELECTROMAGNETIC RADIATION AND CLASSICAL LIGHT SCATTERING The propagation of electromagnetic radiation is described by Maxwell's equations, which determine the behavior of the electric and magnetic fields in space and time. For the simple case of a plane-polarized wave in a homogeneous medium these fields have, at a particular instant in time, the

6

G. Turrell

%^!H

Figure 3 A plane-polarized electromagnetic wave in a homogeneous medium.

forms shown in Fig. 3. Here, %, the electric vector, is directed along the X axis and the magnetic vector !H is parallel to the Y axis. The direction of propagation of the wave is specified by the Poynting vector S = %X!H, and is thus in the Z direction. The electric field, which is of primary importance in the analysis of scattering phenomena, can be represented in this case by "^x^ '^A'exp(-a>o/cZ/c) exp[-\a)Q{nZ/c - t)],

(2)

where OJQ = ITTVQ and VQ is the frequency of the light. The real and imaginary parts of the index of refraction of the medium are given by n and K, respectively; c is the velocity of light in free space and t is the time. This radiation is said to be plane polarized in that the electric field is always in the same direction {X). The amplitude of the wave is %^ and the origin of the Z axis is arbitrary. The factor exp[-ict)o(AzZ/c -1)] in Eq. (2) is periodic in both space (Z) and time, and the velocity of propagation of the wave is given by c/n. The factor exp[-a)o KZ/C)] expresses the loss in electromagnetic energy due to absorption by the medium. Thus, the absorption coefficient or absorptivity which enters into a Beer's law analysis of absorption

The Raman Effect

7

pheomena is proportional to K, the imaginary part of the refractive index of the medium. In free space (a lossless medium) n = \ and /c = 0. The effect of the electric field on a molecule is to polarize the electron distribution. Thus, a dipole moment is induced in the molecule. If the electric field is not too strong, the induced moment is given by y^ = ^%,

(3)

where a is the polarizability of the molecule. Under more intense radiation, terms in %^, %^, etc. must be added in order to account for hyper-Raman effects, which will not be considered here. As both |JL and % in Eq. (3) are vector quantities, the polarizability is a tensor. If magnetic phenomena are not involved, it is composed of nine real elements. The form of the tensor depends on the coordinate system chosen and the molecular symmetry. Because both |x and % are time-dependent, the induced dipole moment oscillates in time, leading to emission of radiation - the classical model of the scattering processes. A simpHfied illustration of this model is instructive. The time dependence of ^j^can be represented by, %x= %%:CO%i27Tvt), where the real part of Eq. (2) has been taken. A diatomic molecule vibrates at a frequency v^ and, assuming simple harmonic motion, its internuclear distance can be written in the form q^ = q^ cos {lirv^t), where q^ is the amplitude of vibration. The polarizability, which in this case is simply a scalar quantity, can be expanded as a Taylor series in q^. Then, n

/ da ,

Mvjo oL^+[^\

(4)

qycosilrrv^i),

where higher terms are neglected for small atomic displacements. Substitution of Eq. (4) into Eq. (3) leads to JJL = %%a^ c o s ilTTV^t) + %%{ - ^ 1

(7^ COS ( 2 W Q O COS (2771^^0

(5)

Va=^e(^,G/c)^vib(e^)

(27)

for a particular vibration described by the normal coordinate Qx- The electronic part of the wave function is of course a function of both the electronic coordinates s of the molecule and the 3A^ — 6 normal coordinates Qj^. This function can then be developed in a Taylor series in the normal coordinates. Substitution of this result into Eq. (26), and, with the use of Eq. (8), leads to a general expression for an element of the scattering tensor in the form

({axY)K ^ = TIf/nl wi i m >V J1L^>M2)_ {0\iJ,x ir}{r\iJ-y\0} -—

VQ

+ iF^

The Raman Effect 23 Following Albrecht (1961), this equation can be decomposed into two parts, the 'A term'. (29) which is the first term of Eq. (28), and a 'B term', which is the sum of two essentially equivalent terms, the second and third in Eq. (28). This derivation involves several approximations which will not be discussed here. However, some general conclusions result from this analysis, e.g. significant enhancement of totally symmetric vibrational modes of a molecule occur when: (i) the absorption in an electronic band is strong; (ii) at least some product (1 |r)(r|0) of the Franck-Condon overlap integrals is numerically important; and, (iii) the exciting frequency VQ is close to the absorption band due to the electronic transition considered. This enhancement effect is attributable to the A term [Eq. (29)]. If the normal mode of vibration corresponding to the observed Raman fundamental is not totally symmetric, the first term in Eq. (28) vanishes, and resonance enhancement can only take place with the aid of the B term. In general, B term enhancement is significantly weaker than that arising from the A term. Thus, it is usually the totally symmetric modes of a polyatomic molecule that are susceptible to significant resonance enhancement. It is perhaps not appropriate to discuss here the question of what is referred to as the 'excitation profile' in resonance Raman spectroscopy. This term describes the intensity of Raman scattering as a function of the frequency of the excitation. While it might be assumed on intuitive grounds alone, that the excitation profile should correspond to the electronic absorption spectrum of the molecule, such is not the case. In fact, it has been shown that the shape of an electronic absorption band and the corresponding excitation profile are related by a Kramers-Kronig transform. The general theory of resonance Raman spectroscopy and, in particular, the calculation of the form of the excitation profile, is extremely difficult. The interested reader is referred to the work of Champion and Albrecht (1982).

XI. POSTLOG In this introductory chapter an attempt has been made to summarize those aspects of Raman spectroscopy which are directly involved in Raman microspectroscopic apphcations. The theoretical treatment of the subject, and, in particular, the mathematical developments, have been minimized.

24

G. Turrell

W h e n e v e r possible, basic references have been given in order to allow the interested reader to improve his or her basic understanding of the theory of the R a m a n effect.

REFERENCES Albrecht, A. C. (1961). /. Chem. Phys. 34, 1476. Anderson, P. W. (1949). Phys. Rev. 76, 647. Behringer, J. (1974). J. Raman Spectrosc. 2, 275. Bellamy, L. J. (1975). The Infrared Spectra of Complex Molecules. Chapman & Hall, London. Brawer, S. (1975). Phys. Rev. IIB, 3173. Brillouin, L. (1922). Ann. Phys. (Paris) 88, 17. Cabanes, J. (1928). Compt. Rend. Acad. Sci. Paris 186, 1201. Carey, P. R. (1982). Biochemical Applications of Raman and Resonance-Raman Spectroscopies. Academic Press, Toronto. Champion, P. M. and Albrecht, A. C. (1982). Ann. Rev. Phys. Chem. 33, 353. Damen, T. C , Porto, S. P. S. and Tell, B. (1960). Phys. Rev. 142, 570. Delhaye, M. and Migeon, M. (1966). Compt. Rend. Acad. Sci. Paris 262, 702; 1513. Delhaye, M. and Dhamelincourt, P. (1974). IVth Int. Conf. Raman Spectrosc, Brunswick, ME, USA. Dollish, F. R., Fateley, W. G. and Bentley, F. F. (1974). Characteristic Raman Frequencies of Organic Compounds. John Wiley & Sons, New York. Gordon, R. G. (1966). J. Chem. Phys. 44, 3083; 45, 1649. Gussoni, M. (1980). In: R. J. H. Clark and R. E. Hester (eds). Advances in Infrared Raman Spectroscopy. Heyden, London, ch. 2. Hameka, H. F. (1965). Advanced Quantum Chemistry. Addison Wesley, Reading, MA. Hirschfeld, T. (1973). / . Opt. Soc. Am. 63, 476. Kramers, H. A. and Heisenberg, W. (1925). Z. Phys. 31, 681. Landsberg, G. and Mandelstram, L. (1928). Naturwissenschaften 16, 557; 772. Long, D. A. (1977). Raman Spectroscopy. McGraw-Hill, New York. Loudon, R. (1964). Adv. Phys. 13, 423. Loudon, R. (1965). Adv. Phys. 14, 621. Maiman, T. H. (1960). Nature (London) 187, 493. Miyazawa, T., Shimanouchi, T. and Mizushima, S. (1958). J. Chem. Phys. 29, 611. Placzek, G. (1934). Rayleigh-Streuung und Raman-Effekt. In: E. Marx (ed.), Handbuch der Radiologic. Academische-Verlag, Leipzig, vol. VL2, p. 205. Porto, S. P. S. and Wood, D. L. (1962). /. Opt. Soc. Am. 52, 251. Raman, C. V. and Krishnan, K. S. (1928). Nature 111, 50. Raman, C. V. and Krishnan, K. S. (1929). Proc. Roy. Soc. Lond. Ill, 23. Rocard, Y. (1928). Compt. Rend. Acad. Sci. Paris 186, 1107. Rosasco, G. J., Etz, E. S. and Cassatt, W. A. (1974). IVth Int. Conf Raman Spectrosc, Brunswick, ME, USA. Rothschild, W. G. (1984). Dynamics of Molecular Liquids. John Wiley & Sons, New York. Schawlow, A. and Townes, C. H. (1958). Phys. Rev. 122, 1940.

The Raman Effect

25

Smekal, A. (1923). Naturwissenschaften 11, 873. Sonnich Mortensen, O. and Massing, S. (1980). In: R. J. C. Clark and R. E. Hester (eds), Advances in Infrared Raman Spectroscopy. Heyden, London, vol. 6. Stoicheff, B. (1963). X Colloquium Spectroscopicum Internationale, University of Maryland, June 1962. Spartan Books, Washington, DC. Turrell, G. (1972). Infrared and Raman Spectra of Crystals. Academic Press, London. Turrell, G. (1989). In: D. J. Gardiner and P. R. Graves (eds). Practical Raman Spectroscopy. Springer-Verlag, Berlin. Weiscopf, V. and Wigner, E. (1930). Z. Physik 63, 54. Wilson, E. B. Jr, Decius, J. C. and Cross, P. C. (1955). Molecular Vibrations. McGraw-Hill, New York.

Characteristics of Raman Microscopy George Turrell, Michel Delhaye and Paul Dhamelincourt

I. INTRODUCTION A general description of the Raman effect was presented in the first chapter of this book, and its apphcation to the analysis of microscopic samples was introduced from an historical point of view. In this second chapter those characteristics which differentiate Raman microspectroscopy from the more conventional techniques will be developed in more detail. The important characteristics of Raman microscopy are directly related to two fundamental optical considerations, namely: (i) the focusing of the incident laser excitation on the sample, and (ii) the collection of the scattered light. These aspects of the microscopic apphcation of the Raman effect will be treated in the following two sections. The specific problem of couphng a microscope to a Raman spectrometer is analyzed in Section V. Finally, in Section VI the confocal effect is described in some detail, as it forms the basis of recent advances in Raman instrumentation, including imaging techniques, which are presented in Chapter 4 of this volume.

II. EXCITATION FOCUSING In conventional Raman spectroscopy the exciting laser beam is usually focused on the sample with the use of a lens of 10-30 cm focal length. The

28

G. Turrell et a/.

laser light is then concentrated in a 'focal cylinder' (Long, 1977). A considerable gain in the intensity of Raman scattering is achieved by this process. It has been shown that the effect of focusing the laser beam does not in this case result in a significant depolarization of the excitation (Turrell, 1985). It should be pointed out, however, that the resulting higher irradiance may in some cases damage the sample being studied. When the laser beam is focused by an objective with a high numerical aperture the diameter of the focal region is ultimately determined by the diffraction limit, and hence by the wavelength of the light excitation. The resulting polarization of the excitation at the sample must be reconsidered. It has been shown by Richards and Wolf (1959) that the electric vector of the excitation in the focal region within a sample is given by ^i(/0 + /2( (1)

—±2 »iii Y

-211

COS if/

It has been assumed here that Z is the direction of propagation of the laser beam and that it is plane-polarized in the X direction before passing through the microscope objective. The angle if/ is measured in the counterclockwise sense with respect to the X axis. The integrals involved in Eq. (1) have been derived for the case of a nonabsorbing, isotropic sample of refractive index n in the form (Bremard et al., 1987a) Io(u,v,n)

=2

D{e)sme Jo X cos OJQ

/i(w,u,n) =2

1 m -h n^ cos ^ -f m cos 6-\-m

usin^ sin^^

D(e)sine JO

exp(iw cos ^/sin ^^) cos^^^ ^d^,

sin^ cos 6 J n^ cos 6-^ m

(2)

vsinS sin Srr,

X exp(iw cos 0/sin^ 6^) cos^^-^OdO

(3)

and OrTi

/2(w,u,n) =2

D{e)sine Jo xcos^72

m n^ cos 0 + m

usin^ sin^^

1 cos d-\- m

exp(iw cos O/sin^ 0^) cos^^^ddO,

(4)

which 6 is the angle of incidence of a given ray of the light excitation, D{6) = A^csc^exp(—sin^^/sin^^^), which represents a Gaussian radial distribution in the laser beam, and A'^ is a normalization constant. A point in the focal region is specified by the dimensionless cylindrical coordinates

Characteristics of Raman Microscopy 29 u = kZ^iv?'d^ and v = k{X^ + Y^Y'^ sin 6^, as well as the angle ifj. Here k = lirlX is the wave number, m = V(n^ - sin^ 6) and the J^s are the Bessel functions of the first kind. The integrals defined by Eqs (2-4) are functions of n, the real, isotropic refractive index of the sample.

III. COLLECTION OPTICS Equation (7) of Chapter 1 shows that the total Raman intensity depends on the solid angle O in which the scattered Ught is collected. In the backscattering configuration fl describes approximately the cones of both the incident and scattered hght. When the cone axis is coUinear with Z and the excitation is assumed to be polarized in the X direction, the expression for the relative Raman intensity becomes (Turrell, 1989) 3 = (aj,xA + aj^A

+ alzB)(2Co

+ C2)

+ (ai^x^ + ctYY^ + oti^zB) C2 + (alxA

+ alyA

+ a|z5)4Ci,

(5)

where

J

'*oo roo

\Ij(u,v,n)\^vdvudu,

j =0,1,2,

(6)

0 Jo The parameters A and B, which are obtained by integration over the scattering cone, are defined by re'm /^ 1 \ A = 7T^\ (cos^ ^ + 1) sin 6> d^ = 77^ - - cos 9'^-cos^ 0'^]

(7)

re'm 12 1 \ sin^ ^ d^ = 277^ - - cos ^;„ + - c o s ^ d'rn \

(8)

and ^ = 277^

w h e r e ^ ^ is the effective half angle of the cone. These p a r a m e t e r s are plotted as functions of n in Figs 1 and 2, respectively. T h e values of the integrals Cj are presented in Figs 3-5 as functions of the index of refraction, n. E q u a t i o n (5) serves as the basis for the interpretation of polarization m e a s u r e m e n t s on isotropic media. If the scattered Hght is analyzed in the direction {X) parallel to that of the polarization of the excitation, the intensity of the scattered light is given by

3|| = {a\xA

+ a\zB) (2Co + C^ + {a\xA

+ {alxA-^alzB)^C^.

+ a\zB) C^ (9)

30

G. Turrell et a/.

3

2

Refractive index, n

Figure I

Parameter A as-afunction of refractive index.

On the other hand if the analysis is perpendicular to the direction of polarization of the excitation, the expression for the intensity becomes 3 ^ = (aj^yA + aj,zB){2Co + C2) + {al^yA + a^yzB) C2 + {alyA

+

alzB)AC,.

(10)

Thus, the nonzero values of 5 , Ci and C2 can be used to evaluate the 'polarization leakage' which is observed in the Raman spectra of single-crystal samples. This problem has been recently summarized (Turrell, 1989). For gaseous and liquid samples the intensities of the scattered light are

Characteristics of Raman Microscopy

31

10

2

3

4

Refractive index, n

Figure 2 Parameter fi as a function of refractive index.

then given in terms of the tensor invariants X*', S^ and 2^ [Eqs (12-14) Chapter 1] by 3|| = [\A1P

+ iSS^ + {U

+

TOS)22]2CO

+ [152° + \AV + iU + ^5)22]4Ci + [M2« + {\A +\B)X^ + i^A + \B) 22] 4C2

(11)

and 3_L=(|2i + Ji22)(^ + B)2Co + [i522 + iA2i + {^oA + *5)22]4Ci + [1^2" + (lA +15) 21 + (^^ + IB) 22] C2,

(12)

respectively. The depolarization ratio defined in Eq. (19) of Chapter 1, by p = 3jy3|| is then calculated from these relations. Equations (9) and (10) have been employed in the interpretation of Raman spectra of both isotropic and anisotropic samples. In the case of isotropic

32

G. Turrell et a/.

Co

2

3 Refractive index, n

Figure 3 Parameter Q as a function of refractive index.

samples correct depolarization ratios can be obtained even when objectives with high numerical aperture are used. However, as a beamsplitter is usually included in the optical system, its transmission characteristics must be evaluated and appropriate corrections introduced (Bremard et al., 1985). If the sample is optically anisotropic, the analysis is considerably more complicated. However, the depolarization effects introduced by the wideangle objectives are not large. They can be evaluated theoretically if the birefringence of the sample is negligible (Bremard et al., 1987b). The depolarization due to highly convergent incident and divergent scattered light is especially important if propagation is in a direction close to that of an optical axis of a crystalline sample. This effect can be minimized by reducing

Characteristics of Raman Microscopy

1

2

33

3

Refractive index, n

Figure 4 Parameter Ci as a function of refractive index. the optical path (depth of focus) within the sample (Bremard et al., 1989). The apphcation of the above analysis in resonance Raman spectroscopy has also been demonstrated (Bremard et al., 1986, 1987a). In this case the nonvanishing of the tensor invariant S^ often results in so-called inverse polarization, in w^hich p becomes very large (cf. Chapter 1, Section X).

IV. ABSORBING SAMPLES

The geometrical problems associated v^ith sample illumination and the collection of Raman-scattered hght were analyzed in the two previous

34

G. Turrell et al.

2

3

4

Refractive index, n Figure 5 Parameter C2 as a function of refractive index.

sections. However, the sample was assumed to be transparent, i.e. nonabsorbing at both the excitation and scattering frequencies. The case of absorbing samples, which has not as yet been sufficiently studied, presents certain practical difficulties. For the traditional 90° configuration, when employed in the observation of Raman spectra of solutions, it has been demonstrated that there is an optimum concentration. Thus, the relative Raman intensity as a function of concentration displays a maximum (Strekas et al., 1974). A simple model is qualitatively consistent with this observation, although quantitative agreement with the experimental measurements has not been obtained (Renaut et al., 1988). It was shown many years ago by Hendra (1967) that useful Raman spectra of very opaque materials such as coal could be obtained in the backscattering configuration. As it is this geometry that is employed in micro-Raman spectroscopy, this observation has become extremely important. Thus, the

Characteristics of Raman Microscopy

35

OBJECTIVE

/

/

/

/

EXCITATION SCATTERING

I--Figure 6 Backscattering geometry.

problem which is presented by strongly absorbing samples, as observed with a Raman microprobe, will be briefly summarized here. The analysis (Turrell, 1989) is made on the basis of the geometry shown in Fig. 6. The laser excitation is assumed to consist of a parallel beam within the focal cyUnder. For a Gaussian radial distribution in the exciting beam, the irradiance at a point within the sample is given by 4 = /e^exp(-C8eZ) exp[-4(Z2 + Y^)ld^].

(13)

The intensity of the light which is scattered by a volume d X d Y d Z within the sample can be expressed in the form d3^a4cdZdydZ,

(14)

and the intensity of the scattered light leaving the sample in the Z direction is then d3s = d3^exp(-c£sZ).

