New application of lasers to chemistry

New Applications of Lasers to Chemistry In New Applications of Lasers to Chemistry; Hieftje, G.; ACS Symposium Series; ...

Author: Gary M. Hieftje

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New Applications of Lasers to Chemistry

In New Applications of Lasers to Chemistry; Hieftje, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.


New Applications of Lasers to Chemistry G a r y M . Hieftje,

EDITOR

Indiana University

Based on a symposium sponsored by the ACS Division of Analytical Chemistry at the 175th Meeting of the American Chemical Society, Anaheim, California, March 14-15, 1978.

ACS

SYMPOSIUM

SERIES

AMERICAN CHEMICAL SOCIETY WASHINGTON, D. C.

1978


85

Library of Congress CIP Data New applications of lasers to chemistry. (ACS symposium series; 85 ISSN 0097-6156) Includes bibliographies and index. 1. Lasers in chemistry—Congresses. I. Hieftje, Gary M. II. American Chemical Society. Division of Analytical Chemistry. III. Series: American Chemical Society. ACS symposium series; 85. QD715.N48 543 78-22032 ISBN 0-8412-0459-4 ASCMC 8 85 1-242 1978

Copyright © 1978 American Chemical Society All Rights Reserved. The appearance of the code at the bottom of the first page of each article in this volume indicates the copyright owner's consent that reprographic copies of the article may be made for personal or internal use or for the personal or internal use of specific clients. This consent is given on the condition, however, that the copier pay the stated per copy fee through the Copyright Clearance Center, Inc. for copying beyond that permitted by Sections 107 or 108 of the U.S. Copyright Law. This consent does not extend to copying or transmission by any means—graphic or electronic—for any other purpose, such as for general distribution, for advertising or promotional purposes, for creating new collective works, for resale, or for information storage and retrieval systems. The citation of trade names and/or names of manufacturers in this publication is not to be construed as an endorsement or as approval by ACS of the commercial products or services referenced herein; nor should the mere reference herein to any drawing, specification, chemical process, or other data be regarded as a license or as a conveyance of any right or permission, to the holder, reader, or any other person or corporation, to manufacture, reproduce, use, or sell any patented invention or copyrighted work that may in any way be related thereto. PRINTED IN THE UNITED STATES O F AMERICA


ACS Symposium Series Robert F . Gould, Editor

Advisory Board Kenneth B. Bischoff

Nina I. McClelland

Donald G. Crosby

John B. Pfeiffer

Jeremiah P. Freeman

Joseph V. Rodricks

E. Desmond Goddard

F. Sherwood Rowland

Jack Halpern

Alan C. Sartorelli

Robert A. Hofstader

Raymond B. Seymour

James P. Lodge

Roy L. Whistler

John L. Margrave

Aaron Wold


FOREWORD The A C S S Y M P O S I U a medium for publishing symposia quickly in book form. The format of the Series parallels that of the continuing A D V A N C E S I N C H E M I S T R Y S E R I E S except that in order to save time the papers are not typeset but are reproduced as they are submitted by the authors i n camera-ready form. Papers are reviewed under the supervision of the Editors with the assistance of the Series Advisory Board and are selected to maintain the integrity of the symposia; however, verbatim reproductions of previously published papers are not accepted. Both reviews and reports of research are acceptable since symposia may embrace both types of presentation.


PREFACE *he papers collected in this volume reflect the content of the two-day invited symposium which was organized under the auspices of the Analytical Division of the American Chemical Society; however, because the presentations contained subject matter not ordinarily associated directly with chemical analysis, a more general symposium title was chosen. This same title is born by the present volume. A rather general title seems especially appropriate here. It is clear that the present and futur only be assessed from th cation to chemistry. By its very nature, analytical chemistry relies heavily on advances in measurement technology and, consequently, on developments in physics and engineering. Accordingly, symposium speakers were invited who addressed the subject of laser application from several vantage points. Physicists, engineers, and chemists all participated and represented not only academic laboratories but government agencies and industrial organizations as well. Consequently, subject material ranged from new developments in laser technology and measurement systems to the use of lasers in the detection of single atoms and molecules. This volume is organized in essentially the same way as the Anaheim Symposium. Although no well-delineated separation among topics occurs, thefirstportion of the volume deals principally with the subject of high-resolution spectroscopy, the second with high-sensitivity analysis, the third with time-resolved or kinetic spectroscopy, and the last with new techniques in laser Raman spectrometry. To a large extent, these divisions represent areas of chemistry in which the laser has had greatest impact. The high spectroscopic resolution afforded by modern lasers makes possible the more accurate identification of molecular and atomic energy levels, affords the specificity necessary to spectroscopically resolve components in a mixture, and permits isotope analysis and separation. Similarly, the high power available from some lasers, coupled with new measurement techniques, has extended analytical sensitivities to the single atom and molecule limit, as indicated earlier. Moreover, the brief pulses emitted by mode-locked lasers provides time resolution below the picosecond regime. Finally, new linear and nonlinear Raman methods made possible by high-power, monochromatic laser sources have extended the range of applicability of Raman techniques, their specificity, and sensitivity. ix In New Applications of Lasers to Chemistry; Hieftje, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.