(15)

36

G. Turrell et al.

H ^

8 = 2 X 10'

5

< H

Z ^

2

Figure 7 Raman scattering intensity plotted against concentration (c) of absorbing solutes.

Equations (14) and (15) lead to the relation d% oc /Oc exp [-c(8s + 8^)Z] AZ exp [ - 4 ( ^ 2 + Y^)ld'^] dXdY,

(16)

whose integration yields approximately 3s--

7 r ( l - l / e ) I^d^ ( l - e x p [ - c ( £ e + £s)^])16 e^ + e.

(17)

Here, L is the effective depth of focus (see Fig. 6). This result is plotted in Fig. 7, where it is seen that for a given value oi e = E^-\- e^ that the relative Raman intensity does not decrease at high concentrations. This result, which has been obtained on the basis of a simple model, has been at least semi-quantitatively confirmed by direct measurement (Renaut, 1988). A problem which arises from sample absorption is their degradation under intense laser Hght excitation. This situation is characteristic of the Raman microprobe. The 'burning' of specimens is certainly due to a combination of thermal effects and photochemical decomposition. However, it is a problem which has not as yet received sufficient attention, although it is a very serious one in Raman microscopy.

Characteristics of Raman Microscopy

37

V. MICROSCOPE-SPECTROMETER COUPLING This section will be devoted to the fundamental problems, as well as the practical design, of the coupling optics between a microscope and a spectrometer or spectrograph.

A. Coupling Conditions As will be shown in the following chapter, the optimum use of the Raman light flux ^ collected from a sample requires that it be transmitted from sample to detector via the successive apertures of the instrument. In general, microscope objectives are aplanetic, so that the rule for maintaining the flux (assuming perfect transmission of all optical elements) is to apply the Abbe invariant n^h-^^m^i (see Fig. 8) at each intermediary image of the sample, as well as at each intermediary aperture along the optical path of the instrument. The Abbe invariant rule ^o^o sin ^o = ^\^'\ sin ^ i . . . = ^o/io sin ^o is indeed equivalent to the conservation of the optical extent f/f (see Chapter 3) of the Raman light beam passing through the instrument without loss in flux. This condition can be expressed by multiplying the square of the Abbe invariant by TT. This operation leads to the expression n^TTS^ sin^ ^o = • • • ^"^Sx sin^ ^j = . . . noTr^o sin^ B[ =

(/)/LR,

(18)

where 5-0, s^ and 5o are the areas of the sample, of any intermediary image of the sample and of the image of the sample from or to the entrance sUt of the spectrometer, respectively. In Eq. (18) L R is the brightness of the Raman source at the sample.

MICROSCOPE AND COUPLING OPTICS

SPECTROMETER

Figure 8. Application of the Abbe invariant rule for each intermediary image and pupil throughout the optical path.

38

G. Turrell et al.

B. Design of Coupling Optics The invariance conditions of the optical extent can be fulfilled with the use of coupling optics which result in good matching between all of the apertures along the entire hght path, from microscope to the spectrometer detector. The matching of the apertures means that the coupling system not only has to conjugate optically all of the intermediary images of the sample with the entrance slit but, at the same time, it must conjugate all of the intermediary apertures with the entrance pupil of the spectrometer. Assuming that in a spectrometer the acceptance angle ^o of the first monochromator is small, the following expression can be written (see Fig. 8), sme()^e()--p/2D,

(19)

where p is the effective size of the grating pupil of the monochromator and D the distance between the pupil and the slit. The Abbe invariant noho sin ^o = ^6 sin ^o can be rewritten in the form 7o

2D

where (N. A.) and y^ are the numerical aperture and the magnification factor of the microscope objective, respectively. The magnification factor of the coupling optics is given by y^. In this formula the inequality expresses the result that the pupil of the monochromator must not be overfilled by the light diffused by the sample. The ratio p/D, which is characteristic of the monochromator aperture, is constant for a given instrument. It is then clear that the magnification factor of the coupHng optics is directly related to the characteristics of the microscope objective. There are then several possibiUties for coupling microscopes with spectrometers. They can be summarized as follows. (i) Use an adjustable zoom for any particular objective mounted on the microscope turret, at the expense of poor transmission. (ii) Design as many interchangeable fixed magnification optical systems as needed for all of the objectives mounted on the microscope turret. (iii) Design a unique optical system whose magnification is adapted to a particular objective and which is suitable for insuring both good spatial resolution and maximum collection of Raman light. Generally, this condition is applied to objectives with high N. A. and high magnification, the others mounted on the microscope turret being chosen according to the (N.A.)/yo restriction given by Eq. (20). The last possibility is, in practice, the best one. Indeed, fixed magnification coupling optics are easy to design and have an excellent transmission factor

Characteristics of Raman Microscopy

39

( r > 0 . 9 ) , when constructed with coated lenses. Several lens combinations are possible. A simple three-lens system, which was designed for the first MOLE instrument (Dhamelincourt, 1979, 1982), is presented in Fig. 9 as an example. Ray tracing indicates clearly both the optical couphng of the image of the sample with the entrance sUt and that of the aperture with the entrance pupil of the spectrometer.

MICROSCOPE

COUPLING OPTICS

Figure 9. Example of a three-lens, optical-coupling system. The ray tracing illustrates the matching of the microscope objective and the spectrometer apertures.

VI. CONFOCAL RAMAN MICROSCOPY A. Introduction The concept of confocal scanning microscopy was introduced by Minsky (1988) in the early 1960s to overcome some of the limitations of the conventional optical microscope. With this technique a significant improvement in both the contrast and the spatial resolution may be obtained when a point source is focused at the diffraction hmit onto the specimen, while the enlarged image of the illuminated spot is analyzed through a pinhole diaphragm. Unlike a conventional microscope, where the entire field is illuminated, the confocal system measures at any one time the intensity of the light reflected or transmitted by a very small area of sample. A reconstitution of a two-dimensional image is performed via numerical data treatment of the photoelectric signals resulting from a sequential analysis by an XY-raster scanning of the sample field. A three-dimensional display may also be produced by combining a series of optical sections recorded sequentially by means of a motorized Z-focus attachment. Detailed theoretical and experimental studies of the properties of confocal microscopes may

40

G. Turrell et a/.

be found in books by Wilson and Sheppard (1984, 1989) and Wilson (1990). Without entering into complicated calculations, the essential features of confocal microscopy may be summarized as follows: (i) An exact optical conjugation onto the sample of the pinhole apertures which are employed for both illumination and detection of the specimen results in combined spatial filtering effects. They produce a narrower 'point spread function' than can be obtained with a conventional microscope. (ii) The stray light background due to the out-of-focus regions of the specimen is strongly attenuated by spatial filtering, so that the main contribution to the signal comes selectively from a thin layer of sample close to the exact focal plane. This capability of optical sectioning undoubtedly constitutes the most important advantage of the confocal configuration, which benefits from both the contrast enhancement and the improved depth of field. The application of the confocal principle to Raman spectroscopy is not straightforward. In fact the first experimental Raman microprobes, conceived and developed in the early 1970s (Delhaye and Dhamelincourt, 1975; Rosasco, 1980), effectively employed two conjugated spatial-filtering diaphragms, one for laser excitation and the other for measurement of Raman scattered light. However, the inherent weakness of Raman signals and the lack of sensitivity of the existing photodetection systems available at that time did not allow the reduction of the pinhole diameters to achieve the diffraction limit. Consequently, the conditions required to obtain optical sectioning with a micrometer axial resolution were not fulfilled. Another limitation, which is particularly severe for Raman microprobing, originates from the very intense laser radiation which is, for the most part, elastically backreflected by the specimen. Unlike the usual scanning, optical microscopes, a Raman instrument must necessarily discriminate between the weak spectral signals and the signal due to the nearby extremely strong excitation radiation. Now, almost 20 years after the appearance of the first prototypes of Raman microprobe instruments, especially designed and optimized systems which benefit from the dramatic improvements in multichannel photodetectors and spectral filtering, have been described (Puppels et al., 1991; Tabaksblat et al., 1992; Barbillat et al., 1992a; Dhamelincourt et al., 1993). Unlike the first generation of Raman microspectrophotometers in which most instrument makers simply added a microscope attachment to an existing monochromator, the commercial instruments which are now available provide the long-term accuracy and stabihty of the optical adjustments that are absolutely necessary to obtain high spatial resolution and depth

Characteristics of Raman Microscopy

41

EYEPIECE

COUPLING OPTICS D2

I

REMOVABLE MIRROR I

PINHOLE SPATIAL FILTER

PINHOLE SPATIAL FILTER

SPECTROMETER ENTRANCE SLIT

BEAMSPLITTER

MICROSCOPE OBJECTIVE

SAMPLE

Figure 10. Schematic diagram of the laser focusing, sample viewing and scattered light collection geometry which is widely employed in micro-Raman spectrometers. discrimination (DILOR, 1991; Delhaye et aL, 1992; Barbillat et al, 1992b, 1994; Manfait et al, 1992). The main features of a Ramann confocal system can be described by the basic layout shown in Fig. 10. The laser excitation beam is first filtered by an illumination pinhole Di. This initial spatial filtering removes the appearance of diffraction rings and speckle noise around the focused spot and results in a clean point source waist which is imaged onto the sample. The scattered Raman radiation is collected by a wide-aperture objective and focused on an adjustable pinhole D2 placed in the image plane of the microscope. A beamsplitter insures a coaxial illumination and Hght collection by the same objective in the backscattering configuration. The pinholes Di and D2 are called confocal diaphragms. Their exact optical conjugation with the point source in the object plane ensures that only the light originating from the sample region, which coincides exactly with the illumination spot, is transmitted to the spectral analyzer and detector. The two effects of spatial filtering, both for illumination and collection, multiply and increase the spatial resolution by ehminating stray light coming from the out-of-focus regions of the sample. This configuration accounts for the ability of the system to provide optical sectioning. In Fig. 11 the variation of the signal intensity is plotted as a function of the axial coordinate Z, when a thin layer of sample is displaced along the

42

G. Turrell et al. J PHOTODETECTOR SIGNAL

AXIAL DISTANCE PHOTODETECTOR

I CONJUGATED PINHOLE / 1 A- DIAPHRAGM

/ BEAMSPLITTER

LASER BEAM

/ ^I 7i^-^-AZ / / ^ Y // .*^^ 'f^^ --'•/•--i__/

FOCAl.PLANE . i' z =o

i

i

1^

-^-

I

^

MICROSCOPE OBJECTIVE THIN SAMPLE SLICE

Figure 11. Effect of sample position on the photodetector signal in the confocal optical configuration.

optical axis. The axial resolving power, or depth of focus, is usually defined as the full width at half maximum (FWHM) of this curve. Decreasing the size of the confocal diaphragm improves both the axial resolution and reflection of stray light. In order to offer maximum versatility of the system, modern confocal Raman microscopes are provided with variable aperture pinholes, which enable an optimum adjustment to be made of the recording conditions. The magnification factor of the microscope may be varied, usually by means of interchangeable objectives mounted on a turret. It is worth noting that optimal performance requires a correct matching of all of the apertures and pupils along the entire light path, both in the microscope and in the spectral analyzer section (see Section V.B). Improper beam matching severely limits the capabilities of the instrument. Consequently, the coupling optics which have been designed by the instrument maker are only optimized to fulfill the matching conditions for certain objectives, and for the beam divergence for a given laser excitation. In particular, overfilling or underfilling of the microscope objective by the laser beam modifies the dimension of the focused spot, as well as the radiance distribution, at the expense of the spatial resolution and/or the signal intensity. The critical importance of a correct adaptation of all optical parameters will be emphasized in the next sections of this book. The numerous applications of confocal Raman microprobing and microscopy will be described and references to the appropriate literature will be given.

Characteristics of Raman IVIicroscopy

43

B. The Confocal Effect

The light scattered by the Raman effect (or fluorescence) is incoherent in nature and, therefore, can be treated with the use of conventional optical laws, assuming that diffraction effects are weak. 1. Raman Light Flux Emitted by a Thin Slice of Sample Consider, in the object plane, a very thin slice of an isotropic and homogeneous sample whose surface SQ is defined by the waist of the focused laer beam (Fig. 12). Assuming a radial profile for the laser excitation, this slice can be considered to be a Raman light source emitting uniformly in all directions. The elementary Raman light flux captured by the objective for a backscattering angle 0 and an elementary solid angle dfi can be expressed as d^^Q =

(21)

L^dH,

where L R and ^% are the Raman source luminance (brightness) and the geometric etendue or throughput of the elementary Raman light tube, as defined by 6 and dfl, respectively. dop/odz, dil

LR =

(22)

where (dcr/dfl) is the differential Raman cross-section for a given band and a given exciting wavelength, p is the molecular density (number of molecules per unit volume), /Q is the laser irradiance at the sample (power per unit area) and dz is the slice thickness. The geometrical extent is given by d2^ = d5Cos^dn,

SAMPLE OBJECT SLICE PLANE

OBJECTIVE-TUBE LENS COMBINATION

BEAM SPLITTER

1

ENTRANCE / EXIT PUPIL ^ PUPIL LASER SOURCE

(23)

IMAGE PLANE

ENTRANCE PUPIL OF COUPLESfG OPTICS

Figure 12. Collection by the microscope objective-tube-lens combination of the Raman flux from a sample slice.

44

G. Turrell et a/.

where ds is the elementary surface around one given point of the surface slice, and 6 and dft have been defined above. The total flux entering the instrument is then

dH,

ds

do = ^ R * ^ = ^ R 0

(24)

0

where ^o is the half-acceptance angle of the microscope objective. Assuming now that dz and 5*0 are small compared to the working distance and the frontal lens diameter, respectively, % may be written in the form ds

sin ^ cos ^d^.

(25)

Thus, after integration d(/)o = L^TTSQ sin^ ^o-

(26)

The Raman light flux entering the instrument is proportional to the square of the numerical aperture of the objective (N.A. = n sin OQ, where n is the refractive index of the object-space medium). Therefore, a significant gain in the light collection efficiency for a thin sample can be achieved with the use of objectives with large numerical aperture. 2. Transmission of the Raman Light Flux through a Confocal

Diaphragm

In the image space, the application of Clausius's law to the combination of objective-tube lens, which is equivalent to the condition for the invariance of the optical etendue of the Raman light tube, yields an expression for the flux entering the pupil of the coupling optics as follows: d<j>'o = TL'^%\

(27)

where r is the transmission of the objective-tube-lens combination, L R = L R M ^ (Kirchoffs law) and %' = 7TSQ%\V? OQ. Here SQ and ^o represent the surface of the image of the slice and the half angle subtended by the marginal ray entering the pupil of the transfer (couphng) optics, respectively. Thus, d'o = /o5o| - ^ 1 p d z - ^ l ^ ) sin^ dl,.

(29)

Introducing P^, the power at sample, the magnification factor of the

Characteristics of Raman /\/!icroscopy OBJECT PLANE

45

IMAGE PLANE

OBJECTIVE-TUBE LENS COMBINATION

SAMPLE SLICE

Figure 13. Ray tracing illustration of the loss offluxthrough the confocal diaphragm for a sample slice outside of the object plane of the objective.

objective-tube-lens combination JQ, and considering that ^o is always small, finally yields the relation TTT

9

,2LI instrument

sample

(30)

where cf)^ and L are the diameter of the entrance pupil of the transfer optics and the distance between this pupil and the image plane of the objectivetube-lens combination, respectively. Thus, thefluxentering the coupUng optics comprises two terms, which are the luminous intensity of the source (Raman scattering by the sample sUce) and the throughput of the objective-tube-lens combination, respectively. However, this expression is valid only when the confocal diaphragm D2 is of a dimension which is at least equal to that of the image of the sample slice produced by the objective-tube-lens combination. When the sample slice is not in the object plane, only a part of the light which passes through the confocal diaphragm D2 is transmitted to the coupling optics. In this case the flux has to be re-evaluated as follows. In the following formulae upper and lower signs have been employed in order to simplify the notation. However, only one sign is valid, depending on the position of the sample slice (see Fig. 13) and the flux is then given by

d06 = A

TTT

dfi r

9 /

(l)c/2

(31)

where y^ is the magnification factor of the objective-tube-lens combination when the sample slice is at a distance A from the object plane. The distance between the slice image and the image plane is given by A' and Tp is the transfer function of the confocal diaphragm Di.

46

G. Turrell et a/.

The function Tp can be expressed in the form Jo°'E(R)ds ^ = :Vc:^ ^ jt^EiR)ds

>

(32)

where i?Lc ^^^ ^ D I ^re the radii of the luminous circle formed in the image plane and of the diaphragm D^, respectively. The illumination distribution in the image plane is represented by E{R). From simple geometrical considerations the relation (c^ex/2)+i^LC ^

(ex/2) + ^^

P

P ± A'

^ ^

rX is the radius of the image slice and can be obtained, where p is the distance between the exit pupil of the objective-tube-lens combination and the image plane. Considering that (/)ex/2 = PfJ^o the relation ^LC = ^ ^ ( ^ 6 A ' + ri)

(34)

/? ± Z\

is obtained, where 0ex is the diameter of the exit pupil of the objective-tubelens combination. Applying the Abbe sine condition to this combination yields the expression ^^^.sin^o^(RAO To

^35^

To

which in turn leads to

The application of the magnification laws corresponding to the objectivetube-lens combination r^ = y^r^, A' = JAJO^ and ( I / ^ A ) = (l/7o) - A//, where / is the focal length of the combination, yields the relation _ y J ( N . A . ) A + /-A] "-^

J-TATOA

•

(37)

P Here, r^ is derived from the focusing law for a Chapter 3) within an isotropic medium, /-A = roV[l + {^o^f],

TEMQO

laser beam (see (38)

where r^ = 0.61A/(N.A.) is the radius of the waist of the focused laser beam

Characteristics of Raman Microscopy

47

inside the sample and A is the laser wavelength. Finally, the flux entering the pupil of the coupling optics may be rewritten as

thus, d 1 if higher-order modes are involved. High-power gas lasers and solid-state lasers, such as the Nd:YAG, have Q values from 1 or 2, up to 200, depending on the technology. The value of Q allows the beam characteristics, and hence the beam focusing, to be specified. The real beam can be treated as Gaussian by substituting an *(2, as defined here, is the same as M^ used in the American Hterature.

Instrumentation

57

'artificial wavelength' QX into the equations employed above for a TEMQO beam. As an example, the beam diameter of a real, focused laser beam is given by

The irradiance at the beam waist is then equal to

It should be noted that in certain commercial instruments the figure of merit Q has been measured.

III. MICROSCOPE OBJECTIVES A. Characteristics

The microscope objective is undoubtedly the heart of the Raman microspectrometer, as it plays the most important role of coupUng both the light source and the spectral analyzer to the specimen. All of the potential information, spectral as well as spatial, is contained in the pupil of the objective and in the primary enlarged image of the sample. In principle, there are no fundamental differences between the optical systems used in conventional Raman instruments for macrosamples and the special optical devices devoted to microprobe analysis, since they both obey the same physical laws. However, micro-Raman systems impose more severe constraints in order to preserve a high spatial resolution and to maximize the signal, with the use of wide-aperture optical components. In particular, an exact matching of all intermediary pupils and images along the optical path is of primary importance, as indicated in Chapter 2. Unlike the classical 90° arrangement, which requires relatively long working distances with the use of two separate lenses, almost all microRaman instruments employ a single, wide-aperture objective in the 180°, or backscattering configuration. A beamspUtter is needed in order to illuminate the sample with the laser beam coaxially, through the objective, and to transmit the backscattered radiation toward the spectral analyzer. The use of commercially available microscope objectives has prevailed in most instruments, as these elements are almost perfectly corrected for aberrations over the relatively narrow spectral range employed in Raman spectroscopy (Wilson and Sheppard, 1984; Keller, 1989).

58

M. Delhaye et al.

Microscope objectives are essentially short-focus, large-aperture aplanetic systems. The cone of light focused or collected by the objective is determined by the numerical aperture (N.A.), which is defined by N.A. =Azsin^,

(21)

where n is the real refractive index of the medium between the object and the lens and B is the semi-angle of the cone for an axial objective point. Commercially available objectives exhibit a maximum semi-angle of ^ — 72°, or N.A. = 0.95 in air, where n = \. In order to introduce aberration corrections these objectives must be used under conditions specified by a fixed magnification factor y and a lens-toimage distance which is usually referred to as the 'tube length'. These specifications are always precisely given by the microscope maker and are sometimes engraved on the barrel of the objective. Consequently, the distance between the object and the front optical element, which is called the working distance (WD), as well as n, the index of refraction of the optical medium in the object space, must be strictly respected. The effective focal length/o is a constant characteristic of a given objective, which is often not specified. It can be deduced with the use of the classical lens equation from the value of 7, the magnification normalized at a fixed tube length p . The latter quantity may vary from 160 to 190 mm, depending on the maker's standards. Infinity-corrected objectives are often preferred for Raman experiments. These lenses are designed to form a parallel output beam when a point fight source is placed at the front focus. An additional convergent achromat, referred to as the 'tube lens', is needed to form a real image of the object. An advantage of this combination is that the effective magnification y^ can be varied without degrading the correction, by choosing the appropriate focal length /t of the tube lens achromat; thus, y^ =/t//o-

B. Efficiency of Light Collection The basic equations which describe the origin of the Raman signal intensity were presented in Chapter 2. The polarization properties of both isotropic and anisotropic media, e.g. oriented crystals, were considered. In practice, the lack of knowledge of the precise geometry of a specimen and the random orientation of |xm-sized particles or heterogeneous sample inclusions makes virtually impossible the prediction of the spatial distribution of the Raman scattering intensity. A limited number of experimental data have been reported which indicate that the 'scattering indicatrix', depicted as a three-dimensional plot of the observed intensity in all directions of space, may be significantly different from that predicted by the theoretical analysis.

Instrumentation

59

Figure 4 The centered spherical indicatrix. In order to provide the user with a practical basis for the comparison of different kinds of microscope objective, two simple geometrical models can be proposed. They are based on two different assumptions, namely: (i) a spherical indicatrix centered on a punctual sample, or (ii) Lambertian scattering for which the intensity vanishes in the plane of the specimen. 1. Isotropic Point Source In this model the sample is assumed to be a point source O which radiates uniformly in all space, i.e. fl = 47? steradians. For consistency with most applications in optics, the traditional calculation refers to the half space where n = 277, although it should be pointed out that with certain special optics a wider soUd angle can be covered. The emission indicatrix is a sphere whose center coincides with the source. Thus, the irradiance / = dc^/d^, expressed in W m~^, at the entrance aperture S of the light collector is constant for all directions in space. The total signal collected in the acceptance cone of the objective is obtained by integrating the Hght flux 0 over the solid angle ft (Fig. 4). For a limited, axially symmetric, conical beam of semi-aperture S the solid angle ft is given by ft = 277(1 - cos e) = 477 sin^ j ,

(22)

so that the light flux becomes ,

•

\

PLANE MIRROR

\ I ^ I

F2

M2

Figure 19 A possible simplification of the system shown in Fig. 18.

coaxial ellipsoidal mirrors, as shown in Fig. 17. In this configuration a point source placed at the common focal point Fi is imaged by each of the halves to the second focal points F2 and F2, which are coincident with the exit aperture at the vertex of the other half. The two output beams, in opposite directions along the principal axis, have to be coupled optically to the laser and the spectral analyzer by two external systems. An interesting application of this principle, which was proposed by DILOR (1987), is shown in Fig. 18. Both sides of a beamsplitter are employed to minimize the Ught losses by directing approximately half of the laser power to the ellipsoid on the left side and the other half to that on the right. The Raman-scattered Ught, which is collected at twice N.A. ~ 1, is transmitted to the spectrometer via the same beamsplitter. A possible simpHfication of this system can be achieved by replacing one of the half-ellipsoids by a plane mirror which supports a microsized sample near its center, as shown in Fig. 19 (CNRS-ANVAR, 1987). Such optical systems transform the cone of light collected over nearly 2TT steradian from the sample with N.A. ~ 1 into an exit beam of lower numerical aperture, e.g. N.A. = 0.8, which can be coupled easily to a conventional lens or mirror objective. The ellipsoidal mirror in this arrangement plays the role of the high-aperture front lens in a conventional microscope objective.

Instrumentation

11

E. Comparison of Microscope Objectives with Camera Lenses A wide variety of high-aperture lenses is commercially available for cinephotography and video cameras, which may be of interest for macro- or long-WD, micro-Raman experiments. In this case attention must be paid to certain important parameters in the selection of such lenses. Their design may be significantly different from that which is appropriate to Raman microscopy. Camera lenses are designed to balance all aberrations so as to cover the useful field, which is determined by the dimension of the photodetector. The dimension is typically from 24 mm X 36 mm down to approximately 8.5 mm X 8.5 mm. The spatial resolution is therefore adapted to the pixel dimension, typically from 50 to lOixm. The spatial resolution of camera lenses is usually given in line-pairs per mm (LPM), as evaluated qualitatively by observing the image of test patterns with the use of a square-wave profile. This profile consists of alternate transparent and opaque bars of equal width. A much more significant evaluation of the image quality is obtained by consideration of the modulation transfer function (MTF), which describes the ability of an optical system to transfer the object contrast to the image. The MTF is measured by forming the real image of a periodic test pattern which exhibits a sine-wave profile with variable spacing. The corresponding contrast or 'modulation' in the image plane is plotted in cycles per mm vs. the spatial frequency. It is worth noting that the art of camera lens fabrication reUes on an optimized balance of residual aberration in a wide-angle field at the expense of axial resolution. Typical resolutions range from 30 LPM for ordinary photographic cameras up to 200 or 500 LPM for photohthography. By comparison, the resolution necessary for micro-Raman measurements is of the order of 1000-2000 LPM. It is concluded, then, that camera lenses are satisfactory for macro- or semi-macro Raman spectroscopy with sample dimensions in the range 10-50 fxm, but are not suitable for diffraction-limited, laser focusing and Raman imaging. The value of the angular semi-aperture Q of camera lenses is generally not available in the manufacturer's specifications. However, it may be determined easily from the photographic //number, which is usually defined as the ratio of the focal length / to the pupil diameter D. Thus, in principle, //number = flD = 1/(2 tan 0).

(38)

Since the effective focal length has to be measured along the optical axis, this definition is not meaningful for very large values of 6, where tan 6 tends toward infinity. In practice, the //number is defined by 1/(2 sin ^), in agreement with Abbe's aplanetic condition. This relation is also in accord with photometric measurements on plane photodetectors, which obey

78

M . Delhaye et a/.

s

ASYMPTOTE

//o .

D H //2 CLH

p^

D CQ

OO

>

"* .^_

*~~ -..^ '^'N

—-

N^

^^

\

^\

\ \

\ \

\

\ \\

v\

W V

\

\

^^\y^

/ -

a

~~" ~~

^\

Oi

o EU

Z 5< ypQ

Z J O PJ — 7

^-intensity profiles versus spectral information. The x spatial information that has been encoded by the Hadamard mask is retrieved by carrying out the inverse, fast-Hadamard transformation (FHT) of the data for each pixel. After computation, a complete set oi x,y,n data stored in the computer can be used to generate single-wavelength images. Usually, only a few wavelengths are needed to describe the specimen. Thus, images are generated more rapidly, as the inverse FHT is performed with only a Hmited number of pixels.

Raman Imaging 189 Cylinder Lens

Projected Image

Entrance Slit

Diffraction Grating

--y n II r u an I » iri I / I sttt r JM If I fill i/tit m tf Ml I ar I lu t r til I m t II ts I m I 01 in ti I $ t ini f9 I I I 1/ i r • / • f n II m I

CCD

Spectroscopically Resolved Hadamard Image

Figure 10 Multichannel Hadamard-transform Raman microscope. In the case of the Hadamard multichannel system, the sample as well as the laser beam are fixed during the measurement; only the mask is moved, in order to achieve spatial encoding of one spatial direction. As the displacement takes place in the image plane of the microscope, there is no need for fine, accurate and reproducible micrometric displacement, as required by scanning systems. The Hadamard technique provides the multiplex advantage (Fellgett) when the instrument is detector-Umited, i.e. when the main source of noise is the detector. The installation with Hadamard encoding and CCD detection behaves differently, as the CCD detector produces very low noise. Thus, the 'photon' (or shot) noise due to the total Hght flux may become preponderant. Then, the advantage of this technique is reduced compared to others such as the scanning methods discussed below.

IV. SIGNAL-TO-NOISE RATIO (S/N) AND COLLECTION TIME In this analysis a very simple model will be assumed in which the detector readout noise is negligible compared to the signal noise. This condition is

190

J. Barbillat

correct in the case of the CCD detectors considered here. It is further assumed that jc-point by >'-point images are studied and the smallest spatial element on the sample is conjugate to a single pixel of the CCD detector. The maximum allowable laser power at the sample is n photons second per spatial element. The total duration for collecting M images at M different wavelengths is T seconds for every method. The background noise is neglected. A. Line Illumination versus Global Illumination 1. The Same Total Laser Power Used in Both Measurements (a) Line illumination The signal originating from a point arises from kn TIxy photons, where k depends on the nature of the sample. This result yields the relation S^ = kK7]nT/xy,

(1)

where S^ is the number of charges at the CCD output mode. The overall optical efficiency determines the value of K and the quantum efficiency of the CCD detector is given by h. The associated noise can be expressed by N,= {kKr)nTlxy)^'^.

(2)

Thus, the signal-to-noise ratio is equal to S,IN, = (kKrjn TIxy) ^'^.

(3)

Actually, the time available for each point is less than TIxy, as the fine has to be moved y times onto the sample in order to scan all of the area to be imaged. (b) Global illumination In this case the magnitude of the signal originating from a point is given by k{nlxy){TIM), which leads to the expression S^ = kK7]nTlxyM.

(4)

The associated noise is equal to yVg = {kKr^nTlxyMY'^.

(5)

which yields 1 1/2

VTVg = {kKr)nTlxyMf'^ " ( y^ I

^^'^^

^^^

Raman Imaging

191

for the signal-to-noise ratio. Thus, global illumination never leads to a better value of S/N, but the sample is in this case well protected against laser damage, as the local irradiance is very low. 2. Each Pixel of the Sample Receives the Maximum Allowable Power in both Measurements (a) Line

illumination

The situation is unchanged, since each point always receives the maximum allowable power, as in the first calculation. The signal-to-noise ratio is still given by {kKiqnT/xyy^^. (b) Global illumination Now, the magnitude of the signal originating from a point is equal to nT/M, leading to the relation 5g = kKr]nT/M.

(7)

The associated noise level is then given by A^g = {kK7]nTIMf'^

(8)

and the signal-to-noise ratio becomes 5g/iVg = {kKr)nTIMf'^ = I ^ j

S,IN^.

(9)

In the real situation the background is seldom negligible, so that correct signal evaluation often requires background subtraction. This operation reduces the time available for each point, as a second image (at least) has to be acquired at a neighbouring wavelength. As a consequence, the S/N is divided by a factor of two, as pointed out by Puppels et al. (1993). This result means that the S/N in global imaging is better than that in the line-scanning technique, as long as the number of desired images is kept smaller than half of the total number of pixels in the image.

V. HADAMARD IMAGING VERSUS GLOBAL ILLUMINATION AND LINE SCANNING A comparison of these techniques is given by Puppels et al. (1993). It is found that in general direct imaging gives a better S/N and that Hadamard imaging

192

J. Barbillat

is to be preferred to direct imaging only in the case of very low signal levels. Moreover, Hadamard imaging does not offer a performance that is superior to Hne-scanning image reconstruction. The reason is that the general Fellgett advantage of the Hadamard multiplexing technique is not achieved when the detector exhibits very low noise, as with the use of a slow-scan CCD detector operating at very low temperatures. The global-illumination techniques provide much more total laser power at the sample than do scanning techniques, although each pixel of the sample does not receive more energy. This result is called the distribution advantage. However, is it a real advantage in the general case? It is not evident that the sample can sustain so much power without damage, as the thermal relaxation is different when a sample is excited point by point or as a whole. The conclusion is that the S/N improvement discussed above is, in practice, certainly lower than the calculated value. Another way to benefit from the distribution advantage of global imaging is to reduce the time required to obtain a set of single-wavelength images, without improving the S/N. The rapid capture of survey images might be ultimately one of the best features of the direct-imaging techniques.

VI. EXAMPLES OF APPLICATIONS Figures 11-17 present some images obtained with the different systems described above. Figure 11 shows the Raman image of a sulfur inclusion within a host matrix of natural strontium sulfate (celestine) obtained with the MOLE microscope (Dhamelincourt and Bisson, 1977). Raman intensity profiles at various wavenumbers on a zircon material (Bowden et al., 1990) are displayed in Fig. 12, while Fig. 13 presents bright-field and Raman images at 998cm~^ of polystyrene spheres (Puppels et al., 1993). Figure 14 illustrates the direct recording of the Raman image of a thin film of rubber-toughened epoxy resin at 1665cm~^ (Garton et al., 1993). The confocal line-scanning image of a polymer film containing polypropylene and polyethylene is shown in Fig. 15. These two polymers are impossible to localize by visual inspection. CORALIS images are obtained with the dedicated software 'Spectrimage' developed by Sharonov and Manfait (1992). The Hadamard imaging of edge-plane microstructures in highly ordered pyrolitic graphite electrodes is illustrated in Fig. 16. The Raman image is reconstructed from the Raman band at 1360cm~^ (Treado et al., 1990). Figure 17 presents images obtained with an acousto-optical imaging system (Treado et al., 1992). They are images of a mixture of dipalmitoylphosphatidylchohne (DPPC) and L-asparagine aggregates which serves as a model for the study of lipid/peptide interaction.

Raman Imaging

193

Figure 11 Raman image of a sulfur inclusion within a host matrix of natural strontium sulfate (celestine). (a) White-light image; (b) 473 cm~^ Raman image of sulfur; (c) 1000 cm~^ Raman image of SrS04.

(b)

Intensity

Position (Microns)

300

400 Raman Shift (cm-1)

Figure 12 Raman intensity profiles at various wavenumbers for a zircon mineral, (a) Video micrograph showing the position of the scanned 150 jjtm line segment on a zircon mineral, (b) Line-scanned Raman spectra from the zircon mineral shown in (a). The light-colored grain from 0 to 50 |uim is the mineral zircon. The dark-colored grain found at the other end of the line (120-150 ^m) is exclusively anatase Ti02.

Raman Imaging

195

Figure 13 (a) White-light image of polystyrene spheres labeled with a fluorescent dye. (b) 998 cm"^ Raman image of polystyrene, (c) 515 nm fluorescence image obtained with blue excitation (420-430 nm).

196

J. Barbillat

Figure 14 Raman image of a thin film of a rubber-toughened epoxy resin at 1665 cm-1.

VII. CONCLUSION In this brief chapter several techniques of Raman imaging have been reviewed and their performances compared. It is clear that there is no absolute answer to the question: What is the best method of imaging, direct scanning or encoding? None of the techniques described above can solve by itself all of the problems encountered in the field of Raman imaging. Depending on the desired result one technique may be preferred to another. It must be decided which advantage is desired in a particular application. If rapidity is of primary importance, direct imaging is to be preferred over image reconstruction, because it provides the desired information in less time. On the other hand, improved spectral resolution, background rejection or optical sectioning capabilities are best achieved with the use of confocal techniques.

( n v ) AM«U«»UI

^

!^

03

O

197

.1§

II C

G

s

^

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CI.

o

03

cd

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II 5 |xm), particularly at normal incidence. For the analysis of thinner films, a method initially proposed by Greenler and Slager (1973) is often used. In this case the sample is deposited as a thin layer on a metallic surface (e.g. Ag or Ni) and laser excitation is applied at glancing incidence. The Raman-scattered light is observed with a microscope objective. It was shown that the optimum angle of incidence for laser excitation is approximately 70°, while the collection efficiency is maximum at around 60° with respect to the surface normal. These parameters depend somewhat on the optical constants of the metallic support (Greenler, 1966). Under these conditions satisfactory Raman spectra of 50 A (5 nm) deposits of benzoic acid were recorded (Greenler and Slager, 1973). The use of a metallic support for the investigation of thin films has become even more interesting with the discovery of surface-enhanced Raman spectroscopy (SERS). The phenomenon of surface enhancement will be treated in Chapter 9, where its importance in the Raman microscopic analysis in biological systems and medicine is emphasized.

B. Integrated Optics A more sophisticated approach to the investigation of thin films by Raman spectroscopy has developed from the early work of R. Dupeyrat and coworkers (Levy et al., 1974). The method involves an integrated optical system in which the sample film serves as a waveguide or dielectric wall. The basic theory of this method has been reviewed by Rabolt and Swalen (1988). In the initial experiments a thin-film sample of refractive index ^2 was deposited on a support surface. The laser beam was focused on the sample through a coupling prism, as shown in Fig. 18. If the angle of incidence and the polarization direction are properly chosen, the light propagates as a guided wave in the sample film. A microscope objective is employed to collect the scattered light. The image of the entrance sHt of the monochromator is adjusted so that its width is equal to the diameter of the propagating laser beam and its length corresponds to the distance L (Fig. 18). The coupling of the excitation to the waveguide sample is determined by the boundary conditions which are functions of the refractive indices of the

Raman Microscopy and Other Local Analysis Techniques

237

Laser beam Sample film Prism

///////7/////////////

/

Figure 18 Application of the waveguide technique to Raman studies of films.

media. It was shown by Levy and Dupeyrat (1977) that one or several modes of either the TE (transverse electric) or TM (transverse magnetic) polarizations can propagate in a film of thickness a according to the relation 2kn20-cos 62 - ^12 - ^23 = 27rm,

m = 0,1,2,,

(1)

where k=27r/\o, with AQ equal to the vacuum wavelength of the light, S12 and ^23 are the phase shifts at interfaces 1-2 and 2-3, respectively, and $2 is the angle of reflection within the waveguide (see Fig. 19). The phase shifts depend on the refractive indices rii and ^3, and propagation constant a = n2sin02fora given polarization (TE or TM). For the TE modes, as an example, Eq. (1) yields the family of curves shown in Fig. 20 for a typical case in which rii = 1.46, ^2 = 1.50 and ^3 = 1. For a given value of m, it is seen that if the film thickness a decreases, a approaches rii and the energy penetrates more and more deeply into the support. However, when a increases, a tends towards the value of /t2 ^nd the energy is contained in the waveguide layer. In the latter limit, the field of evanescent or exponentially damped waves becomes equal to zero. It is thus possible to obtain separately or simultaneously the Raman spectrum of the waveguide sample or its support by varying the thickness of the sample layer. The advantages of the waveguide method in Raman spectroscopy were pointed out by Levy and Dupeyrat (1977), who estimated that it could yield an intensity gain of a factor of 2000 with respect to the backscattering geometry under similar sampling conditions. Furthermore, unlike the glancing-incidence geometry, the waveguide technique allows a choice of polarization directions to be made. The TE modes can be excited by polarizing the laser beam in the Z,X plane (see Fig. 19) and choosing the proper angle of incidence. Rotation of the polarization so that the electric vector is parallel to the Y axis, accompanied by a small adjustment of the angle of incidence, results in propagation of a TM mode. It should be noted, however, that there is always a lower limit to the thickness of a film which

238

M. Truchet et a/.

///////y/. Figure 19

112

Analysis of light propagation in a film deposited on a substrate.