Unfortunately, not all individuals who participated in the Anaheim Symposium have contributed to this volume. However, an excellent cross section of topics and authors appears and serves to represent the scope of current applications of lasers to chemistry and the potential importance of lasers in the future of chemistry and chemical analysis. Indiana University Bloomington, Indiana August 7, 1978

GARY M . HIEFTJE

χ


1 Selective Excitation of Probe Ion Luminescence (SEPIL) J O H N C. W R I G H T , F R E D E R I C K J. G U S T A F S O N , and L A U R A C. P O R T E R Department of Chemistry, University of Wisconsin, Madison, W I 53706

There have been numerous s t u d i e s t h a t illustrate the e x c e l l e n t d e t e c t i o n l i m i t s obtained when a l a s e r e x c i t a t i o n source generates o p t i c a l emission from a sample (1-7). When working a t such low c o n c e n t r a t i o n l e v e l s , one must b a t t l e the new problems of contamination and i m p u r i t i e s . The l a s e r can provide a potential advantage here as w e l l because o f the very narrow s p e c t r a l bandpasses o f modern l a s e r s . I f the a n a l y t i c a l system has sharp line o p t i c a l t r a n s i t i o n s whose wavelength depends upon the partic u l a r a n a l y t e , the narrow bandwidth can provide a high degree o f selectivity f o r the analyte of i n t e r e s t . A common method o f p r o v i d i n g the narrow t r a n s i t i o n s i s t o convert the a n a l y t i c a l sample t o a gas as is commonly done in atomic a b s o r p t i o n and atomic fluorescence experiments (7). The techniques f o r accomp l i s h i n g t h i s transformation have a long h i s t o r y i n the field and there can be little doubt about the eventual success. We have been studying the feasibility of a very d i f f e r e n t method o f a c h i e v i n g narrow t r a n s i t i o n s t h a t are c h a r a c t e r i s t i c o f an a n a l y t e . The lanthanide ions have narrow l i n e t r a n s i t i o n s , even i n condensed phases, as a r e s u l t o f s h i e l d i n g o f the o p t i c a l l y a c t i v e 4f e l e c t r o n s h e l l by the outer 5s 5p o r b i t a l s (8,9). A small c r y s t a l f i e l d s p l i t t i n g i s produced by the immediate surroundings. T h i s s p l i t t i n g can be used as a s h o r t range s p e c t r o s c o p i c probe o f the condensed phase. In p a r t i c u l a r i f analyte ions are a l s o present i n the condensed phase near a l a n thanide probe i o n , a c r y s t a l f i e l d s p l i t t i n g w i l l be produced that i s c h a r a c t e r i s t i c o f the presence of that a n a l y t e . n

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0-8412-0459-4/78/47-085-001$05.00/0 © 1978 American Chemical Society


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In an a r b i t r a r y condensed phase, lanthanide ions w i l l encounter a number o f d i f f e r e n t surroundings e i t h e r because o f d i f f e r e n t ways the ions o f the phase can be ordered around the lanthanide i o n or because other ions (analytes) have a l s o entered the phase. The a b s o r p t i o n and fluorescence spectra can t h e r e f o r e become q u i t e complex because o f the d i f f e r e n t s p l i t t i n g s o f the lanthanide ions i n d i f f e r e n t surroundings. This complexity may be s i m p l i f i e d i f a tuneable l a s e r i s used to e x c i t e a t a wavelength t h a t matches an absorption l i n e o f a lanthanide i o n with a p a r t i c u l a r surroundings (10). Since the lanthanide ions with d i f f e r e n t surroundings have d i f f e r e n t c r y s t a l f i e l d s p l i t t i n g s , these ions w i l l not be e x c i t e d . The fluorescence spectrum then c o n t a i n s only l i n e s from a lanthanide with one type o f surroundings. T h i s process i s c a l l e d s e l e c t i v e l a s e r e x c i t a t i o n o r s i t e s e l e c t i v e spectroscopy o f one s i t e may be obtaine l i n e while scanning the dye l a s e r e x c i t a t i o n wavelength. Thus a high s e l e c t i v i t y f o r a p a r t i c u l a r analyte may be obtained by e i t h e r e x c i t i n g o r monitoring s p e c t r a l t r a n s i t i o n s o f the l a n t h a nide i o n t h a t has the analyte o f i n t e r e s t i n the immediate surroundings . In order t o make t h i s idea p r a c t i c a l , one must solve the chemical problem o f b r i n g i n g about an a s s o c i a t i o n between an analyte i o n and a lanthanide i o n . There are a number o f methods that can be used t o accomplish t h i s . The simplest method i s t o make the condensed phase i t s e l f from lanthanide ions, i . e . a pure lanthanide compound. Any f o r e i g n i o n t h a t enters the c r y s t a l l i n e l a t t i c e o f such a compound w i l l have to perturb a lanthanide i o n . In a second method, the presence o f analyte ions could l e a d to the establishment o f a second c r y s t a l l i n e phase which w i l l p a r t i c i p a t e i n the p a r t i t i o n i n g o f t r a c e amounts o f lanthanide i o n s . I f two phases a r e present, one c o n t a i n i n g the analyte and the other an innocuous d i l u e n t , the lanthanide spectra w i l l r e f l e c t the presence o f both phases. A t h i r d method introduces both the lanthanide and a n a l y t e ions a t t r a c e concentrations i n a host compound and r e l i e s upon i n t e r a c t i o n s between them to form assoc i a t e s o r complexes. The i n t e r a c t i o n s can be the r e s u l t o f chemical bonding o r o f d i f f e r e n c e s i n i o n i c r a d i i or charge s t a t e between analyte and lanthanide. F o r example, s u b s t i t u t i o n o f a t r i v a l e n t lanthanide f o r a d i v a l e n t cadmium i n cadmium molybdate produces a charge imbalance which c o u l d be compensated by r e p l a c i n g t h e hexavalent molybdenum by a quadravalent niobium. The e f f e c t i v e p o s i t i v e charge o f the lanthanide i o n ( r e l a t i v e t o the l a t t i c e ) and the e f f e c t i v e negative charge o f the niobium


1.

Probe Ion Luminescence

WRIGHT E T AL.