-

1.A9

m=0

/

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/

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y

m = 3 ^'^^^'^

1.A8 c o

o en

O

Q.

o

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-

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10 Reduced f i l m thickness ,

a/X.

Figure 20 Propagation constant a = «2 sin 62 as a function of reduced sample thickness for TE modes. In this typical case ni = 1A6, n2 = 1.5^ and ^3 = 1.

Raman Microscopy and Other Local Analysis Techniques

239

Laser beam Figure 21 The multilayer integrated optical system. Excitation is at an angle 6i with respect to the vertical axis. can be used as a waveguide. In the above example for m = 0, it is equal to 0.56A, as determined by the intersection of that curve with the line a = rii.

In subsequent work by Cipriani et at. (1974) a multilayer structure was developed which can be employed to obtain the Raman spectra of thinner films. The analysis of this system is based on Fig. 21; the four layers are characterized by their respective refractive indices and thicknesses. In this configuration the exciting laser beam enters the system from below through medium 1 at an angle of Si with respect to the vertical (Z) axis. Media 2 and 3 are both thin films, while the semi-infinite medium 4 is usually air. The Raman scattering is observed as before, from above, with a microscope objective. The parameters of the various media are chosen to produce inhomogeneous fields in media 2 and 4, and homogeneous ones in medium 3. For Hi, ^3 and n^ real, the necessary conditions are realized if 6i is larger than the critical angle defined by sin~^(^4/ni). Equation (1) represents the resonant condition in the thin layer (3) if 3^2 and 623 are replaced by 623 and 634, respectively. The phase shift ^23 depends on the presence of medium 1, and, as 0-2 approaches infinity, the system becomes (aside from notation) equivalent to that of Fig. 19. For a typical multilayer system, the energy density in layers 2, 3 and 4, relative to that in layer 1, is represented in Fig. 22. It is apparent that a very large increase in energy density can be obtained in film 2. It is estimated that in this case the Raman scattering is approximately 3000 times greater than that obtained with the use of the backscattering configuration (Levy and Dupeyrat, 1977). Although the scattering intensity is about the same as in the waveguide method described above, the thickness of the film can be significantly reduced (Rabolt et al, 1979). C. Applications

The integrated optical technique outlined above has been developed and applied to the Raman spectroscopic investigation of various thin-layer

240

M. Truchet et al.

Relative

energy density

Figure 22 Relative energy density as a function of vertical distance in a multilayer system.

systems (Rabolt et al., 1980). It has also been employed to obtain Raman spectra of films of the Langmuir-Blogett type (Barbaczy et al., 1987; Rabe et al., 1987). The spectra are indicative of molecular orientation on surfaces and are sensitive to order-disorder transitions. This technique has been recently combined with Fourier transform Raman spectroscopy to study organic films, polymer laminates and molecular composites (Zimba et al., 1990). In these applications excitation in the near-infrared region with a YAG laser (A = 1.064|xm) was employed in order to eliminate fluorescence interference (see Chapter 3).

REFERENCES Barbaczy, E., Dodge, F. and Rabolt, J. F. (1987). Appl Spectrosc. 41, 176. Barry, B. and Mathies, R. (1982). 7. Cell Biol. 94, 479. Benoit, D., Grillon, F , Maurice, F., Roinel, N., Ruste, J. and Tixier, R. (1987). Microanalyse par sonde electronique: spectrometrie des rayons X. ANRT, Paris. Berthod, A., Laserna, J. J. and Winefordner, J. D. (1987). Appl. Spectrosc. 41, 1137. Carreira, L. A., Rogers, L. B., Gross, L. P., Martin, G. W., Irwin, R. M., Von Wandruska, R. and Berkowitz, D. A. (1980). Chem. Biomed. Environ. Instr. 10, 249.

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Castaing, R. (1951). Doctoral thesis, Conera Paris. Castaing, R. and Deschamps, J. (1958). La Recherche Aeronautique. ONERA, Paris. pp. 41-51. Castaing, R. and Slodzian, G. (1962). / . Microsc. 9, 395-410. Chapput, A., Roussel, B. and Montastier, J. (1979). C. R. Acad. Sci. C289, 283. Chapput, A., Roussel, B. and Montastier, J. (1980). / . Raman Spectrosc. 9, 193. Chen, C. Y. and Morris, M. D. (1988). Appl Spectrosc. 42, 515. Cheng, Y. F. and Dovichi, N. J. (1988). Science 242, 562. Chong, C. K., Mann, C. K. and Vickers, T. (1992). / . Appl Spectrosc. 46, 249. Cipriani, J., Racine, S., Dupeyrat, R., Hasmonay, H., Dupeyrat, M. Levy, Y. and Imbert, C. (1974). Optics Comm. 11, 70. Delhaye, M. and Truchet, M. (1987a). In: R. H. Geiss (ed.), Microbeam Analysis. San Francisco Press, San Francisco, pp. 163-164. Delhaye, M. and Truchet, M. (1987b). Patent 87 09883 (CNRS-ANVAR). Dhamelincourt, P., Delhaye, M., Truchet, M. and Da Silva, E. (1991). / . Raman Spectrosc. 22, 1. D'Orazio, M. and Schimpf, U. (1981). Anal. Chem. 53, 809. Eberhart, J. P. (1976). Methodes physiques d'etude des mineraux et des materiaux solides. Douin, Paris. Eloy, J. F. (1980). Proc. 5th Int. Symp. High Purity Mat. Sci. Techn., DDR Akad. Wiss., Dresden, 1980. Freeman, R. D., Hammaker, R. M., Meloan, C. E. and Fateley, W. G. (1988). Appl. Spectrosc. 42, 456. Garrell, R. L. (1989). Anal. Chem. 61, 401. Giles, P. L. (1975). Cathodoluminescence. In: P. Echlin and P. Galle (eds). Biological Microanalysis. SFME, Paris,.pp. 357-370. Greenler, R. G. (1966). /. Chem. Phys. 44, 310. Greenler, R. G. and Slager, T. L. (1973). Spectrochim. Acta 29A, 193. Hendra, P. J. and Loader, E. J. (1967). Nature 216, 789. Hendra, P. J. and Loader, E. J. (1968). Nature 111, 637. Hillenkamp, F. and Kaufmann, R. (1981). Fresenius Z. Anal. Chem. 308, 1-320. Hillenkamp, F., Kaufmann, R., Nitsche, R. and Unsold, E. (1975). Appl. Phys. 8, 341. Iriyama, K., Ozaki, Y., Hibi, K. and Ikeda, T. (1983). / . Chromatogr. 254, 285. Koglin, E. and Planar, J. (1989). Chromatography 2, 194. Koizumi, H. and Suzuki, Y. (1987). / . High Res. Chromatogr & Chromatogr. Comm. 10, 173. Koningstein, J. A. and Gachter, B. F. (1973). / . Opt. Soc. Am. 63, 892. Lafage-Szydlowski, N. (1984). Thesis 'Docteur Ingenieur', University of Lille. Lehner, C , Sawatzki, J., Koglin, E., Kramer, H. and Kawai, N. T. (1994). In: N. T. Yu and X. Y. Li (eds), Proc. Int. Conf. Raman Spectrosc. The Hong Kong University of Science and Technology, Hong Kong. Additional volume, p. 289. Levy, Y. and Dupeyrat, R. (1977). /. Phys. (Col. C5) 38, 253. Levy, Y., Imbert, C , Ciperiani, J., Racine, S. and Dupeyrat, R. (1974). Optics Comm. 11, 66. Magnan, C. (1961). Traite de Microscopie Electronique, vol. 1. Hermann, Paris. Merlin, J. C. and Delhaye, M. (1987). In: J. Stepanek, P. Anzenbacher and B. Sedlacek (eds). Laser Scattering Spectroscopy of Biological Objects (Studies in Physical and Theoretical Chemistry). Elsevier, Amsterdam, vol. 45, p. 49. Ni, F., Thomas, L. and Cotton, T. M. (1989). Anal. Chem. 61, 888. Quintana, C. (1980) Biol. Cell. 39, 151. Rabe, J. P., Swalen, J. D. and Rabolt, J. F. (1987). / . Chem. Phys. 86, 1601.

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Rabolt, J. F. and Swalen, J. D. (1988). In: R. J. H. Clark and R. E. Hester (eds), Spectroscopy of Surfaces. John Wiley & Sons, Chichester, p. 1. Rabolt, J. R , Santo, R. and Swalen, J. D. (1979). AppL Spectrosc. 33, 549. Rabolt, J. R , Santo, R. and Swalen, J. D. (1980). AppL Spectrosc. 34, 517. Roqueta, C , Merlin, J. C , Fournier, C. and Hecquet, B. (1988). In: R. J. H. Clark and D. A. Long (eds), Proc. Int. Conf. Raman Spectrosc. (London). J. Wiley & Sons, Chichester, p. 717. Ruste, J. (1975). J. Microsc. Biol. Cell. 22, 151-162. Slodzian, G. (1964). Doctoral thesis. University of Paris. Todoriki, H. and Hirakawa, A. Y. (1984). Chem. Pharm. Bull. 32, 193. Truchet, M. and Delhaye, M. (1988). J. Microsc. Spectrosc. Electron. 13, 167175. Truchet, M. and Delhaye, M, (1992). In: W. Kiefer (ed.), Proc. Xlllth Int. Conf Raman Spectrosc, Wurzburg, Germany. John Wiley & Sons, Chichester, pp. 1068-1069. Zarrin, R and Dovichi, N. J. (1985). Anal. Chem. SI, 2690. Zimba, C. G., Hallmark, V. M., Turrell, S., Swalen, J. D. and Rabolt, J. F. (1990). / . Phys.Chem. 94, 939.

Application to IVIaterials Science Paul Dhamelincourt and Shin-ichi Nakashima

1. INTRODUCTION When Raman spectroscopy is applied to the analysis of microscopic particles or microscopic volumes within a heterogeneous sample, it is usually assumed that all of the physical phenomena involved in the Raman scattering of Hght from macroscopic samples are the same. However, it is well known that scattering from samples whose dimensions become comparable to the wavelength of the excitation represents a special case. Calculations based on the Lorenz-Mie formahsm (Kerker, 1969) show that a strong increase in the internal electric field may occur as a result of morphology-dependent resonances when microsamples of well-defined geometries (e.g. spheres, spheroids and cylinders) are illuminated by a plane wave at certain wavelengths. Physically, these resonances result when a wave traveling inside the microsamples is in phase with itself after having been internally reflected at the interface with the surrounding medium. The analysis of such resonances, which occur for certain values of the size parameter X= ITTYIX, where r is the radius of the microsample, is very well documented (Owen et al., 1982). Calculations show that these resonances should lead to very sharp peaks in the Raman and fluorescence spectra of |xm-sized samples (dimensions up to several tens of juim) that are not predicted in bulk samples of the same composition. These peaks would result either from resonance-induced enhancement of the Raman scattering efficiency itself and/or enhancement of the fluorescent background when fluorescent species are embedded in the sample. Fortunately, the resonances described above have never been observed in the usual micro-Raman experiments. Their absence is explained both by imperfect particle or heterogeneity geometries (complex structures inhibit phase relations between travelUng waves) and the strong optical coupling into the sample substrate, which is for a heterogeneous sample that part of the

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P. Dhamelincourt and S. Nakashima

sample which is not illuminated. Both factors preclude the observation of the morphology-dependent effects. Hence, resonances in the inelastic scattering (Raman and fluorescence) spectra of juim-sized samples have only been observed when special experimental conditions (e.g. levitated liquid droplets, glass and polystyrene perfect spheres supported by smooth substrates, and ends of cylindrical optical fibers) have been employed. In these cases the samples have well-defined geometries with virtually no optical coupling into the substrate, if present (Thurn and Kiefer, 1985). Morphology-dependent effects have not, thus far, impaired the analytical usefulness of micro-Raman spectroscopy. However, at fxm sample dimensions orientation effects are far more important (see Chapter 1, Section VIII). Indeed at that scale most of the microcrystals or crystalline domains have well-defined crystallographic axes. Hence, the relative intensities of the Raman bands (compared with those observed in polycrystalUne bulk samples) depend strongly on the orientation of the samples with respect to the incident polarization, as defined by the electric field direction of the laser beam at the sample and the polarization vector of the scattered fight. If no analyzer is used, the latter parameter is determined by the axis of the entrance slit of the monochromator. Care must be taken when qualitative (and quantitative) comparisons of the spectra of microsamples are made with reference spectra obtained from polycrystalline bulk samples, especially for the Raman bands which correspond to totally symmetric modes of vibration. Finally, for samples which contain crystallites whose dimensions are far below the exciting wavelength (nanophases ranging from several tens to a few hundred nm), band shifts and broadening are expected due to finite crystalUte size (relaxation of the K = 0 selection rules), surface pressure and nonstoichiometry. New bands may also be observed which are due to surface and aggregation effects (Bobovich and Tsenter, 1982; Pigenet and Frevet, 1980).

II. INORGANIC SOLIDS A. Catalysts L

Introduction

Metal oxide yAl203-supported catalysts to which an NiO or CoO promoter is added have been extensively studied because of their important industrial applications. In particular, after activation, they are employed in hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) of petroleum and coal products. The precursor oxides, which are the catalysts before activation by sulfidation, are generally prepared by the pore-filUng method with the use

Application

to Materials Science

245

of yAl203 extrudates according to the following steps (Payen et al., 1986): (i) Impregnation with ammonium heptamolybdate solution, (ii) Drying at temperatures above 373 K, followed by calcination at 773 K for several hours, and (iii) Promoter impregnation with Ni or Co nitrate solution, drying and final calcination at 773 K. The metal and promoter loadings are given in oxide (i.e. M0O3, NiO, CoO) weight per cent. 2. Characterization by Vibrational Spectroscopy In addition to surface characterization techniques (see Chapter 5) such as X-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS), vibrational spectroscopy has proven to be a very useful method for characterizing the supported phases themselves. As these catalysts are not transparent below 1000 cm"^ due to absorption by the 7AI2O3 support, Raman spectroscopy is preferred to infrared spectroscopy for obtaining data. Conventional laser Raman spectroscopy has been used to analyze oxide-supported catalysts, but a large number of the results reported in the literature have been obtained with the use of micro-Raman spectroscopy. The advantages of a Raman microprobe for the study of 7Al203-supported catalysts are as follows: (i) In the case of absorbing samples the efficiency of the excitation and collection of hght is far higher than in the conventional instrument (see Chapter 3), (ii) The confocal configuration used is very efficient in reducing background emission, and (iii) The micro-Raman instrument offers the possibihty of employing controlled temperature and controlled atmosphere cells. Such cells for optical microscopy are currently available or can be made in the laboratory. They facihtate the study of sohds in situ under a wide variety of atmospheric conditions and at temperatures ranging from that of Uquid nitrogen to 1500 K. 3. Raman Spectroscopic Analysis of Precursor Oxides The understanding of the function of alumina-supported oxomolybdate catalysts in their oxide states owes much to results obtained by the use of Raman spectroscopy. It was shown by Brown et al. as early as 1977 that even

246

P. Dhamelincourt

and S. Nakashima

for loadings lower than that required to saturate the first monolayer, good Raman spectra could be obtained. According to Payen et al. (1986), the observed Raman spectra are complex in nature, as they involve mainly surface effects and loading inhomogeneity. A significant amount of work has appeared in the literature and an overall interpretation of the Raman bands associated with supported oxides has emerged (Payen et al., 1987), namely: (i) At loadings of 14wt% M0O3), heptamolybdate (HM) aggregates are present. Similar results are observed when loading 7AI2O3 with tungsten and vanadium. In situ measurements at temperatures up to 773 K show that the species M and HM are stable; this stability is thought to result from interaction with the support. During thermal treatment the dehydration and rehydration of the supported species can be followed by the shift of the stretching vibration of the Mo-Ot bond, which occurs between 900 and 1090 cm~^. (Ot denotes a terminal oxygen atom which is not Unked to other atoms.) In the VI"^ oxidation state, molybdenum may adopt various configurations between tetrahedral and octahedral. Hence the shift of the Mo-Ot bond stretching vibration which corresponds to the monomer or heptamer species will depend not only on the ligand heterogeneity (relative to the number of bridging oxygens), but also on the coordination heterogeneity (unsaturated Mo^^ due to ligand vacancies). Both the ligand and coordination heterogeneity effects lead to an increase in the Mo-Ot stretching frequency (see Fig. 1). Thus, by recording spectra in the 900-1090 cm~^ range, it is shown that: (i) Calcination around 773 K reinforces the link between the H and HM species and the support through dehydration. The presence of a promoter (Ni or Co) enhances the phenomenon by faciUtating dehydration. (ii) Calcination at higher temperatures leads to the formation of AI2 (M004)3.

(iii) Ageing of the catalysts after calcination is a hydration process which always leads to the supported hydrated species

Movjf °' . OH (iv) This hydration-dehydration scheme seems to be general and has been observed in other yAl203-supported oxides such as WO3 and V2O5.

Application nb 0

nb nb nb 0 0 0 \ 1/

/ l \ 0 0 0 nb nb nb

920 cm-1

0 \

920 cm-1

1 1

XX

XX

OH

/

IVI06+ ,,

// w0 0

00 I1 I1

II 1 1

XX

XX

HO

1 1

^

Mo6-^,


M06 +

//l\\

0 0 0 00 1 1

X X X

M06 +

1 1

1 1

1 1 1 1

X X X X X 1000 cm-1

Figure 1 Characteristic Raman frequencies of the molybdenum ion in various oxidation states. The subscripts b and nb denote bridging and nonbridging oxygen atoms, respectively.

4. Bronsted Acidity of Supported Oxides Pyridine is a model molecule which is often used to measure the surface acidity of soHds. In the hydrated form, precursor oxides possess a hydroxyl group bonded to the supported metal. Pyridine chemisorption through the formation of the pyridinium ion is a good way to demonstrate the acidic

248

P. Dhamelincourt and S. Nakashima 1014

(c)

b)

(a) N)cm-i

Figure! Pyridine chemisorption on a Ni-W catalyst, (a) Fresh catalyst, (b) after increasing time of contact with pyridine, (c) after desorption of physisorbed pyridine by N2 purging. character of the OH group. With the use of a controlled atmosphere and controlled temperature cell, Raman measurements can be made during the flow of the N2-pyridine mixtures on powder catalyst (Payen et al., 1982). On molybdenum- or tungsten-supported catalysts, the same observations have been made (see Fig. 2). At first, a band at 1014 cm"^ appears which is characteristic of the chemisorbed pyridine (pyridinium ion). Then, on increasing the contact time, bands at 1032 and 990cm~^ appear which are characteristic of physisorbed pyridine. Subsequent purging with pure N2 leads to the desorption of physisorbed pyridine only (the bands at 1032 and 990cm~^ disappear, whereas that at 1014 cm~^ is unchanged). Therefore, the supported polymolybdate or polytungstate species have acidic Bronsted sites if no pretreatment to desorb water has been performed prior to the absorption experiments. 5. Sulfidation of Precursor Oxides The precursor oxides have to be sulfided (activation step) before being used in the catalytic reactor. This activation process yields supported sulfide

Application to Materials Science

249

(a) (cm - i i

15

M-

380

10J

385

yJ

\

J

L.