3

w i l l cause Coulombic a t t r a c t i o n s which w i l l favor formation o f a lanthanide-niobium complex i n the l a t t i c e . The p e r t u r b a t i o n s o f the l o c a l c r y s t a l f i e l d s by the niobium w i l l produce c r y s t a l f i e l d s p l i t t i n g s on the lanthanide i o n t h a t are unique and permit the use o f s e l e c t i v e l a s e r techniques to e x c i t e only lanthanide ions that have nearby niobium i o n s . The i n t e n s i t y i s p r o p o r t i o n a l to the number o f niobium i o n s . Any o f the lanthanide ions which are f l u o r e s c e n t can be used f o r these procedures. In a p r a c t i c a l a n a l y s i s , i t would be b e s t to choose the lanthanide i o n and the s p e c i f i c t r a n s i t i o n s t h a t gave the best s e l e c t i v i t y and s e n s i t i v i t y . In order to l i m i t the number o f v a r i a b l e s t h a t need to be optimized, we have chosen to use europium as the lanthanide i o n i n a l l o f our s t u d i e s because of s e v e r a l unique p r o p e r t i e s t h a t make Eu p a r t i c u l a r l y s u i t a b l e f o r p r e l i m i n a r y work. Th F i g u r e 1 (8). ^The groun e x c i t e d s t a t e DQ are both s i n g l e t l e v e l s (a J=0 s t a t e can have^ only an Mj =0) and t h e r e f o r e f l u o r e s c e n t t r a n s i t i o n s from D Q " * F Q o r a b s o r p t i o n t r a n s i t i o n s from ^ F - ^ D Q have only one t r a n s i t i o n . Thus, when one monitors the "^p^ f l u o r e s c e n c e t r a n s i t i o n s with an instrument of s u f f i c i e n t l y l a r g e bandpass to i n c l u d e t r a n s i t i o n s from the Eu ions i n a l l p o s s i b l e c r y s t a l l o g r a p h i c s i t e s as one scans a tuneable l a s e r over the r e g i o n of " ^ F Q - ^ D Q t r a n s i t i o n s , the r e s u l t i n g spectrum w i l l c o n t a i n o n l y one l i n e f o r each type o f c r y s t a l l o g r a p h i c environment t h a t Eu encounters i n the sample. T h i s procedure permits a r a p i d c h a r a c t e r i z a t i o n o f the s i t e s present i n a sample by t a k i n g a s i n g l e scan. The advantage i s p a r t i a l l y o f f s e t because the p o s i t i o n o f the ^ F Q - ^ D Q t r a n s i t i o n r e s u l t s only from a second order p e r t u r b a t i o n and i t s i n t e n s i t y i s lower because i t i s forbidden i n f i r s t order. There i s a l s o a p o s s i b i l i t y o f having the l i n e from a Eu with a nearby analyte a c c i d e n t a l l y overlapping l i n e s from Eu ions i n other s i t e s . These problems can be e l i m i n a t e d by using the other Eu t r a n s i t i o n s such as ^ Q ~ ^ 2 ky using other lanthanide i o n s . Q

F

>

D

o

r

LANTHANIDE ANALYSIS Our research i n t o the f e a s i b i l i t y o f implementing these ideas as an a n a l y t i c a l methodology has been d i v i d e d i n t o two s e c t i o n s development of trace methods f o r lanthanide i o n a n a l y s i s where the a s s o c i a t i o n with an analyte i s not a f a c t o r (the lanthanide i o n i t s e l f i s the analyte) and development of methods f o r other ions where the analyte ion must be a s s o c i a t e d with the l a n t h a n i d e . The o v e r - a l l problem can then be approached i n smaller s e c t i o n s while simultaneously p r o v i d i n g new a n a l y t i c a l methods. The procedure f o r performing a lanthanide a n a l y s i s c o n s i s t s of adding Ca(N0 )2 to the s o l u t i o n c o n t a i n i n g unknown amounts o f lanthanide ions and p r e c i p i t a t i n g with the a d d i t i o n of NH4F (11). The lanthanide ions c o - p r e c i p i t a t e with a very f a v o r a b l e 3


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T O CHEMISTRY

-,21.61

Energy cm: xio 1

21.60

3

20 21.59 17.34

r

6 5432-

o 1

ô-

MA! Eu

Eu

MA:EU,X

3+

Figure 1. The Eu electronic energy levels of the free ion are shown on the left. The splittings and shifts that occur when the Eu ion is placed within two different crystalfieldsis shown on the right for the D and Ό manifolds. Note the scale expan sion required to see the crystal field splittings. The MA:Eu represents Eu doped in a httice MA and the MA:Eu,X repre sents Eu and an analyte X in association in the MA Ixittice. 3+

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1.

WRIGHT E T

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d i s t r i b u t i o n c o e f f i c i e n t i n the CaF2. T h i s step serves both as an e x t r a c t i o n , s e p a r a t i o n , and p r e - c o n c e n t r a t i o n . The p r e c i p i t a t e i s f i l t e r e d , washed, and i g n i t e d . The i g n i t i o n step converts the f l u o r i d e i n t e r s t i t i a l charge compensation o f the c o - p r e c i p i t a t e d lanthanide ions to an oxygen charge compensation. T h i s conversion a l s o reduces the number of d i f f e r e n t s i t e s present and i n c r e a s e s the o s c i l l a t o r strengths o f the o p t i c a l t r a n s i t i o n s . The procedure works q u i t e w e l l . No s e p a r a t i o n steps are r e q u i r e d f o r i s o l a t i n g i n d i v i d u a l lanthanide i o n s , l i n e a r c a l i b r a t i o n curves are obtained up t o c o n c e n t r a t i o n s o f 1 ppm, and d e t e c t i o n l i m i t s o f 200 p a r t s i n 1 0 are p o s s i b l e . The l a t t e r l i m i t i s expected to be lowered by a t l e a s t an order of magnitude because o f s e v e r a l improvements i n our d e t e c t i o n e l e c t r o n i c s . 1 5

ANALYTES ASSOCIATED WITH LANTHANIDES The simplest metho the lanthanide i o n and an analyte was by forming an ordered s t r u c ture o f a lanthanide compound i n the presence o f an a n a l y t e . This method i s i l l u s t r a t e d i n F i g u r e 2 f o r E u ( S 0 ) 3 produced by r a p i d evaporation o f s o l u t i o n c o n t a i n i n g 0.5 mole % Na^PO^ r e l a t i v e to SO^. The spectrum i n F i g u r e 2 was obtained by scanning a dye l a s e r over the e x c i t a t i o n r e g i o n o f the ^ F Q ^ ^ D Q t r a n s i t i o n while monitoring the ^ D Q ~ ' ^ 2 fluorescence with a very wide bandpass. Each peak represents a s i n g l e Eu s i t e . The two small peaks to the l e f t o f the main i n t r i n s i c peak are only obtained with PO^~ i n the o r i g i n a l s o l u t i o n and they have an i n t e n s i t y which i s p r o p o r t i o n a l to the P0^~ c o n c e n t r a t i o n . The same_behavior i s obtained i f A s O ^ i s p r e s e n t although the two AsO^ peaks occur a t d i f f e r e n t wavelengths from those of PO^~. Although the analyte peaks do not look very s i g n i f i c a n t i n compari s o n with the main i n t r i n s i c peak and would seem s u s c e p t i b l e to being l o s t i n the main l i n e a t lower c o n c e n t r a t i o n s , t h e i r absol u t e i n t e n s i t y i s c o n s i d e r a b l e . In f a c t , the i n t e r f e r e n c e from the main peak can be e l i m i n a t e d e i t h e r by tuning the l a s e r to e i t h e r of the e x c i t a t i o n wavelengths o f the PO4- peaks and s e l e c t i v e l y e x c i t i n g f l u o r e s c e n c e from only the Eu s i t e s with P04~ nearby or by s e t t i n g a monochromator with a narrower bandpass_to monitor a f l u o r e s c e n c e t r a n s i t i o n o f Eu s i t e s with nearby PO^ and o b t a i n i n g a s i n g l e s i t e e x c i t a t i o n spectrum. E i t h e r method produces s p e c t r a completely f r e e o f the main i n t r i n s i c l i n e s and permits one to f o l l o w the P0^~ peaks to much lower c o n c e n t r a t i o n s . w

2

>

n

i

c

n

n

a

s

b

e

e

n

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f

=

-

A s s o c i a t i o n between the lanthanide and analyte ions can a l s o be achieved i f the two ions provide a mutual charge compensation f o r each other i n a l a t t i c e o f ions with d i f f e r e n t valences (12). T h i s method i s more complex than the previous example because s e v e r a l a d d i t i o n a l f a c t o r s become important. The Coulombic i n t e r a c t i o n s w i t h i n the charge compensating p a i r t h a t promote t h e i r a s s o c i a t i o n are opposed by the l a t t i c e d i s t o r t i o n s t h a t might


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r e s u l t i f the p a i r becomes a s s o c i a t e d and by entropy considerat i o n s which favor randomization of the i o n s . I f these a s s o c i a t e s have favorable f r e e energies, they w i l l cause an increase i n the d i s t r i b u t i o n c o e f f i c i e n t s f o r the p a r t i t i o n i n g o f the ions i n t o the c r y s t a l l a t t i c e . A d d i t i o n a l l y , both ions can be compensated by native d e f e c t s that a r e present i n the l a t t i c e g i v i n g r i s e to i n t r i n s i c s i t e s that comprise the s p e c t r a i n the absence o f a n a l y t e s . One can again have a s s o c i a t e s o f e i t h e r lanthanide or analyte ions with the native defects i n a number o f d i f f e r e n t arrangements. A lanthanide i o n w i l l be i n v o l v e d i n a competition between a s s o c i a t i o n with an analyte, a s s o c i a t i o n with a n a t i v e d e f e c t , and d i s s o c i a t i o n . Each o f the p o s s i b l e e q u i l i b r i a w i l l a l s o a f f e c t the native d e f e c t e q u i l i b r i a o f the host l a t t i c e . T h i s s i t u a t i o n can be compared d i r e c t l y with the f a m i l i a r e q u i l i b r i a that a chemist considers f o r complexation o f EDTA with a t r a n s i t i o n metal i n an formation constant can and ammonia l i g a n d c o n c e n t r a t i o n . In the same way, success o f t h i s method r e q u i r e s favorable " c o n d i t i o n a l formation constants" f o r the lanthanide-analyte a s s o c i a t e s . There are d i f f e r e n t l i m i t i n g cases that can be encountered f o r d i f f e r e n t values o f the e q u i l i b r i a constants. 1.

Analyte-lanthanide

a s s o c i a t i o n i s strong.

2.

Lanthanide-native d e f e c t a s s o c i a t i o n i s strong while a n a l y t e native d e f e c t a s s o c i a t i o n and analyte-lanthanide a s s o c i a t i o n are weak.

3.

Both lanthanide-native d e f e c t a s s o c i a t i o n and a n a l y t e - n a t i v e d e f e c t a s s o c i a t i o n a r e strong.

4.

Lanthanide a s s o c i a t i o n with e i t h e r analyte o r native defects i s weak.