L 12

8

% M0O3 (b)

L (nm) 4

-L

J-

8 %Mo03

12

Figure 3 (a) Wavenumber and width of the E2g band and (b) average length of an M0S2 crystaUite versus Mo loading.

particles. The same controlled atmosphere and controlled temperature cell can be used to make in situ Raman analyses of the surface phases appearing during the sulfidation of oxides by H2/H2S or N2/H2S mixtures. During sulfidation intermediate sulfides (MS3) and complex oxysulfides [(MoS2(S2)n)^~] have been identified (Payen et at., 1989). After complete sulfidation, nanocrystallites of M0S2 and WS2 are formed whose dimensions correlate well with the oxide loading for hydrated catalysts (see Fig. 3).

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and S. Nakashima

6. Conclusion Raman spectroscopy is a sensitive method of monitoring the state of 7Al203-supported catalysts before (precursor oxides) and after activation by sulphidation.

B. Ceramics

1.

Introduction

The Raman microprobe is particularly well adapted to the analysis of ceramics, as it allows the investigation of highly localized volumes in ceramic microstructures. The dimensions of these volumes are comparable to typical grain sizes. This situation is in contrast to that of conventional X-ray diffraction techniques, where the probed volume cannot be localized. Moreover, microphases and inclusions can usually be observed directly under the microscope with the use of conventional illumination techniques (because of differences in reflectivity or color). These elements are normally not visualized by scanning electron microscopy. 2. Polyphase Ceramics (a) Silicon and boron nitride Two polymorphs of silicon nitride (Si3N4) are known to exist {a and 0). The powdered a form is usually the starting product employed in the manufacture of hot-pressed or injection-moulded silicon nitride alloys. The j8 structure is formed during hot pressing. Thus the a-to-j8 phase transition is often used as a monitor of the extent of the hot pressing process. As the spectra of the two structures are completely different (a-Si3N4 is characterized by bands at 262, 365, 514, 670 and 850 cm~^; j8-Si3N4 exhibits a triplet with components at 185, 210 and 230 cm~^), they provide a convenient means of distinguishing the structures. In the same way silicon oxynitride (Si2N20), which often coexists with the nitrides, is easily identified by its Raman spectrum (bands at 187 and 254cm~i). The ultra-hard borazon ceramic is the cubic phase of boron nitride (j8-BN or Z-BN) produced at high pressure and temperature. This material is characterized by the TO and LO modes of the cubic lattice which appear at 1055 and 1306 c m ~ \ respectively. Conversion of the cubic phase to the hexagonal one can be easily seen because the latter, which is isoelectric with graphite, has only a single band at 1367 cm~^.

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(b) Partially stabilized zirconia (PSZ) Zirconia (Zr02) is normally monoclinic at room temperature and transforms to the tetragonal and cubic forms at higher temperatures. However, these higher-temperature structures can be stabiUzed by mixing the starting material with additives such as alumina, magnesia, ceria or yttria. Multiphase ceramics based on the constrained, metastable, tetragonal zirconia constitute a class of technically important materials due to their mechanical properties (flexural strength, toughness, wear resistance, etc.). In recent years considerable effort has been employed in the research and development of these ceramic materials for use as engine components. The toughening mechanism in PSZ depends on a volume expansion and sheer strain that occur when tetragonal zirconia transforms irreversibly into the monocHnic form. This transformation is initiated by the stress fields that form ahead of any crack propagating in the material. The size of the transformed zone is an important parameter used to model the enhanced toughness derived from the transformation. Thus, the determination of the size of the transformation zone is of prime importance and imphes that the relative concentration of the monoclinic phase should be obtained with a good spatial resolution. The monochnic and tetragonal polymorphs of zirconia have distinct and characteristic Raman spectra. In particular, over the range 100-300 cm"^ the monoclinic doublet at 181 and 192 cm~^ is well separated from the tetragonal bands appearing at 148 and 264 cm""^. This result permits the monoclinic concentration to be measured with the use of a relation proposed by Clarke and Adar (1982), namely tl81_j_2l92 ^m

77/0^148 , Ci264\

, ^ 1 8 1 , ^^192

V-^^'

where 3 ^ and 3^ are the integrated intensities of the characteristic bands of each phase (refer to superscripts) and F is a correction factor to allow for the increased Raman cross-section of the monochnic phase with respect to the tetragonal one. Most of the experiments reported in the hterature use the Vickers hardness tester to produce cracks in the ceramic material. The focused laser beam is first positioned on the indentation cracks and then at successively greater distances from the crack. At each point, a Raman spectrum is recorded and the monochnic concentration is evaluated. Plots of the relative monochnic concentration as a function of distance from the stress-induced crack can be estabhshed for different indentations and materials. With the use of a Raman imaging technique, Veirs et al. (1990) obtained maps which give the monoclinic fraction in phase-transformed zones surrounding cracks induced in magnesia-stabihzed zirconia.

252

P. Dhamelincourt

3. High-Tc

and S. Nakashima

Ceramics

(a) Introduction Since the high critical temperature superconductors were discovered in 1986, there has been intense interest in their investigation and characterization by Raman spectroscopy. Many superconducting oxides are now known with various critical temperatures. However, the most interesting ones are those with Tc above the liquid N2 temperature (Tc > 77 K) because of their practical applications (see Fig. 4). Their technology is the same between 77 K and room temperature. (b) Structure of high-T^ superconductors and Raman spectroscopy The common feature of all high-r^ superconductors is that these compounds are copper-oxide-based ceramics for which CUO2 planes are present in a more-or-less oxygen-deficient perovskite structure (see Fig. 4). In these CUO2 planes the electrons which are missing from the closed oxygen shell are responsible for the superconductivity. Thus, most of the Raman work on high-Tc materials has been devoted to phonon characterization because of its possible application to the investigation of the mechanism of superconductivity and, more particularly, to the study of vibrations in the *superconducting' CUO2 planes. [For a review in the field see Ferraro and Maroni (1990).]

T{K) 130 A2M2Ca„_^Cu„02„44 n = 1-3 A = BiorTI M = Sr or Ba

120 110

r

oj

100 O

YBa2Cu30y

90 80 LIQUID

N2 BARRIER

70 k

(a) Figure 4 Superconducting oxides with T^ above liquid nitrogen temperature, (a) Composition and (b) structure.

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253

In addition to the fundamental studies of phonon and related electronic properties, Raman spectroscopy has been extensively used to evaluate the quality of variously prepared high-Tc materials (e.g. bulk powders, thin films and single crystals). (c) Interest in Raman microspectroscopy The potential appUcations of high-r^ superconducting materials are mainly in the field of superconducting microelectronics [superconducting, quantuminterference devices (SQUIDs), superconducting microwave and sub-mm devices, etc.]. But this possibility impUes the fabrication of very-high-quality superconducting thin films. Micro-Raman spectroscopy cannot be used directly as a test of superconductivity, as unequivocal connections between specific vibrational modes and superconductivity have not been made. However, this technique is particularly well adapted for controUing the microchemical structure of the films (compositional heterogeneity and impurity phases), as well as the quality of the epitaxy. The MBa2Cu30(7_;^.) compounds, which have been the most investigated, provide a good illustration of the power of micro-Raman spectroscopy as a controlling technique. These compounds are synthesized with the use of the ternary system BaO(BaC03)-M203-CuO, where M may be any of the rare-earth metals (Y, Gd, Ln, etc.). The yttrium compounds are the best-known members of the series, where Yi 2,3 is the common name given to the yttrium-based compounds (YiBa2Cu3). When x is close to zero, these compounds are superconducting at Tc = 90 K and have an orthorhombic structure. On the other hand, when x is close to unity they are semiconductors with a tetragonal structure. There is now a general agreement that the wave numbers of the five Raman-active Ag modes of the orthorhombic phase are: 502 cm-^' 436 cm-^^ 335 cm" ^ 146 cm-^^ 115 cm-^^

0(IV) 0(II)-Cu(2)-0(III) 0(II)-Cu(2)-0(III) Cu(2) Ba

(axial motion) (in-phase bending motion) (out-of-phase bending motion) (axial motion) (axial motion)

In the early stages of this work many studies were made on poorly controlled materials. Thus, the assignments of phonon symmetries were often ambiguous, or even erroneous, due to the presence of impurities (see below). (d) Characterization of impurity phases Impurity phases are byproducts of the processes which lead to the preparation of superconducting MB CO materials. Micro-Raman spectroscopy may

254

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Table 1 Observed Raman wavenumbers of MB CO materials. Phase

Color

Wavenumbers (cm~^)

Y2CU205

Blue-green Green Black Black

210, 315, 390, 480, 605 265, 330, 395, 516 640 640, 585

YzBaCuOj BaCu02 BaCuO(2+J

be used to characterize impurity phases inside the targets used in sputtering techniques (e.g. DC or RF diode, DC magnetron, or laser), as well as inside the superconducting thin film itself. Though many of the impurity phases have a number of Raman Hnes close to, or coincident with those of MBCO, they are now well characterized (Etz et aL, 1991), as shown in Table 1. (e) Stoichiometry

monitoring

The success of sputtering techniques is a result of their ability to produce homogeneous, stoichiometric thin films on substrates. Post-deposition annealing is generally made in order to optimize the superconducting properties of these films. The critical temperature T^ is very sensitive to the oxygen stoichiometry. Furthermore, thermal annealing under atmospheric-oxygen pressure ensures the reoxygenation of the oxygen-deficient sputtered films. Some modes of vibration of the sample are very sensitive to the oxygen content. In particular, the mode at 500 cm~^ can be used to monitor the homogeneity of the stoichiometry of the films. A number of Raman studies have shown that the frequency shift of this mode is well correlated with the oxygen content (Burns, 1991; Huong, 1991). This observation has been used to test in situ the homogeneity of the film on the jjim scale. (f) Epitaxial quality Oriented superconducting thin films present a very selective anisotropy in their polarized Raman spectra. In particular, spectra recorded with incident and scattered polarization along the c axis {zz spectrum) are quite different from those with polarization along the a ov b axes {xx or yy spectra). With the aid of polarization measurements, it is possible to determine the orientation of any surface and thus to establish the orientation of the film on the substrate. (g) Conclusion Although micro-Raman spectroscopy does not provide a direct test for superconductivity, it is an excellent tool for characterizing the quality of thin superconducting films deposited on substrates.

Application to Materials Science 255 C. Protective Coatings

1. Polycrystalline Diamond Coatings The fabrication of diamond films by chemical vapor deposition (CVD) and, more recently, by plasma-enhanced, chemical vapor deposition (PECVD) at low pressure, has opened potential applications in numerous hightechnology areas. A considerable effort has been made in the perfection of these techniques (Bachmann et al., 1991). Diamond films are produced in order to take advantage of the well-known properties of this substance, which include high thermal conductivity, hardness, chemical inertness and electrical resistance. However, in optical applications it is their transparency that is important, not only from the UV to the far IR, but also in the X-ray region. Micro-Raman spectroscopy provides several key advantages for the investigation of carbon films deposited with the use of any of the CVD techniques. In addition to its spatial resolution, which permits the study of individual microcrystals as well as thin films, Raman spectroscopy can distinguish the various forms of carbon. Thus, carbon with sp^-type bonding (diamond), carbon with sp^-type bonding (graphite and carbonaceous materials) and carbon in mixtures of these two types of bonding (diamondlike carbon) can be characterized by their Raman spectra (Sarvides, 1986). Films prepared by vapor deposition (evaporated or sputtered carbon) are typically diamond-Uke, amorphous carbon films (DLC). On the other hand, films prepared by PECVD methods (DC, RF or microwave plasmas) are either crystalline diamond or DLC films, depending on the conditions of deposition, i.e. nature of the plasma, nature and temperature of the substrate, flow rate and current density (Piano and Adar, 1987). Diamond and perfect graphite are each characterized by a single Raman line which appears at 1332 and 1580 cm "^, respectively. However, when the graphite lattice is disordered, a second line appears at 1360 cm~^ which grows in intensity with increasing disorder (Beny-Bassez and Rouzaud, 1985). Furthermore, both bands broaden as the disorder increases (see Fig. 5). The Raman spectra of DLC differ notably from those of graphite and amorphous carbon (Sarvides, 1986). The DLC spectra are characterized by a very broad band centered at 1530 cm~^, with a more-or-less distinct shoulder at about 1400 cm" 1 (see Fig. 5). An important feature of the Raman spectrum is that it is very sensitive to carbon materials having sp^-type bonding. The Raman cross-section of these materials is far higher than that of diamond, thus small amounts of graphite or diamond-Uke carbon mixed with diamond are easily detected. For example, films that appear to be purely polycrystalline diamond on the basis of electron diffraction results often exhibit bands that correspond to disordered carbon (graphitic or DLC).

256

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1500 Wavenumber

Figures Characteristic Raman spectra of carbon materials, (a) Pyrolitic carbon (highly oriented graphite), (b) polycrystalline graphite, (c) amorphous carbon and (d) diamond-like carbon.

2000

1500 Wavenumber

1000

500

(cm-l)

Figure 6 Raman spectra of a diamond coating on an Si substrate, (a) Single microcrystal, (b) grain boundary. An example of the analysis of a polycrystalline diamond film deposited on an Si substrate is shown in Fig. 6. The spectrum recorded from one microcrystal (Fig. 6a) exhibits the well-characterized sharp diamond line at 1332 c m ~ \ together with very weak bands which are characteristic of graphitic carbon. On the other hand, the spectrum recorded from an area where microcrystals are not adjacent (Fig. 6b) is characteristic of DLC mixed

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with diamond. It is worth noting that the spectum of DLC always exhibits an increase in the background due to luminescence emission induced by diamond-lattice imperfections (Etz et al., 1988). Micro-Raman spectroscopy is thus a rapid and sensitive method of characterizing the quality of diamond films and other carbon coatings. The Raman microprobe technique was apphed to the characterization of diamond films by Bonot (1990), who measured the Raman spectra of individual crystaUites of various shapes. For diamond crystallites which are well faceted, the spectra show only the Raman component, but the bands are broader than those obtained from natural diamond. Ager et al. (1991) have studied the frequency and shape of Raman bands for a number of crystallites in diamond films grown by chemical vapor deposition. With the use of a two-dimensional detector they obtained 500 data points from different positions on each of the single films grown under different conditions. It was found that the Raman frequencies and bandwidths are correlated and that the films with higher frequencies have larger bandwidths. 2. Silica Coatings EthylsiHcate paints, charged or not with zinc particles, provide excellent protection against corrosion of steel structures attacked by water or chemicals. The sol-gel transformation of ethylsilicate leads to amorphous sihca, a very inert material. However, it can be apphed only to steel which has been previously sand-blasted in order to permit a mechanical linkage between the silica coating the steel surface. Recently, a new process has been developed (Dhamehncourt et al., 1989; Mayot et al., 1989) which permits both unpolished and polished steel to be coated with ethylsilicate paints. After dipping thlesteel structure in a bath of phosphoric acid, an ethylsilicate prehydrolyzate is vaporized at ambient temperature, resulting in bonding of an amorphous-silica coating to the metal. The chemical phosphatation pretreatment insures that the sol-gel transformation starts from the metal surface. By introducing the correct water vapor pressure in the medium surrounding the film, the reactions are carefully controlled to ensure that the film becomes dense without bursting as residual solvents are released during the densification. The coatings obtained in this manner (with thicknesses varying from 10 to 100 |xm according to the conditions of deposition) offer exceptional electrical insulation and thermal shock strength. Micro-Raman spectroscopy can be used to monitor the extent of silica formation and to characterize the nature of the compound formed at the substrate-coating interface (Mayot et al., 1990). Ethyl residues are well characterized by sharp bands appearing between 3000 and 1000 cm" ^, whereas amorphous silica exhibits wide bands near 500 and 1100 cm~^.

258

P. Dhamelincourt and S. Nakashima

3500

3000

_L

_L

2500

2000

Wavenumber

1500

1000

500

( cm-i)

Figure 7 Micro-Raman study of the interface between sheet steel and siHca coatings showing the strong condensation of ethyl polysilicate induced by the phosphatation pretreatment. Figure 7 presents the results of an analysis of a section of polished sheet-steel a few hours after the ethylsilicate paint deposition. It shows that at the surface the dissolution of the iron phosphate [Vivianite, Fe3(P04)2. 8H2O] has induced a strong densification of ethylsihcate in the first few ixm above the surface. In this region the spectra of the ethyl residues are barely observable. The process creates an interphase between the metal and the silica network which is responsible for the adhesion of the coating. Near the surface the densification process is not achieved. This result is clearly evidenced by the presence of the strong Raman bands of the ethyl residues.

III. MICROELECTRONICS AND SEMICONDUCTORS A. Introduction The Raman microprobe provides a powerful technique for the investigation of semiconductor materials and the analysis of problems in microelectronic devices. This method is a nondestructive one which is important for the characterization of semiconductors with composite structures, ceramics consisting of grains, heterogeneous and device structures, etc. A Raman microprobe measurement is not limited to the study of a local

Application to Materials Science 259

point in bulk materials and small particles. Recent developments in Raman technology have enabled one- or two-dimensional images to be obtained (see Chapter 4). Raman imaging provides information on the spatial distribution of physical quantities in materials such as strain, atomic fraction in mixed crystals, impurity concentrations, free carrier concentrations and local crystallographic orientation. This information is useful not only to evaluate the quahty of a sample, but also to infer the relevant dynamical processes, e.g. growth of crystallites, atomic diffusion and reactions at interfaces or surfaces. In an earher report (Nakashima and Hangyo, 1989) some results were presented on semiconductor characterization with the use of Raman microscopes. This section describes some further developments in this area, focusing attention on the Raman imaging technique. B. Raman Microprobe Measurements

Some precautions are necessary in the application of Raman microprobe measurements. Therefore, several of the problems which are relevant to micro-Raman studies will be briefly described in the following paragraphs. 1. Heating Effects The temperature rise in materials due to laser illumination under a microscope presents a serious problem for the evaluation of strain from observed Raman frequency shifts. The temperature variation not only produces shifts of Raman peaks, but local expansion of the heated region also causes additional strain (Liarokapis and Anastassakis, 1988). The frequency variation of the first-order Raman line of Si is about 0.02 cm~^ per degree at room temperature. This shift corresponds to that of the Si Raman line under a pressure of 0.1 GPa. Accordingly, the determination of the local strain in crystals requires that the Raman microprobe measurements be carried out at minimum laser powers. A point-illumination method is widely used for Raman microprobe measurements of semiconductors because high spatial resolution can be obtained. However, this method results in heating, and possible sample degradation, even when low laser power levels are used. Particular care should be exercised in the measurement of powders and thin films on insulators, because their thermal diffusion is poor. As the laser power level is decreased, Raman peaks shift in general toward lower frequencies. Optimum laser power can be determined if a level can be found below which the Raman peak does not shift. Huang et al. (1990) observed the Raman spectrum of Si with the use of a power of 0.05 mW fxm~^ in order to avoid the heating effect.