Each o f the d i f f e r e n t cases has a d i s t i n c t i v e i n f l u e n c e on the observed s p e c t r a . The f i r s t case i s the a n a l y t i c a l l y u s e f u l one i n which new l i n e s appear i n the s p e c t r a t h a t are d i r e c t l y r e l a t e d to the presence o f an a n a l y t e . T h i s case i s shown i n F i g u r e 3 f o r CdMo0 with Eu and Nb present i n t r a c e q u a n t i t i e s . These s p e c t r a were obtained by monitoring the D Q - * F transition with a very broad bandpass while scanning the wavelength o f a dye l a s e r over the p o s s i b l e F Q - * D Q t r a n s i t i o n s . The l i n e s charact e r i s t i c o f Nb can be s e l e c t i v e l y e x c i t e d or monitored to o b t a i n s p e c t r a o f only those Eu ions with nearby Nb. 4

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For case 2, the a d d i t i o n o f analyte to a m a t e r i a l can cause marked changes i n t h e spectrum of a Eu doped system because o f the disturbance i n native d e f e c t concentrations. The a d d i t i o n o f analyte i s accompanied by an increase i n the n a t i v e d e f e c t concent r a t i o n r e q u i r e d to charge compensate t h a t a n a l y t e . I f there i s l i t t l e a s s o c i a t i o n between the analyte and i t s native d e f e c t , the concentration o f the unassociated n a t i v e d e f e c t w i l l become l a r g e


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7


WRIGHT E T A L .

Figure 2. The excitation spectrum of the F -» Ό transition in Èu (SO^) with PO ~ added obtained by monitoring the fluorescence from D -» F with a broad 7

5

0

0

9

3

u

5

7

0

578

579 Wavelength (nm.)

2

of

h

CdMo0 :Eu 4

CdMo0 !Eu,Nb 4

580.2

580.6

581.0

581.4

Wavelength (nm.) 7

5

Figure 3. The excitation spectra of the F -> F> transition in CdMoO with europium alone and with europium and niobium added in trace quantities. This spectra was obtained by monitoring the D -» F fluorescence with a broad bandpass instrument. The additional line associated with the niobium is readily observed. The niobium concentration is 1 mol%. 0

0

h

5

7

0

2


s

8

NEW

APPLICATIONS O F

LASERS T O

CHEMISTRY

thus depressing the concentration o f the native defects compensat i n g the Eu i o n . T h i s lowered c o n c e n t r a t i o n can be p a r t i a l l y r e s t o r e d by d i s s o c i a t i o n of Eu-native d e f e c t a s s o c i a t e s which i s r e f l e c t e d by the l o s s of l i n e i n t e n s i t y from these a s s o c i a t e d Eu s i t e s . An example o f t h i s i s shown i n F i g u r e 4 f o r PbMoO^ with Eu and As present i n t r a c e amounts. No new l i n e s appear that can be r e l a t e d to As but there i s a strong i n f l u e n c e o f As on the spectrum. I t should be emphasized at t h i s p o i n t that the explanation given above can only be c l a s s i f i e d as h i g h l y s p e c u l a t i v e a t t h i s p o i n t because of the lack o f research i n t h i s area i n our l a b o r a t o r y and others. The s p e c t r a t h a t are observed i n the t h i r d case are not w e l l d e f i n e d . The a d d i t i o n o f analyte can have no a f f e c t on the Eu spectra i f the a s s o c i a t i o n of analyte and i t s n a t i v e d e f e c t i s strong enough that the defects i s not a f f e c t e d not be t h a t strong and changes are expected although not as marked as case 2. No new l i n e s appear that are c h a r a c t e r i s t i c of the analyte. The f i n a l case corresponds to no changes i n the Eu spectra because o f the a d d i t i o n of an analyte s p e c i e s . The Eu i s i s o l a t e d and not i n t e r a c t i n g with e i t h e r analyte nor n a t i v e d e f e c t compens'ations. The model presented above i s an i d e a l i z e d one i n many respects that i s meant only to focus a t t e n t i o n on the important aspects o f t h i s method o f a n a l y s i s . I t neglects many v a r i a b l e s such as s e v e r a l competing e q u i l i b r i a with other n a t i v e d e f e c t s or i m p u r i t i e s , e q u i l i b r i a with the surrounding atmosphere, and the changes that occur i n d i s t r i b u t i o n c o e f f i c i e n t s because o f the a d d i t i o n of an analyte s p e c i e s . I t does p o i n t out the p o s s i b i l i t y of c o n t r o l l i n g the important d e f e c t e q u i l i b r i a f o r o p t i m i z i n g an a n a l y s i s procedure by analogy to the EDTA e q u i l i b r i a where the pH c o n t r o l s the a n a l y t i c a l l y important e q u i l i b r i a . For c r y s t a l l i n e l a t t i c e s , i t may be p o s s i b l e to c o n t r o l the native d e f e c t concent r a t i o n s to increase " c o n d i t i o n a l formation constants" f o r the lanthanide-analyte a s s o c i a t e s making a p a r t i c u l a r procedure f e a s i b l e or o p t i m a l . CONCLUSIONS These methods represent a departure from conventional f l u o r escence approaches t h a t become f e a s i b l e with the a d d i t i o n o f the l a s e r to the l i n e o f a n a l y t i c a l instruments. The approach i s very promising f o r s e v e r a l reasons. I t i s i n h e r e n t l y a method with very low d e t e c t i o n l i m i t s and can be used over a wide dynamic range of concentrations as demonstrated i n the experiments on t r a c e lanthanide a n a l y s i s . I f the p r e p a r a t i o n step involves a


1.

9


WRIGHT E T A L .

580

581

Wavelength (nm.)

Figure 4. The excitation spectra of the F -» Ό transition in FhMoO with europium alone and with europium and arsenic added in trace quantities. This spectrum was obtained by monitoring the D —» F fluorescence with a broad bandpass instrument. It can be seen that the addition of the arsenic analyte does not produce any useful lines but it does have a marked effect on the intrinsic europium sites. The arsenic concentra tion is 1 mol%. 7

5

0

0

5

k

7

0

2


10

NEW

APPLICATIONS OF

LASERS T O

CHEMISTRY

p r e c i p i t a t i o n , t h i s step w i l l serve as a method of preconcentrat i o n o f the s o l u t i o n i n t o the l a t t i c e . There are two steps i n the method t h a t provide a high s e l e c t i v i t y f o r p a r t i c u l a r analytes thus p o t e n t i a l l y e l i m i n a t i n g the need f o r p r i o r s e p a r a t i o n steps. The p r e p a r a t i o n w i l l exclude o t h e r ions with i o n i c r a d i i and charges incompatible with the c r y s t a l l a t t i c e . The narrow l i n e - w i d t h s o f s p e c t r a l t r a n s i t i o n s permit the s e l e c t i v e e x c i t a t i o n o f only the Eu ions with the analyte o f i n t e r e s t nearby. We have examined a number o f d i f f e r e n t chemical systems to d e t e r mine the range o f a p p l i c a b i l i t y o f such methods. Thus f a r , we have shown a n a l y t i c a l l y u s e f u l l i n e s can be found i n 15 of the elements as w e l l as the lanthanid extended to i n c l u d e a m a j o r i t p e r i o d i c t a b l e . We thus b e l i e v e the method i s widely a p p l i c a b l e and has some powerful advantages but c o n s i d e r a b l y more work i s r e q u i r e d before the method can be c o n s i d e r e d an a d d i t i o n to the a n a l y t i c a l chemist's bag of t r i c k s .

ACKNOWLEDGMENTS We would l i k e to thank the N a t i o n a l Science Foundation f o r the support of t h i s r e s e a r c h under grant number MPS74-24394.

LITERATURE CITED 1.

S t e i n f e l d , J . I . , Tunable Lasers and T h e i r A p p l i c a t i o n in A n a l y t i c a l Chemistry, C.R.C. Crit. Rev. A n a l . Chem. (1975) 5, 225-241.

2.

Fairbank, W.M., Hänsen, R.W. and Schawlow, A.L., Absolute Measurement o f Very Low Sodium-Vapor D e n s i t i e s Using Laser Resonance Fluorescence, J . Opt. Soc. Am. (1975) 65, 199-204.

3.

Hurst, G.S., Nayfeh, M.H. and Young, J.P., A Demonstration o f One-Atom D e t e c t i o n , Appl. Phys. L e t t . (1977) 30, 229-231.

4.

L y t l e , F.E. and Kelsey, M.S., C a v i t y Dumped Argon-Ion Laser as an E x c i t a t i o n Source i n Time-Resolved F l u o r i m e t r y , A n a l . Chem. (1974) 46, 855-860.

5.

Richardson, J.H., W a l l i n , B.W., Johnson, D.C. and Hrubesh, L.W., S u b - P a r t - P e r - T r i l l i o n D e t e c t i o n o f R i b o f l a v i n by Laser Induced Fluorescence, A n a l . Chem. (1976) 98, 620-621.


1.

WRIGHT

ET

AL.


11

6.

Bradley, A.B. and Zare, R.N., Laser F l u o r i m e t r y . Sub-PartPer Trillion D e t e c t i o n o f S o l u t e s , J. Am. Chem. Soc. (1976) 98, 620-621.

7.

F r a s e r , L.M. and Windfordner, J.D., L a s e r - E x c i t e d Atomic Fluorescence Flame Spectrometry, A n a l . Chem. (1971) 43, 1693-1696.

8.

Dieke, G.H., "Spectra and Energy L e v e l s o f Rare Earth Ions i n C r y s t a l s " , I n t e r s c i e n c e P u b l i s h e r s , New York (1968).

9.

Wybourne, B.G., " S p e c t r o s c o p i c P r o p e r t i e s o f Rare E a r t h s " , I n t e r s c i e n c e P u b l i s h e r s , New York (1965).

10.

T a l l a n t , D.R. and Wright, J.C., S e l e c t i v e Laser E x c i t a t i o n of Charge Compensate (1975) 63, 2074-2085

11.

Gustafson, F . J . and Wright, J.C., Ultrat r a c e Method f o r Lanthanide Ion Determination by S e l e c t i v e Laser E x c i t a t i o n , A n a l . Chem. (1977) 49, 1680-1689.

12.

Wright, J.C., Trace A n a l y s i s o f Nonfluorescent Ions by S e l e c t i v e Laser E x c i t a t i o n o f Lanthanide Ions, Anal. Chem. (1977) 49, 1690-1701.

RECEIVED

August 24, 1978.


2 Applications of Tunable-Diode-Laser IR Spectroscopy to Chemical Analysis J. F. BUTLER, K. W. NILL, A. W. MANTZ, and R. S. ENG Laser Analytics, Inc., 38 Hartwell Ave., Lexington, MA 02173