260

P. Dhamelincourt and S. Nakashima

2. Oblique Incidence When a wide-aperture objective lens is used a large fraction of the laser beam enters the sample at large angles with respect to the surface normal; this oblique incidence results in an apparent breakdown of the Raman polarization selection rules (Turrell, 1984; Mizoguchi and Nakashima, 1989). This situation is the same for the scattered hght (see Chapter 2). Therefore, these effects should be taken into account in the determination of crystallographic orientation of crystals by Raman microprobe polarization measurements. The use of an objective lens with a small numerical aperture or the rejection of the light at large oblique incidence with the use of a suitable diaphragm is desirable for polarization measurements. 3. Depth Profiling The depth resolution for semiconductors which are opaque to laser Ught is limited by the optical penetration depth. In order to obtain Raman spectra with high depth resolution, the following methods have been used: (i) One observes Raman spectra with the use of various exciting Unes which have different penetration depths. The resulting bandshape change is analyzed with the aid of a model based on the convolution of the penetrating depth of the light and the depth dependence of the Raman bandshape (Shen and Pollak, 1984; Hang et al., 1987). (ii) With the use of beveled samples, Raman spectra of the beveled edge are observed as a function of position by translating the sample under the microscope. The spatial variation of the spectrum is analyzed by the convolution method similar to method (i). Depth profihng of a beveled specimen has been studied with strain and disorder in the GaAs epitaxial layer on Si (Huang et al., 1987), strain in Ge;».Sii_;»;/Si-strained superlattices (Chang et al., 1988), strain in laser-annealed amorphous Si (Inoue et al., 1986), interface disorder of a GaAs/Si heterostructure (Mlayah et al., 1990) and composition in Al;^.Gai_;^. as mixed-crystal layers (Abstreiter et al., 1978). (iii) Depth profiles are obtained from Raman measurements by the use of successively etched surfaces (Kakimoto and Katoda, 1982; Holtz et al., 1988; Roughani et al., 1989). C. Ion Implantation and Annealing Ion beams are widely used in the fabrication of semiconductor devices. Ion implantation is an important technology for impurity doping. The damage to crystals resulting from ion implantation, as well as the recovery of crystallinity after annealing, has been studied by Raman spectroscopy.

Application

1

"1

r

r

to IVIaterials ~i

Science

261

r

CrystdUine Si Implanted with As* 2x10^0111^2

1.5X10 Cm^,.. ^..^u;,;;^,..:^^.,^.

..^..A. 7U \ < ^ O

n

'

-2

9x10 cm

6x10 cm

^,•^^../•;Vw.tf•y-••V.^V*^*•*T•*Jf''^"»^'>

(a) SEEDED SOI

-Si02

UNSEEDED SOI

Si02

(b)

Figure 15 Structure of laser-recrystallized silicon-on-insulator (SOI), (a) Seeded SOI and (b) unseeded SOI. is no opening in the Si02 film (Fig. 15b). The Seco-etched samples showed that there are a number of grains with small and large areas. The large grains lie in the central region of the recrystallized stripes and the small grains are in the circumference of the stripes. The average size of the large grains is about 20 X 200 |xm^ and that of the small grains is a few |xm. Crystallographic orientations of the laterally seeded and unseeded SOI are determined by the polarization Raman microprobe technique. As shown in Fig. 16, the seeded SOI exhibits a variation of the crystal axes with distance from the seeded region along the direction of laser scanning. At the seeded region, the orientation of the recrystallized film is the same as that of the substrate. The (001) axis is normal to the surface. Going away from the seeded region, the crystal axis varies gradually and at the point B, which is 2 mm from the seeded region, the (001) axis of the film is inclined by about 45° with respect to the surface normal. Figure 17 shows the orientations of vectors normal to the surface for various small grains in unseeded SOI. It can be seen that the normal vectors gather in a certain region. One of the reasons for this tendency may be related to the interface interaction between the silicon film and Si02 layer. Local crystallographic orientations have also been measured for laser-recrystallized silicon films by Kolb et al. (1991), Hopkins et al. (1984), Nakashima et al. (1983) and Nakashima and Hangyo (1989).

272

P. Dhamelincourt

and S. Nakashima

Figure 16 Variation of the (001) axis of the recrystalhzed film along the scanning direction of the laser. The measurement was made at intervals of 100 |xm.

Figure 17 Distribution of the surface orientations of small grains in unseeded SOL The open circles show the unit vector along the normal to the surface.

Application to Materials Science 273 Yoshikawa et al. (1991) applied the technique of Raman microprobe determination of crystal orientation to diamond films grown on cubic BN. They confirmed by this experiment that a single crystal of diamond grows on the (100) surface of cubic BN.

E. Distribution of Free Carriers The control of conductivity (carrier concentration, mobility) in semiconductors is important in device fabrication. Contactless and nondestructive characterization of the concentration and mobility of free carriers is desirable. Raman spectroscopy is a potential technique for this purpose. However, plasmons formed by the free carriers do not couple with Raman active modes in centrosymmetric semiconductor crystals such as Ge and Si. Plasmons were observed in highly doped Ge, but their intensities were low (Cerdeira et al., 1984). In noncentrosymmetric crystals such as zincblende and wurtzite-type semiconductors, the plasmon and an LO phonon form a hybridized mode, the so-called LO-phonon plasmon-coupled (LOPC) mode. This mode is Raman active and has two branches, L+ and L_. The Raman frequency, intensity and shape of the LOPC mode depend strongly on the plasma frequency o)^ and the damping constant y. Both o)^ and y are related to the carrier concentration n and mobility ^t, respectively, through the relations w^ = ATTn^l{eo,nf)

(7)

y=T-i = e/(m»,

(8)

and

where e^ is the high-frequency dielectric constant and m* is the effective mass of the carriers. The analysis of the Raman band of the LOPC mode, therefore, enables the carrier concentration and mobility in semiconductors to be measured. The optically determined carrier concentrations and mobihties are consistent with the values obtained from Hall effect measurements (Irmer et al., 1983; Yugami et al, 1987). The Raman microprobe technique can be applied to the characterization of the nonuniform distribution of dopants in compound semiconductors, which are introduced by atomic diffusion, heteroepitaxial growth at high temperatures and ion implantation. The method is also useful in the evaluation of electrical activity of dopants introduced by ion implantation and subsequent annealing. The distribution of carrier concentration and carrier mobility in GaP hght-emitting diodes (LED) have been obtained by Nakashima et al. (1988). The LED diode used had a p"^-n-n"^ junction structure, as shown in Fig. 18.

274

P. Dhamelincourt

and S. Nakashima

20

1

1

AO 60 DISTANCE ( p m )

1

1——1

1

r

I

l

l

1A

- (b)

-

j

12

r 10

M

^ 8-

^ 6 S 4 u. 2 n

-

j

\

H \ ^ \

\

1

rp—O-O—O——O——O

1

20

20

1

1

1

1

_ J

C

J

1

80

100

AO 60 80 DISTANCE ( j j m )

100

AO DISTANCE

60 (urn)

Figure 18 (a) The intensity, (b) bandwidth and (c) peak frequency of the plasmonLO phonon-coupled mode plotted as a function of distance from the outer surface of the GaP LED (Nakashima et aL, 1988).

Application

to Materials Science — 1

1

-|

1

(b)

275 \—

-

o

300

'>

1

\

1 ^

e

\ \ / >o-o-o

u

>• =1-200 1— _j

p*

CQ O

I

n

100

1 "•

_

> n 1 20

AO 60 80 OISTANCE(pm)

100

L 20

^

_!_

1 _ _

AO 60 80 DISTANCE ( p m )

1

100

Figure 19 Distributions of (a) carrier concentration and (b) mobility in GaP LED obtained from the analysis of the results in Fig. 18 (Nakashima et at., 1988).

The Raman spectra were measured at various points in a cross-section of the diodes. In a GaP carrier, damping is large (wpT0.6) is usually necessary for efficient collection of the scattered light, so that a useful spectrum, with adequate signal-to-noise ratio, can be collected in a reasonable time. This condition was especially important for work with scanning Raman spectrometers, although it is now less of a consideration. With the advent of instruments equipped with diode-array detection (where considerably more signal averaging is obtained in an equivalent time) or highly sensitive charge-coupled device (CCD) detectors, as well as Fourier transform instruments, this problem has effectively been eliminated. In any case, it is alleviated with the use of the high-N.A., dry objectives commonly used in optical microscopy, which have working distances of a few mm. These objectives can be used in most studies of powdered or bulk samples at ambient conditions. However, this method is no longer possible in the study of samples included in other minerals at depths greater than 75-100 luum with respect to the host surface, or located inside

Earth, Planetary and Environmental

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291

in a high-pressure cell (usually a diamond-anvil cell) or a heating furnace, which necessitate much longer working distances. These appHcations require the use of specially designed, long working distance, dry objectives, with as high a numerical aperture as possible, coupled with efficient detection of the Raman signal. Often the loss of light intensity, both from the incident and the scattered beams, is large when dry objectives are employed, because of the high refractive index of the minerals studied (usually in the range 1.5-1.7). This problem can be particularly critical in the observation of inclusions. Immersion objectives with oil or water as immersion media can be used to resolve this problem. The advantage of water immersion objectives is that water does not give rise to a strong Raman signal, provided that axial spatial filtering is employed. Their major disadvantage is the rapid evaporation of the water. Oil immersion objectives use an alkane as an immersion medium. However, the high intensity of their C—H stretching bands around 2900 cm~^ renders this medium of little use for fluid-inclusion studies if an alkane (usually methane) is being investigated. The use of immersion objectives requires a large, flat surface to retain the immersion Hquid, a condition which is not always fulfilled, especially in gemmological studies. Fluorescence is often a major problem in the application of Raman spectroscopy in the earth sciences. The major difficulty is due to electronic fluorescence, excited by the incident laser light, which is often much more intense than the weak Raman signal. This problem is particularly important in the case of iron-containing minerals, or for fluid inclusions with hydrocarbons present (Wopenka et al., 1990). In some cases, prolonged exposure to the incident laser beam will 'bleach' the fluorescence, and a Raman spectrum can be obtained. This effect is most often observed with organic materials (Pasteris, 1988). In other cases it is possible to use a laser wavelength which does not excite electronic transitions. For example, useful spectra of iron-containing minerals and glasses can be obtained with the use of red or yellow excitation (Griffith, 1969a,b, 1974; Mao et aL, 1987; Sharma and Cooney, 1990, 1992; Wang et aL, 1991a). Because the intensity of the Raman scattering is dependent on the inverse fourth power of the excitation wavelength [see Eq. (7) Chapter 1], the use of red laser light results in a severe reduction in signal-to-noise ratio, thus longer acquisition times are required. However, many of the photomultipUers, diode-array or CCD detectors which are available for Raman spectroscopy are optimized for maximum sensitivity in the red spectral region. One elegant solution to this problem is the use of a Michelson interferometer to obtain a Fourier transform Raman spectrum (Chase, 1987). In this experiment, a near-IR laser (for example, the 1.06 |xm fundamental radiation of an Nd:YAG laser) is used to excite the Raman spectrum of the sample. This excitation has insufficient energy to induce transitions between electronic states in many

292

P. F. McMillan et al.

minerals; so fluorescence is avoided. The low signal-to-noise ratio per scan is compensated by the multiplex advantage of the Fourier transform method, thus extremely high-quality spectra of samples have been obtained in very short times (Chase, 1987). Another method of overcoming the fluorescence problem is to carry out time-resolved Raman experiments (Kamogawa et at., 1988; Sharma, 1989). Because the Raman effect is essentially instantaneous, whereas electronic fluorescence usually has a much slower response, it is possible to discriminate between the Raman and the fluorescent signals with the use of a gated detection system. Another interesting method for extracting the Raman signal from the spectra of strongly fluorescent samples is based on the digital analysis of the random noise in the total collected signal (Durham, 1989). Micro-Raman spectroscopy with the diamond-anvil cell can be difficult due to the fluorescence of the diamond windows, which often overpowers the weak Raman signal. The various solutions to this problem include the careful selection of diamonds (type II) for low fluorescence in the spectral range of interest, direction of the incident laser beam at approximately 45° with respect to the collection optics, and careful spatial filtering before the spectrometer entrance (Hemley et at., 1987a). Raman spectroscopy at high temperatures is difficult because of the strong thermal emission of the sample, for which the intensity at a given wavelength increases as the fourth power of the absolute temperature. Once more, long working distance objectives are required, especially for temperatures above a few hundred degrees Celsius, to avoid degradation of the objective (cooUng of objectives can be useful in this case), and spatial filtering is required. In fact, micro-Raman spectroscopy is a technique of choice for high-temperature studies. Because the incident beam is focused on, and collected from, a ixm-sized region of the sample, blackbody radiation from the remainder of the sample and the furnace assembly can be considerably reduced or eUminated by spatial filtering after the microscope. Time-resolved methods can be used to discriminate further between the Raman spectrum, which is excited only on laser irradiation of the sample, and the blackbody radiation background, which is emitted continuously at high temperatures (Sharma, 1989). Finally, studies of isolated small particles, especially ones which are highly colored or are inherently unstable (or metastable) at ambient conditions (e.g. jxm-sized particles of carbon polymorphs, high-pressure phases, minerals containing transition metals), are often complicated because the sample is damaged or destroyed under laser irradiation via absorption of the incident beam. Obvious solutions to this problem are to reduce the laser power or to choose an excitation wavelength which is not absorbed. Another technique which can be useful is to embed the sample in a transparent matrix material such as KBr to ensure good thermal contact with the matrix and thus to dissipate the heat generated.

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III. MINERALOLOGY AND PETROLOGY A. Phase Identification in Natural and Synthetic Samples One of the first steps in the characterization of a natural or synthetic rock sample is the identification of the phases present. This consideration is particularly important in conjunction with geological field studies, as well as in experimental petrology, so that the pressure and temperature at which a given mineral assemblage was formed, and the P-T trajectory which it subsequently underwent, can be established. Such P-T determinations carried out on individual, natural samples are then often used in a second step to reconstruct the P-T conditions on a regional scale, for example, during formation of a mountain belt. In the classic method, all of the mineral and other phases present in a rock sample are identified, along with their textural and spatial relationships. In addition, some assumptions about the equilibrium conditions are usually made. This analysis allows the pressure and temperature of the rock formation to be reconstructed from a prior knowledge of the phase diagram. Much information about the phases present and their relationships can usually be obtained by applying classical optical techniques with the use of the petrographic microscope, although there are limitations to this type of study. Some of these restrictions can be overcome by applications of micro-Raman spectroscopic techniques, especially since the characteristic Raman spectra of different mineral classes are becoming well known and understood (Griffith, 1987; McMillan and Hofmeister, 1988; Sharma, 1989). If an amorphous phase is present, optical microscopy shows only the presence of an isotropic material with a particular refractive index. MicroRaman spectroscopy, on the other hand, can be used to gain detailed information on the structural state of the glass (or gel), its degree of hydration, and can sometimes indicate the presence of sub-microscopic crystals within the glass. This capability can be particularly useful in the study of shocked phases. 7. Identification of Crystalline

Polymorphs

One area in which micro-Raman spectroscopy is an essential complement to optical microscopy is in the identification of crystalline polymorphs, which are difficult to distinguish with the use of optical techniques. The presence of certain polymorphs in a given mineral assemblage can provide important constraints on the P-T conditions encountered by a rock, especially during metamorphism. The Al2Si05 polymorphs (silHmanite, andalusite, kyanite) form, perhaps, the best known examples. These substances can usually be readily recognized and differentiated using optical techniques, but not in all

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Sillimanite

M-^uJjJ Kyanite

Andalusite

1200 1000

800

600

400

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Raman shift (cm"'') Figure 1 Micro-Raman spectra of Al2Si05 polymorphs (Mernagh and Liu, 1991a).

cases. They can, however, be easily distinguished by their characteristic Raman spectra, as shown in Fig. 1 (lishi et at., 1979; McMillan and Piriou, 1982; Salje and Wernecke, 1982; Mernagh and Liu, 1991a). In contrast, it is usually difficult to distinguish the Ti02 polymorphs (brookite, rutile and anatase) using optical microscopy. However, the differences in the symmetry of the crystals result in completely different Raman spectra, which permit their unambiguous identification by micro-Raman spectroscopy (Beny et al., 1989). The identification of high-pressure polymorphs of Si02 has played a significant role in elucidating the P-T histories of natural and synthetic minerals subjected to high static or dynamic pressures. For example, the identification of coesite in a clinopyroxene from Norwegian eclogitic rocks in the Caledonides (Fig. 2) demonstrated that the rock formation pressure was at least 30kbar (Smith, 1984). This observation has had a profound impact on the controversial origin of eclogites in Norway. Boyer et al. (1985) have also used micro-Raman spectroscopy to study coesites in a range of natural eclogites, and to identify sub-microscopic grains of the low-pressure Si02 polymorph, quartz, presumably formed by reversion of the highpressure sample. Gillet et al. (1984) used micro-Raman spectroscopy in conjunction with TEM techniques to identify coesite inclusions within pyrope grains in metasedimentary rocks from subducted continental crust, and proposed an elastic model to explain the preservation of the high-pressure

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|521 CS

>1-

C/)

zLU

12 2

< DC

178 CS 151 1

a^ 118 1 1-

1 1 1 1 400 600 800 1000 RAMAN SHIFT (cm-'')

o ro

\0A_ MPa|

ir

< _i Ui

'200'

'

300

1 400

1 500

1

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RAMAN SHIFT (cm-'')

Figure 7 Micro-Raman spectra of high-pressure phases in the Si02-MgO system, (a) Stishovite at atmospheric and high pressure (Hemley, 1987). (b) /3- and 7-Mg2Si04 (McMillan and Akaogi, 1987). (c) MgSi03 ilmenite (McMillan and Ross, 1987). (d) MgSi03 garnet, majorite (McMillan et al., 1989). (e) MgSi03 perovskite at atmospheric and high pressure (Hemley et al., 1989b).