Tunable diode l a s e r spectroscopy has become a widely used and important technique f o r u l t r a - h i g h r e s o l u t i o n i n f r a r e d mea surements (1). A r e s o l u t i o n in the order o f 10 cm makes it p o s s i b l e , f o r example, t o study fully r e s o l v e d lineshapes o f Doppler broadened l i n e s o f low pressure molecular gases. Such measurements, which can be made r a p i d l y and e a s i l y with a tunable diode l a s e r spectrometer, are virtually impossible by other t e c h niques. Tunable diode l a s e r s are a l s o being used i n s e n s i t i v e air p o l l u t i o n monitors i n both p o i n t (2) and long-path c o n f i g u r a t i o n s (3). The purpose o f the present paper is t o review and d i s c u s s new a p p l i c a t i o n s o f tunable diode l a s e r spectroscopy in s e v e r a l areas r e l a t i n g specifically t o chemical a n a l y s i s and measurement, i n c l u d i n g n o n l i n e a r spectroscopy and photochemistry. These l a s e r s have a number o f unique f e a t u r e s o f p a r t i c u l a r i n t e r e s t i n chemical a n a l y s i s . Some o f these f e a t u r e s and t h e i r general uses are summarized i n Table I . -4

-1

Low L e v e l Detection Concentrations o f molecular gases can be measured t o excep t i o n a l l y low l e v e l s using tunable diode l a s e r IR absorption spectroscopy. As one example o f t h i s a p p l i c a t i o n , a group a t McMaster U n i v e r s i t y has constructed a h i g h l y s e n s i t i v e a i r p o l l u t i o n monitor using the apparatus diagrammed i n Figure 1 (2^) . The a i r sample flows i n t o the m u l t i - t r a v e r s a l absorption c e l l a t r e duced pressures. An AC modulation superimposed on the DC b i a s current induces a small r e p e t i t i v e scan o f the l a s e r emission frequency, a l l o w i n g the use o f d e r i v a t i v e spectroscopy. The McMaster group was able t o measure absorbance l e v e l s as low as α = 10~ by using the second d e r i v a t i v e . Figure 2 i l l u s t r a t e s the use o f t h i s apparatus f o r the d e t e c t i o n o f atmospheric SO2. The McMaster group reported s e n s i t i v i t y l e v e l s as low as 3 ppb 5

0-8412-0459-4/78/47-085-012$05.00/0

© 1978 American Chemical Society In New Applications of Lasers to Chemistry; Hieftje, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.

2.

Tunable-Diode-Laser

BUTLER E T A L .

IR Spectroscopy

13

TABLE I TUNABLE DIODE LASER FEATURES RELEVANT TO ANALYTICAL INSTRUMENTATION

High S p e c t r a l P u r i t y (10

cm

)

High s p e c i f i c i t y e l i m i n a t e s or g r e a t l y reduces e r r o r s due t o i n t e r f e r i n g absorption l i n e s and the complexity o f a l g o rithms r e q u i r e d t o compensate f o r background absorptions. S p e c t r a l Region 3-30 ym r e g i o n contains absorption s p e c t r a o f n e a r l y a l l molecular substances. Tunability P r e c i s e s p e c t r a l l i n e s or regions may be s e l e c t e d . Information i n the lineshape can be u t i l i z e d . Ease o f Rapid

Modulation

High frequency operation reduces turbulence and p a r t i c u l a t e noise. Novel s i g n a l p r o c e s s i n g techniques can be employed. High Brightness Nearly opaque substances can be analyzed. Transmission over long o p t i c a l path i s f e a s i b l e . —6 Small E m i t t i n g Area (5 χ 10

2 cm )

Small, constrained regions can be probed. Long-path measurements are f a c i l i t a t e d .


N E W APPLICATIONS O F LASERS T O C H E M I S T R Y

CHOPPER

DIODE

PUMP

DIODE

LASER

CURRENT MODULATOR REF. L O C K - IN

X-Y

AMPLIFIER

RECORDER

Figure 1. Diagram of a tunable -diode-laser pollution monitoring sys tem. Sensitivity is enhanced by the use of a multi-traversal, low-pressure cell and second-derivative detection.

Figure 2. Typical measurement data for the tunable-diode-laser instrument diagrammed in Figure 1. A calibration measurement of 20 ppb of S0 in Ν is followed by a measurement of air containing 8 ppb of S0 . 2

2

2


2.

BUTLER

ET

AL.

Tunable-Diode-Laser

15

IR Spectroscopy

f o r SO2. T h e i r p r e d i c t e d s e n s i t i v i t y l e v e l s f o r other p o l l u t a n t s measured by t h i s technique are summarized i n Table I I . As another i l l u s t r a t i o n o f the p o t e n t i a l low l e v e l d e t e c t i o n c a p a b i l i t y of tunable diode l a s e r spectroscopy, consider the mea surement o f CC>2 c o n c e n t r a t i o n . Researchers from Laser A n a l y t i c s and the New York State Department of Health (£,_5) have shown t h a t the i s o t o p i c s h i f t s i n CO2 absorption l i n e s are e a s i l y r e s o l v e d with diode l a s e r s (Figure 3) and have experimentally v e r i f i e d an absorption s t r e n g t h f o r strong l i n e s of about S = 2.1 χ 10"" cm" molecule~ cm . From the AFGL l i n e l i s t i n g s (6), i t i s found t h a t the optimum CC>2 l i n e f o r d e t e c t i o n purposes i s the Ρ (18) l i n e a t 2210.885 cm"" , and t h a t the p r i n c i p a l i n t e r f e r i n g species i n a i r i s N2O. Assume a d e t e c t i o n system i n c o r p o r a t i n g a 6 - l i t e r m u l t i t r a v e r s a l c e l l , a 200 m o p t i c a l path, and α£ = 10" . In a back ground of 7.6 Torr of a i r the minimum d e t e c t a b l e c o n c e n t r a t i o n of C02 i s found to be sponds to a t o t a l mass 1.73 χ 10~ p i c o c u r i e s i n e q u i v a l e n t u n i t s o f r a d i o a c t i v i t y . In pure CO2 without a background of a i r , c o n c e n t r a t i o n as low as 975 molecules/cm corresponding to a t o t a l mass of 4.36 χ 10"^ p i c o grams or 6.48 χ 10""4 p i c o c u r i e s can, i n p r i n c i p l e , be detected. 14