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the mineralogy and rheology of the transition zone (Akaogi et al., 1989; Rubie, 1989; Guyot et al., 1991). In addition to carrying out in situ, high-pressure experiments of the forward transitions, Ming et al. (1991) have described the importance of studying the back-transformation of the high-pressure phases at ambient or low pressures. McMillan etal. (1991) used micro-Raman spectroscopy to study a sample of )8-Mg2Si04 which had been heated for several minutes at 580°C. They observed a spectrum similar to that of the low-pressure phase (forsterite, a-Mg2Si04), but exhibiting additional peaks in the 600-700 cm~"^ region. These features are characteristic of the presence of SiOSi linkages, indicating that the back-transformed sample contained some Si207 units, as in the high-pressure phase, in addition to isolated Si04 groups. Another interesting reversion study on a high-pressure mineral has been carried out by Durben and Wolf (1991) on the geophysically important perovskite phase of MgSi03. These authors used micro-Raman spectroscopy to study the variation of the Raman spectrum of this phase with increasing temperature at atmospheric pressure. In this investigation they found that above approximately 300°C MgSi03 perovskite begins to revert to a glass. In effect, this process puts the crystaUine lattice under compressive stress, resulting in shifts in the Raman bands towards higher frequencies. This observation places important constraints on high-temperature structural data measured on MgSi03 perovskite at atmospheric pressure. 2. High-pressure and High-temperature In Situ Studies Micro-Raman spectroscopy used in conjunction with the diamond-cell has been one of the most powerful techniques for characterizing Earth and planetary materials at ultrahigh (megabar) pressures, equivalent to those found deep within planetary interiors. The reason for this success has been due to the fact that such extreme pressures can be produced on very small samples (tens of |xm or less) in the laboratory; hence, micro-sampUng techniques are required to probe the material in situ under these conditions (Hemley et al, 1987a; Hemley and Porter, 1988). For phases which are likely to be present deep within the Earth, it is essential to characterize their structural and dynamic behavior at high pressures and temperatures. Micro-Raman spectroscopy is a technique of choice for such studies, especially for in situ work at high pressure with the diamond-anvil cell (Sharma et al., 1985; Hemley et al., 1987a; McMillan and Hofmeister, 1988; McMillan, 1989; Sharma, 1989). From available phase equilibrium data, high-pressure phases of Si02 are likely to be present within the mantle (Fei et al., 1990; Gasparik, 1990; Ringwood, 1991; Thompson, 1991). The best-characterized high-pressure phase of Si02 is stishovite, which has the rutile structure. The atomic displacements associated with the lowest-frequency (B^g) Raman mode of rutile-structured minerals correspond

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Figure 8 Variation of the Raman frequencies of stishovite with pressure (Hemley, 1987). to those required for a transition to the CaCl2 structure (Nagel and O'Keeffe, 1971). Hemley (1987), who used micro-Raman spectroscopy to investigate the vibrational spectrum of stishovite at pressures up to 33 GPa, observed that the frequency of the Big mode decreased ('softened') with increasing pressure, suggesting that a transition to a CaCVstructured, post-stishovite phase of Si02 might occur at pressures in the 100 GPa range (Fig. 8). This transition has now been observed by Kingma et al, (1993b). Hemley (1987) also observed evidence of a phase transition in coesite at 22-25 GPa. This work has recently been extended by Williams et al. (1993). Micro-Raman spectroscopy with the diamond-anvil cell has also been used to study the dynamics of MgSiOs perovskite up to 26 GPa (Hemley et al., 1989b). Gillet et al. (1993b) studied CaTi03 perovskite over a similar pressure range. Concerning the pressure dependence of lower-pressure phases which are stable in the crust and upper mantle, a number of workers have used Raman and micro-Raman techniques to investigate the vibrational behavior of forsterite and other olivines (Besson et al., 1982; Gillet et al., 1988, 1991; Chopelas, 1990; Liu and Mernagh, 1990), the Al2Si05 polymorphs (Mernagh

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and Liu, 1991a), several garnets (Mernagh and Liu, 1991b; Gillet et al., 1992), in addition to the extensive work which was carried out on a-quartz (Asell and Nicol, 1968; Dean et aL, 1982; Hemley, 1987; Jayaraman et al., 1987; WiUiams et al., 1993). There has also been much interest in the high-pressure structural and dynamic properties of carbonate minerals, in an effort to understand the phase stabihty of carbonates in the mantle, and the carbon budget and oxidation state of the deep Earth (Irving and Wylhe, 1975; Kushiro et al., 1975; Katsura and Ito, 1990; Blondy et al., 1991). Several micro-Raman investigations of carbonates have been carried out, at pressures in excess of 30 GPa (Gillet et al., 1988; Liu and Mernagh, 1990; Kraft et al., 1991; Biellman and Gillet, 1992; Gillet et al., 1993a). The study of calcite is particularly interesting, because two transitions to metastable forms of CaC03 (calcite-II and calcite-III) have been observed in the 1.4-2.0 GPa range (Fong and Nicol, 1971; Gillet et al., 1988; Hess and Ghose, 1988; Liu and Mernagh, 1990; Biellman and Gillet, 1992). In contrast, dolomite and magnesite show no evidence for any phase transitions up to the highest pressures employed (Kraft et al., 1991; Biellman and Gillet, 1992); nor does the stable, high-pressure aragonite phase of CaC03 exhibit such transitions. As noted earlier, micro-Raman spectroscopy is particularly adapted for in situ, high-temperature studies of minerals. This technique has been employed in such investigations of jxm-sized crystals of the high-pressure Si02 polymorphs coesite and stishovite (Gillet etal., 1990), and forsterite (Sharma, 1989; Gillet et al., 1991) and its germanate analogues (Gillet et al., 1989; Piquet et al., 1992), MgSiOs, CaGeOs and CaTi03 perovskites (Wolf et al., 1990a; Durben and Wolf, 1991; Durben etal., 1991; Gillet etal., 1993), and Ca-Mg carbonates (Gillet et al., 1993a). Sharma (1989) has also presented in situ, high-temperature, micro-Raman data for several polymorphs of MgSi03 enstatite. These last spectra are interesting in that they were obtained with a time-resolved technique which was used to eliminate the blackbody radiation background. These high-temperature observations are particularly important for exploring the intrinsic anharmonicity of vibrational modes, which can have an important effect on the high-temperature thermodynamic properties of these systems (Gillet et al., 1989a, 1990, 1991; Piquet et al., 1992). In particular, the work on the silicate and germanate oHvines has revealed some interesting indications of dynamic disorder at high temperatures which can be associated with a rapid increase in the mineral heat capacity just below the melting point, termed 'pre-melting' by Richet and Piquet (1991). Relatively little work has been done on rock-forming minerals under combined high-pressure, high-temperature conditions, although research is currently underway on this important topic in several laboratories. With the use of conventional (resistive) heating, Kraft et al. (1991) used micro-Raman spectroscopy with a diamond-anvil cell to investigate the vibrational spectrum

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of dolomite at pressures up to 11.5 GPa and temperatures to 550 K. In a similar study using conventional (macro-) Raman spectroscopy, Arashi (1987) investigated the monoclinic-orthorhombic phase transition in the important ceramic Zr02 under high P-T conditions. Most recently, Gillet et al. (1993b) have used micro-Raman spectroscopy in a diamond-anvil cell heated with a CO2 laser to follow the phase changes in CaTi03 perovskite to pressures and temperatures of 12 GPa and 1600 K, respectively. In addition to the intrinsic value of the micro-Raman data for identification and structural characterization of mineral phases, the vibrational data are essential for testing theoretical models, both empirical and ab initio, of their lattice-dynamical properties. These calculations are particularly important for understanding the phase stability and structural behavior of minerals at high temperatures and pressures which are often well out of the range of experimental measurements (Bukowinski and Wolf, 1986; Cohen etal., 1987; Hemley et al, 1987b, 1989b; Price et al, 1987; Wolf and Bubowinski, 1987; Wall and Price, 1988). Micro-Raman spectroscopy has also played an important role in establishing a basis for similar calculations for low-pressure mineral phases. For example, Sato and McMillan (1987) used the microRaman technique to obtain vibrational spectra of fxm-sized grains of isotopically substituted (^^Si-^^Si and ^^O-^^O) quartz (Fig. 9). These data were then used to test the results of a lattice-vibrational calculation on a-quartz, with the use of valence force constants derived from ab initio cluster calculations (McMillan and Hess, 1990). Measurements performed on BeO show that the wurtzite structure of the mineral (bromellite) is stable to at least 40 GPa (at 300 K), in agreement with theoretical calculations (Jephcoat et al, 1988).

3. Calculation of Thermodynamic

Properties

Because the lattice vibrations provide the primary sink for thermal energy in crystal structures, a knowledge of the complete vibrational spectrum, usually expressed as the phonon (vibrational) density of states [g(a>)], permits a calculation of the vibrational heat capacity Cy{T), and associated thermodynamic properties (Salje and Viswanathan, 1976; Kieffer, 1979a,b,c, 1980, 1982; Salje and Wernecke, 1982). In general, the total, vibrational density-of-states function is not known, and must be modeled from available experimental data. Kieffer (1979a,b,c, 1980, 1982) and Salje and Viswanathan (1976) have developed methods for constructing such models of the g{(o) functions with the use of information from Raman and infrared spectra, as well as elastic constant measurements. Heat capacity calculations have been carried out with this method for a wide variety of important rock-forming minerals, including high-pressure phases (Kieffer, 1979a,b,c, 1980, 1982; Akaogi etal, 1984; McMillan and Ross, 1987; Gillet etal, 1989b,

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SiO:

30SiO' CO

z

Sil802

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500

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RAMAN

SHIFT

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Figure 9 Raman spectra of the isotopic species of a-quartz, Si02, ^^Si02 and Si^^02 (Sato and McMillan, 1987).

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MODEL I

(Jmor^K-^) ANHARMONIC MODELS 175 h

160 h

115 300 0

(a)

105

644

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WAVENUMBER (cm"^)

(b)

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TEMPERATURE (K)

Figure 10 Calculation of the heat capacities of forsterite. (a) Models of the density of states consistent with the spectroscopic data (optical modes). The values shown in the boxes are the number of modes in that continuum, (b) Harmonic and anharmonic values of Q calculated with the densities of states shown in (a). D.P. represents the Dulong-Petit limit (Gillet et a/., 1991).

1990, 1991; Fei et al, 1990; Madon et ai, 1991; Hofmeister and Chopelas, 1991; Hofmeister and Ito, 1992). In these works the micro-Raman technique was essential to obtain the spectra of the iJim-sized particles which were available, especially of the synthetic, high-pressure phases, and to assign the observed spectra reliably to the phase of interest. Gillet et al. (1989a, 1990, 1991) have recently made a significant advance in the application of such vibrational heat capacity calculations - especially in the high-temperature limit - by expHcitly considering the anharmonicity of the vibrational modes (Fig. 10). For this calculation, the temperature- and pressure-induced shifts of the vibrational frequencies are measured separately and used to obtain the intrinsic mode-anharmonicity parameters for use in the heat capacity calculation. The anharmonicity is shown to have a large effect on the calculated heat capacities at high temperature, as they are significantly higher than those calculated with the use of the harmonic model. The effects on thermal expansion have also been explored by Hemley et al. (1991). In all of this work micro-Raman spectroscopy proved to be a convenient tool for the study of these high-temperature and high-pressure vibrational properties.

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D. Phase Transitions in Minerals 1. Displacive Phase Transitions One of the classic applications of Raman spectroscopy in solid-state physics and chemistry has resulted from the study of displacive phase transitions through the observation of soft modes. A soft mode is an anharmonic vibration whose atomic displacements indicate the displacive changes associated with the phase transition. They can be driven thermally, by increasing the pressure, or by compositional changes (Raman and Nedungadi, 1940; Cochran, 1960, 1961; Scott, 1974; Samara and Peercy, 1981; Jayaraman, 1983; Ferraro, 1984; Wang 1984; Hemley et al, 1987a; Wong, 1987). The first Raman investigation of temperature-induced mode softening was carried out for the a-j8 quartz transition by Raman and Nedungadi (1940), who observed that the broad, 207 cm~^ band decreased rapidly in frequency with increasing temperature, to disappear at the a-j8 phase transition temperature (Fig. 11). They proposed that the atomic displacements associated with the 207 cm ~^ band might mimic the a-)8 displacive phase transition, which involves a co-operative rotatory movement of Si and O about the three-fold screw axes in the quartz structure. There have since been many experimental studies of the lattice dynamics of quartz through its a-/3 phase transition with the use of a variety of spectroscopic, thermodynamic and structural techniques; this basic picture is generally confirmed (Shapiro et al., 1961 \ Scott, 1968; Axe and Shirane, 1970; HochH and Scott, 1971; lishi, 1978), although the details of the phase transition near the transition temperature are considerably more complex than originally thought. For example, it has been shown that the a-)8 quartz phase transition does not proceed directly, but that the a and j8 phases are related by a series of incommensurate phases (DoUno, 1986). A second complication arises with the nature of the high-temperature phase, which may have either a dynamic or statistically averaged structure above the phase transition temperature (Dohno et al, 1983; McMillan and Hess, 1990). A second class of geophysically important compounds to which soft-mode Raman spectroscopy has been applied is perovskite; these compounds exhibit a rich series of phases with different degrees of structural distortion, starting from the ideal cubic structure. These phases are connected by displacive phase transitions (Cochran and Zia, 1968; Lockwood and Torrie, 1974). The earliest Raman work was carried out on SrTi03 perovskite, which showed a cubic-tetragonal phase transition at 110 K. The cubic perovskite phase has no allowed, first-order Raman bands; thus, the Raman spectrum is characterized by broad bands due to second-order vibrational transitions (Nilsen and Skinner, 1986). The transition to the tetragonal phase is marked by the appearance of a set of sharp Raman peaks, which are the allowed first-order Raman spectrum of the lower-symmetry phase (Nilsen and Skinner, 1986;

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Ai 215 cm"* (207)

\ , ^ \

oSi

B 599.1°C 542,6°C 395.6°C 182.8°C 33.2C —I— 500

I

400

I

1

I —

300 200 100 Raman Shift (cm~ )

Figure 11 The a-p displacive phase transition in quartz. (A) displacement vectors for the 207 cm~^ mode of a-quartz, and for the a-jS quartz structural displacement (McMillan, 1985). (B) High-temperature behavior of the 207 cm~^ mode (Shapiro et al., 1967). (C) Plot of the square of the Raman shift (v^) versus T-T^ for the 207 and 147 cm~^ modes, as shown in (b) and (c) of part A. Fleury et al., 1968). There is currently intense interest in the structure and dynamics of MgSi03 perovskite. Recent micro-Raman studies at high pressure and room temperature (Hemley et al., 1989b, 1990), and atmospheric pressure at high temperature (Wolf et al., 1990a; Durben and Wolf, 1991), have suggested that this is a major constituent phase of the Earth's mantle (Jeanloz and Thompson, 1983). No evidence has been found for such second-order displacive transitions in MgSi03 perovskite, although a firstorder phase transition appears to have been observed (Wang et al., 1991b). Gillet et al. (1993b) have recently used micro-Raman spectroscopy to investigate the behavior of CaTiOs perovskite at high pressure (up to 21GPa) and high temperature (up to 1450 K). They found evidence for the orthorhombic-tetragonal-cubic phase transitions suggested by both calorimetry and X-ray diffraction.

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There have been several Raman and micro-Raman studies on the influence of pressure on compounds with the rutile structure. There is a known high-pressure form of titanium dioxide (Ti02-II), which was originally suggested to have the a-Pb02 structure (BendeHani et al., 1966). Nicol and Fong (1971) and Samara and Peercy (1981) observed a softening of the lowest frequency (Big) mode of Ti02 with pressure using in situ Raman measurements. The displacements associated with this mode would lead to a transition to the CaCl2 structure, if the mode became dynamically unstable, suggesting that this structure might be possible for the high-pressure phase (Nagel and O'Keeffe, 1971). However, it has been shown (Mammone et at., 1980) that the Raman spectrum of Ti02-II is inconsistent with the CaCl2 structure. The latter authors concluded that the high-pressure phase has, in fact, the a-Pb02 structure. The same type of Big-mode softening has been observed in Sn02 by Peercy and Morosin (1973), and for Si02 by Hemley (1987), and by Kingma et al. (1993a,b).

2. Order-Disorder

Transitions

(a) Graphite and related compounds Raman spectroscopy has also been used in the analysis of order-disorder processes relevant to mineralogy and geochemistry. The study of graphiterelated carbonaceous compounds provides one of the simplest applications. Graphite is formed mainly in the continental crust, either from precipitation from a fluid phase, or, more commonly, from some organic precursor embedded in sediments under conditions of increasing temperature and pressure during burial to metamorphic conditions. This process is termed 'graphitization'. The degree of disorder in the poorly crystallized graphitic material can be an important petrogenetic indicator. Raman spectroscopy has been adopted as a useful tool for quantitative characterization of the degree of order in graphites. Because the carbonaceous particles can be very small, of the order of only a few |xm, the Raman microprobe technique is ideally suited for the study of natural and synthetic samples undergoing this graphitization process (Beny-Bassez and Rouzaud, 1985; Pasteris, 1988; Pasteris and Wopenka, 1991). Micro-Raman spectroscopy is also of interest because it is an in situ, nondestructive technique that does not require any extraction procedure during which the organization state of the material might be altered. In addition, the spatial scale probed by micro-Raman spectroscopy is of the order of that probed by medium- to high-resolution electron microscopy; thus, the onset of ordering can be examined in the samples (McMillan, 1984a). Perfectly ordered graphite has P63/mmc = Z)6h space-group symmetry. There are two Raman-active vibrational modes with E2g symmetry: one at

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42cm~^ which can be described as a ghding of adjacent aromatic planes (E2g^) with respect to each other, and the second at 1575 cm" ^ (E2g2)' corresponding approximately to C—C stretching within the hexagonal layers (Tuistra and Koenig, 1970; Song et al., 1976). An overtone vibration is also observed near 2700 cm~^. The graphite structure can be disordered in several ways: by relative rotation of adjacent layers, by puckering the planar sheets, or by disorder in the interlayer spacing (Rouzaud et al., 1983; Buseck and Bo-Jun, 1985). Any disorder has a marked effect on the Raman spectrum. With increasing disorder, the E2g2 band broadens and moves to higher frequency, and a new band appears at 1350 cm"^. The relative intensity of this additional band has been correlated with a structural 'correlation length' (La), determined from X-ray data, which corresponds to the average interatomic layer separation in the disordered structure (Tuistra and Koenig, 1970; Lespade et al., 1982, 1984). These effects have been explained by considering the phonon dispersion curves of perfectly ordered graphite (Lespade et al., 1982). If the crystal is uniformly disordered, this interpretation is equivalent to a consideration of a larger unit cell (or smaller Brillouin zone), so that the vibrational density of states makes a more important contribution to the Raman spectrum. The disordered structure is often identified by the appearance of the so-called Boson band at low frequencies. Micro-Raman data have been correlated with observations by optical and transmission electron microscopy on graphitizable and nongraphitizable reference-carbon series, as a function of heat treatment by Beny-Bassez and Rouzaud (1985); see Fig. 12a. In this work the graphitization process was monitored by the L^ value, which was directly measured from lattice fringes and dark-field micrographs obtained by electron microscopy, as well as the intensity of the 1350 cm~^ Raman band relative to that near 1600 cm~^. In the series of oxygen-poor, graphitizable anthracene cokes, the graphitization is characterized by a three-step process (Fig. 12b). The end-product of this heat-treatment is highly ordered graphite, obtained at 2700°C, accompanied by complete disappearance of the 1350 cm~^ default band. In contrast, nongraphitizable carbon heated at atmospheric pressure always exhibits the 1350 cm~^ defect band, even for samples heated to 3000''C. The role of pressure during the graphitization process has been demonstrated experimentally by Beny et al. (1986), who rapidly transformed nongraphitizable cokes into graphite at 1800°C and 5 kbar pressure. Because the degree of order in these samples is sensitive to temperature and pressure, and can be monitored with the use of Raman spectroscopy, the micro-Raman spectra of natural, graphitized samples can provide a useful metamorphic indicator. First-order micro-Raman spectra of eight carbonaceous samples from metasediments show a progressive decrease in the width of the £2g2 band, a decrease in its frequency to 1575 cm"^, and a decrease of the intensity ratio 3(1360 cm~^)/3(1580cm~^) from 1.2

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0

10^/L

2900°C 80

2500°C 70

J.

2000°C 1800°C 1500°C

60

^^^^^^^

1300°C

50

Vf^^.w/'''f^

***^

""N^^iv* 800°C

40 30 A

^^^j

% H 600°C

/ Vw^^^'^v. . ^

20 A

10

^''--- SEMI-COKE 1600

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20 30

40 50

60

-^1350

Figure 12 (a) Raman spectra of anthracene cokes heat-treated at different temperatures, (b) 10^/La versus 'S^i35o- The correlation length L,^ was determined from X-ray data; 51350 is the ratio of the integrated intensities of the 1350 and 1600 cm~^ bands (Beny-Bassez and Rouzaud, 1985). to 0.1 on passing from the low-grade metamorphic (prehnite-pumpellyite) to the high-grade metamorphic (staurolite) environments (Beny and JehHcka, 1991). In contrast, breaks in the evolution of the Raman spectra of graphitic carbons, documented by changes in the intensity ratio 3 (1350 cm~^)/3 (1580cm~^) of a series of metapelites, seem to be correlated with changes in silicate mineralogy (Pasteris and Wopenka, 1991). These authors suggested that the release of fluid, accompanied by locahzed stresses on carbonaceous grains due to changes in grain size, might produce changes in carbon crystallinity. These studies indicate that a great number of parameters control the graphitization process: the nature of the organic precursor, the pressure and any deviative stresses, the mineral reactions, the precipitation of graphite directly from the fluids, and perhaps other factors. This result opens up a new area in the investigation of metamorphic terrains with the use of a combination of micro-Raman spectroscopy and highresolution microscopy to analyze the carbonaceous species present. In a different type of study, the disordering of an initiafly perfectly ordered graphite sample in an alteration zone associated with a uranium deposit was monitored by micro-Raman spectroscopy (both first- and second-order

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Raman spectra), high-resolution TEM and X-ray diffraction (Wang et at., 1989). It is worth noting that the X-ray diffraction technique was not sensitive enough to monitor the increase in disorder of the graphite. In the micro-Raman spectra, the increase in disorder is marked by a frequency increase of the E2g2 band, an increase in its FWHM, and by an increase in the intensity ratio 3(1350 cm" V3(1580cm~^). This result is consistent with a decrease in the L^ parameter from 1000 to 50 A for the most altered graphite (from 100 to 5nm). (b) Silicates and other minerals McMillan et al. (1984b) investigated the Raman spectra of a series of well-characterized synthetic cordierite samples with differing degrees of Al-Si order. They found systematic changes in the spectra, including narrowing and splitting of peaks, with increasing Al-Si order. These changes coincided with structural changes at distances of the order of 100 A, as determined by electron microscopy. These changes appeared well before any transformation was apparent with optical microscopy or X-ray diffraction. Putnis (1980a,b) and Clemens et al. (1987) used micro-Raman spectroscopy, combined with high-temperature solution calorimetry, X-ray diffraction and high-resolution TEM, to investigate Al-Si and stacking disorder in phlogopite. McMillan et al. (1989) combined micro-Raman and infrared spectroscopy with ^^Al NMR spectroscopy to study cation-site ordering in high-pressure garnets along the Mg3Al2Si30i2-Mg4Si40i2 join. The disordering of ions over the A and B sites in spinels results in the appearance of additional Raman peaks due to a reduction in local symmetry (Fraas et al, 1973; McMillan and Hofmeister, 1988; McMillan et al, 1989). Malezieux et al. (1983) used the micro-Raman technique to investigate a series of natural spinels with different degrees of structural order. Cynn et al (1991) employed Raman spectroscopy to determine the cation ordering in MgAl204 spinel in situ at high temperatures, while Hofmeister and Chopelas (1991) studied garnet solid solutions. Salje (1985) has pioneered the apphcation of Landau's theory to the problem of Al-Si disorder in alkaU feldspars, and has described a method for using the temperature dependence of Raman-vibrational intensities as a parameter for quantifying both Al-Si and alkali-cation ordering in these phases (Salje, 1986). This technique will certainly prove to be a powerful one in future studies of ordering in aluminosiUcate and other minerals. 3. Pressure-induced

Amorphization

One of the more novel appHcations of micro-Raman spectroscopy is its use in the study of amorphization phenomena, specifically in pressure-induced

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amorphization. Raman scattering has been particularly useful in this regard because vibrational measurements are sensitive to both long- and short-range order in materials; i.e. the vibrational frequencies are determined to a large measure by local bonding properties, whereas the number and symmetry of bands are dictated by the long-range order in the crystal. The changes in both are therefore useful indicators of the loss of order at different length scales in the materials, as these observations complement the results of more direct probes of crystallinity, such as X-ray diffraction. Following the discovery using Raman spectroscopy of this amorphization phenomenon in ice (Mishima et al., 1984), the transformation was subsequently confirmed by Hemley et at. (1989a) with synchrotron radiation, which demonstrated the disappearance of the crystalline diffraction peaks, and by micro-Raman spectroscopy. This phenomenon was first observed in the Si02 polymorphs a-quartz and coesite at 25-35 GPa with the use of micro-Raman, diamondcell techniques (Hemley, 1987; Kingma et al., 1993b). In some recent studies of the transition in a-quartz Si02 and Ge02, micro-Raman spectroscopy has been combined with TEM to investigate the microstructural changes which accompany the amorphization process (Verhelst-Voorhees and Wolf, 1992; Wolf et al., 1992; Kingma et al., 1993b). Other minerals that have been studied by micro-Raman spectroscopy which are observed to undergo such pressure-induced crystal-amorphous transitions include cristobalite (Halvorson and Wolf, 1990; R. J. Hemley, unpubhshed), serpentine and portlandite, Ca(OH)2 (Meade etal., 1992) and Ge02 (Wolfed al., 1992), respectively. A number of minerals also undergo intermediate, metastable crystalline-crystalline transitions prior to amorphization. Examples include coesite (Hemley, 1987), cristobalite (Palmer er a/., 1992; Gratz etal., 1993), and serpentine and portlandite (Meade etal., 1992). In addition, there has been a growing number of observations of pressure-induced amorphization of related (e.g. mineral-like) materials with the use of micro-Raman, diamond-cell techniques (Fujii et al., 1985; Sankaran et al., 1988; Jayaraman et al., 1992; Serghiou and Hammack, 1992). Finally, micro-Raman measurements have also been carried out at atmospheric pressure on mineral samples amorphized by shock compression in the laboratory (Velde et al, 1989; Clough et al., 1992; McMillan et al, 1992a).

E. Micro-Raman Studies of Condensed Gases Perhaps most important to planetary science, as well as to condensed matter physics, is the behavior of hydrogen under ultrahigh pressures. Specifically, the understanding of the nature of the theoretically predicted transition of solid hydrogen to high-pressure metallic states requires detailed information on the structural, dynamical, and electronic properties of the material at ultrahigh pressures (100 GPa) (see Mao and Hemley, 1992, for a review).

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The characterization of the soUd under these extreme conditions has reUed exclusively on spectroscopic techniques used in conjunction with the diamond cell. Among these investigations, vibrational micro-Raman spectroscopy has been particularly useful, and has provided unique information on the behavior of hydrogen to near 300 GPa. As such, these measurements provide an important complement to other techniques, including X-ray diffraction, optical absorption and reflection, and infrared spectroscopy. Raman excitations in solid hydrogen involve intramolecular vibrational transitions (vibrons), lattice-mode excitations (phonons), and rotational bands (rotons). Measurements of the vibron frequency (VQ = A\55cmr^) provide a sensitive probe of the state of bonding in the molecular solid. The first Raman studies carried out up to 60 GPa with a diamond-anvil cell demonstrated that the frequency of the Raman-active vibron decreases with pressure above 30 GPa (Sharma et al., 1980). Subsequent investigations with the use of beveled, diamond anvils showed that the negative pressure shift continues to at least 147 GPa (Mao et al., 1985) and to - 2 5 0 GPa, as shown in subsequent work performed at 77 K (Hemley and Mao, 1988). These measurements demonstrate that the molecular bond is stable, although weakened, at these pressures. Recent Raman and infrared measurements indicate that the negative pressure shift can be understood in terms of a dramatic increase in intermolecular coupUng with pressure (Brown and Daniels, 1992; Hanfland et aL, 1992; Loubeyre et al, 1992; Silvera et al., 1992). At the highest pressures (corresponding to vibron frequencies at 3725 cm~^) there is also evidence for resonance enhancement at visible wavelengths, consistent with changes in electronic properties at these pressures, as measured by direct optical methods (Mao and Hemley, 1989). The Raman measurements of the vibron have also been instrumental in the discovery of a phase transition in sohd hydrogen at 150 GPa and low temperatures (Hemley and Mao, 1988; Fig. 13). The transition is characterized by a major, discontinuous shift in the vibron frequency at —100 cm~^ at a temperature of 77 K. A large number of subsequent experiments have been performed to determine the extent to which the transition is associated with an orientational ordering, or a structural or electronic transition (Hemley and Mao, 1989, 1990; Lorenzana et aL, 1989, 1990; Hemley et aL, 1990). An electronic transition could include the metallization process itself or the formation of a localized (e.g. excitonic) state. Measurements as a function of temperature reveal that the discontinuity decreases with increasing temperature up to a triple point. Direct measurements as a function of temperature show that the triple point is at —130K (Hemley et aL, 1990). Recent work shows that an analogous behavior occurs in deuterium (Mao and Hemley, 1994). Bands in the low-frequency Raman spectrum of molecular hydrogen (10.5). The bicarbonate ion is a weak Raman scatter, and has not been detected in fluid inclusions. All of these factors contribute to the observation that the main polyatomic ions identified in fluid inclusions to date are only S04~ and HS~ (Rosasco and Roedder, 1979). The sulfate concentration in primary fluid inclusions from upper triassic evaporates has been measured by Dubessy et al. (1983). In this work it was shown that the sulfate concentration was too low to have resulted from simple evaporation of a modern sea-water. The authors suggested a supply of calcium in the evaporating basin, probably due to dolomitization of carbonates found on the southern border of the German basin near the Alps. More recently, the bisulfate ion has been identified in the aqueous phase of complex fluid inclusions containing a mixture of liquid N2 and CO2 in the volatile phase (Dubessy et al, 1992a; Fig. 14). A calibration of the intensity

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ratio of the two bands has allowed an estimation to be made of the concentration of an extremely acidic fluid, with pH around zero. This is the first time that such highly acid geological fluids have been found. The samples were collected from a quartz vein cross-cutting itabyrites in the Tron quadrilateral' (Brazil). This observation raises two interesting geochemical questions. Did the fluids result from the oxidation of pyrite? Do the oxidizing fluids represent leachates of sulfate-bearing evaporate rocks? Unfortunately, the strong lateritization of the vein host rocks obliterated all of the information which could have led to an explanation of these unusual fluids, among the most acidic geological fluids ever documented. Hydrated monatomic ions, such as Na"^, K"^, Ca^"^, Mg^"^ and Fe^^, give rise only to weak bands in the 350-600 cm~^ spectral range. They are assigned to vibrations of the cation relative to the oxygens of the water molecules of the inner hydration sphere (Brooker, 1986; Dubessy, 1986). However, these bands are not useful for the identification of the particular cations present in an inclusion because they are very weak and are usually obscured by Raman bands or any, even weak, luminescence of the host crystal. On cooHng, the cations (R) mentioned above, together with chloride, the dominant anion of most geological fluids, nucleate salt hydrates, which can be described by the general formula R^C1„./7H20. These salt hydrates have different structures depending on the nature of the hydrate-forming cation, and have characteristic Raman spectra. The nucleated hydrate can then be easily identified if a microthermometric stage is coupled with the microRaman spectrometer, allowing the collection of the Raman spectra at different temperatures. Raman spectra have been obtained for the following hydrates: NaC1.2H20, CaCl2.6H20, MgCl2.6H20, MgCl2.12H20, KCl.MgCl2.6H2O, FeCls. I2H2O (Dubessy et al., 1982), LiCl. 5H2O (Dubessy et al., 1992a) and probably also CaCl2.4H20 (Schiffries, 1990). The identification of different hydrated crystals at different temperatures permits, first, a choice of an appropriate simplified projection in a ternary system (H20-Salt 1-Salt 2) to be made, and second, the reconstruction of the Uquid-composition path to the disappearance of the last soHd phase. The latter can be used to give a semi-quantitative estimate of the fluid composition. An illustration of the method is provided by the analysis of fluid inclusions in a quartz sample from the Bushveld complex (Schiffries, 1990). A new class of Hquid-absent fluid inclusions, containing halite (NaCl), antarticite (CaCl2. 6H2O) and probably a polymorph of CaCl2. 4H2O in the presence of a vapour phase, was documented at room temperature (Schiffries, 1990; Fig. 15). Upon heating, antarticite melted incongruently at 29°C. More than 25 vol% of the cavity is filled by the hquid at 31°C. With increasing temperature, the remaining hydrate (CaCl2.4H20) melted between 31 and 38°C, leaving as a soUd phase only halite, which disappeared at approximately 200°C. This phase behavior upon heating implies a high concentration of

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3100

3300

3500

3700

Raman shift (cm-i) Figure 15 Raman spectra of the OH stretching mode region of antarticite daughter crystals with decreasing temperature (Schiffries, 1990).

dissolved solids (>52wt%) and a high Ca/Na ratio. Grishina et al. (1992) studied another series of liquid-absent inclusions, trapped inside a metamorphic evaporitic halite, which exhibits similar behavior upon heating. As emphasized by Schiffries (1990), aqueous fluid inclusions that do not contain a liquid phase at room temperature may commonly be overlooked, or misinterpreted as mineral inclusions, although they are relics of highly saline fluids. Therefore, the investigation of salt hydrates by micro-Raman spectroscopy, coupled with microthermometric analysis, is essential for the identification of calcium-rich fluids. The chloride anion is a hydrogen-breaker; thus it strongly modifies the shape of the O—H stretching band of the aqueous phase (Walrafen, 1964, 1966). In contrast, cations have been found to have little effect on the frequency at maximum intensity, or the shape of the O—H stretching band. This observation has been recently used as a tool for the determination of the concentration of chloride in fluid inclusions (Mernagh and Wilde, 1989). These authors experimentally determined a 'skewing parameter' of the O—H stretching band with the use of synthetic-fluid inclusions in the NaCl, KCl, MgCl2 and CaCl2 systems. Based on the fact that the Raman spectra of aqueous solutions of different halides at various concentrations intersect one another at approximately 3300 c m ~ \ Mernagh and Wilde (1989) divided the spectrum into two regions. The skewing parameter is a function of two integrated areas: Xis equal to the integral between 2800 and 3300 cm~^, and

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Y is equal to the integral between 3300 and 3800 cm~^. The regression formula is given by wt%NaCl = a ^ f ^ ( 2 - ^ ) - ^ ,

(1)

where /? = 3(3400 c m " ^ 3 ( 3 2 0 0 cm"^) and a and j8 are experimentally determined regression parameters obtained from solutions of known concentrations for each spectrometer. The technique can be used up to halite saturation and can detect as Httle as 1 wt% NaCl in solution with a relative error of 15%. In the absence of other methods for the analysis of individual ions inside fluid inclusions, this technique is very useful, especially when the ice-melting temperature cannot be measured because of the presence of clathrates, or, in the case of complex systems, if phase diagrams are not available. Other solids, such as carbonates (Jrad et al., 1989), bicarbonates, phosphates and sulfates, are strong Raman scatters, and can be easily identified in fluid inclusions. An example of such solid identification in multiphase, fluid inclusions in a gold deposit is illustrated in Fig. 16 (Guilhaumou et al., 1990). Solids as small as 1 |xm in diameter can be identified. SiUcates such as K-feldspar, quartz and muscovite can also be easily determined (Coelho, 1990), unless they are iron-rich, in which case they absorb the exciting radiation and are often fluorescent. In contrast, NaCl and KCl have no first-order Raman spectra. Thus, these minerals cannot be identified by this technique. Carbon is often identified by its Raman spectrum by focusing the laser beam on the wall of the cavity, even though it is not visible under optical-microscopic examination. A very thin carbon coating around the wall of the cavity is thus indicated. As visible light is not significantly absorbed, the thickness is probably less than a few nm. 2. Hydrocarbon-fluid

Inclusions and Diagenetic Fluids

These inclusions are reUcs of the secondary hydrocarbon migration formed during the diagenesis of organic-matter-bearing sediments. They are also found in oil reservoirs, and could contain trapped hydrocarbons which are not necessarily identical to present-day oil; they thus can provide information on the time evolution of petroleum chemistry. There are very few studies in which micro-Raman spectroscopy has been found suitable for the characterization of hydrocarbon-fluid inclusions (Guilhaumou et al., 1981; Goffe, 1982; Pironon and Barres, 1990). The comparison of the Raman spectrum obtained from a natural fluid inclusion with that of a filled, synthetic inclusion permitted Pironon and Barres (1990) to identify the principal alkane present to be n-heptane. This identification was also consistent with micro-infrared data. Usually, the color of hydrocarbon inclusions is yellow

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to brown, indicating the absorption of light in the visible spectral range. They are, therefore, highly fluorescent under laser excitation with either the visible radiation provided by the Ar"^ laser (488 or 514.5 nm) or the Kr"^ laser (647.0 nm). The Raman spectrum is completely hidden by the highly fluorescent background, making analysis of the inclusion impossible with conventional techniques. The development of methods of treating fluorescence noise, or fluorescence rejection with the use of time-resolved, Raman spectroscopy, as described in the Introduction to this chapter, will be invaluable in future work.

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Inclusions can also be destroyed as a result of light absorption, provoking photochemical reactions and heating, and inducing chemical reactions such as hydrocarbon cracking. The experience of these authors has shown that these mixtures are fluorescent if they contain hydrocarbons heavier than CH4, even at low concentration, as indicated by a homogenization temperature of approximately -66°C. This result strongly suggests that the thermal stability of fluorescent organic molecules is roughly similar to the thermal stability of ethane, rendering micro-Raman spectroscopy useless for such hydrocarbon-fluid-inclusion analysis with visible excitation, at least with the use of conventional techniques. Fluids of the Terres Noires in the French southeastern Alps provide an exception to this empirical correlation. Guilhaumou et at. (1988) have correlated the regional distribution of wet and dry gases with vitrinite reflectance, and clay-mineral paragenesis. Methane, with a few mol% of ethane and perhaps propane, was identified in a region where the trapping temperature was in the 140-180°C range. Methane is the only hydrocarbon in the region where the temperature was between 180 and 230°C. Hydrocarbons heavier than methane are no longer stable at temperatures above 190°C, as shown from observations of fluid inclusions in oolitic limestones from core samples at 6.5 km depth (Guilhaumou et al., 1984). A notable exception to these generalities is the preservation of hydrocarbons to 300-320°C, as documented from fluid-inclusion analysis in high-pressure metasediments (Goffe, 1982). The attainment of high pressures due to nappe piling (up to 6 kbar), as inferred from mineral assemblage, strongly inhibited thermal cracking of the hydrocarbons. Fluorescence, and, more generally, light absorption by a material under monochromatic Hght illumination of energy EQ = hvQ, occurs if the energy difference between the electronic ground level and the excited electronic level of the illuminated material is similar in magnitude to, or smaller than, EQ. Therefore, an increase in the wavelength of the exciting radiation, resulting in a decrease of EQ, would be expected to eliminate the fluorescence. Thus, near-infrared excitation at 1.06 jjim provided by a continuous YAG laser has been used to study hydrocarbon inclusions (Pironon et aL, 1991). The FT Raman spectra were recorded with a Bruker IFS 66 spectrometer equipped with a Raman module FRA 106. The laser beam enters a classical optical microscope through an optical fiber. The fluid-inclusion sample was flat and jagged, but large in size (400 jxm); it was included in halite. It was filled with an aqueous phase with small crystals of anhydrite, a hquid-hydrocarbon phase with low fluorescence under UV iUumination, and a vapor bubble. The fluorescence on near-IR excitation was much lower than the fluorescence obtained under 514.5 nm excitation, but was still present. The Raman spectrum exhibited only the symmetric and antisymmetric —CH2 and —CH3 vibrations between 2800 and 3000 cm~^ and two small peaks of the C—H bending vibrations at 1300 and 1445 cm~^ (Fig. 17). The skeletal deformation vibrations in the 100-700 cm~^ region were not detected. The poor quality

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