18

1

1

2

li+

1

5

llf

2

3

P a r t i a l Pressure

Determination

Laser s p e c t r o s c o p i c a n a l y s i s o f f e r s a d i r e c t , non-perturbing and general method o f determining p a r t i a l pressures i n gaseous mixtures. As i l l u s t r a t e d i n the H2SO4 study discussed below, p a r t i a l pressures can be measured by t h i s new technique to much lower l e v e l s and under more severe c o n d i t i o n s (e.g., 200°C hot s u l f u r i c acid) than would be p o s s i b l e by other methods. A Laser A n a l y t i c s group has i n v e s t i g a t e d p a r t i a l pressures of H2O, SO3 and H2SO4 vapors over a z e o t r o p i c aqueous s o l u t i o n o f H2SO4 under a program sponsored by the U.S. Environmental P r o t e c t i o n Agency (7_). The p a r t i a l pressures were determined by mea s u r i n g absorption l i n e strengths of SO3 and H2O above the s o l u t i o n and comparing them with l i n e strengths of c a l i b r a t i o n sam p l e s . The H2SO4 pressure was then deduced from a t o t a l pressure measurement using a U-tube manometer f i l l e d with l i q u i d s u l f u r i c acid. F i g u r e 4 shows a diode l a s e r scan of the vapor over an H2SO4 bath near 1416 cm" f o r a sample temperature of 165°C. The broad absorption d i p i n the center i s due t o atmospheric water vapor i n the 2.2m unpurged o p t i c a l path e x t e r n a l to the sample cell. One low pressure water l i n e i s apparent a t the center o f t h i s d i p ; the other l i n e s are due to SO3. The use of diode l a s e r spectroscopy allows s i n g l e l i n e s of SO3 or H2O t o be s e l e c t e d and t h e i r strengths measured. P a r t i a l pressures o f SO3, H2O and H2SO4 obtained i n t h i s study are summarized i n Table I I I . The d i s s o c i a t i o n constant Κ was c a l c u l a t e d from the data of Table I I I and i s i n c l u d e d i n 1

P


NEW

APPLICATIONS

O F LASERS T O C H E M I S T R Y

TABLE I I MINIMUM DETECTABLE CONCENTRATION FOR A TUNABLE DIODE LASER SYSTEM

Pollutant

Approximate Frequency (cm" )

S0

1140

0

2

1

Sensitivity (ppb) 3

105

3

N0

1150

2

C0

1075

300

H0

1135

50

NH

1050

2

2

2

3

0.05

PAN

1150

-0.3

CH

4

1300

0.03

S0

2

1370

0.3

N0

2

1600

0.02

NO

1880

0.03

CO

2120

0.01

C0„

2350

0.001


2.

BUTLER

Tunable-Diode-Laser

ET AL.

17

IR Spectroscopy

Figure 3. Τ unable-diode-laser transmission spec trum near 4.5 ^m showing the C 0 1-0 band R(23) absorption line at 2224.713 cm' , the C 0 ρ s band Τ(62) line at 224.033 cm , the C 0 [ΟΙ ! 5 W).

Figure 4. Schematic of the twophoton absorption process. Ger ade and ungerade states are denoted |g> and |w>, respec tively. |i> represents the inter mediate level while ω and ω are the frequencies of the two photons. 2

2


36


c i r c u l a r e x i s t s w i t h two f r e q u e n c i e s . B e c a u s e t h e two p h o t o n s a r e a b s o r b e d s i m u l t a n e o u s l y , t h e i r r e l a t i v e p o l a r i z a t i o n s map d i r e c t l y i n t o t h e symmetry o f t h e e l e c t r i c s u s c e p t i b i l i t y , a n d i t i s t h e r e f o r e p o s s i b l e t o p e r f o r m p o l a r i z a t i o n measurements i n f l u i d s o l u tion. The m a t h e m a t i c a l d e r i v a t i o n b y M c C l a i n (28) p r o v e s r i g o r o u s l y t h a t t h e p o l a r i z a t i o n t e r m s do n o t v a n i s h when t h e m o l e c u l e s a r e r a n d o m l y o r i e n t e d , a n d t h e r e f o r e t h e symmetry i n f o r m a t i o n i s retained. B y c o n t r a s t , i n o n e - p h o t o n s p e c t r o s c o p y , symmetry a s s i g n m e n t s c a n o n l y b e made f r o m s i n g l e c r y s t a l s t u d i e s . Again, t h e a n a l o g y b e t w e e n i n f r a r e d a n d Raman c a n b e made. i Raman s p e c t r o s c o p y t h e p o l a r i z a t i o n i n f o r m a t i o n i s r e t a i n e d because t h e e x c i t a t i o n and e m i s s i o n a r e simultaneous. I n o r d e r t o r e l a t e t h e m e a s u r e d c r o s s - s e c t i o n s t o t h e symm e t r y o f t h e t r a n s i t i o n , i t i s necessary t o determine t h e spectrum u n d e r t h r e e d i f f e r e n t p o l a r i z a t i o n c o n d i t i o n s a n d compare t h e r e s u l t s t o t h e known p a t t e r n p r a c t i c e t a b l e s have bee I t i s n e c e s s a r y t o have p r i o r knowledge o f t h e p o i n t group o f t h e m o l e c u l e b u t i t s h o u l d be n o t e d t h a t t h e two-photon experiment can be u s e f u l i n u n c o v e r i n g e n v i r o n m e n t a l e f f e c t s w h i c h l o w e r t h e symmetry o f t h e m o l e c u l e . Two-photon s p e c t r o s c o p y i s p a r t i c u l a r l y u s e f u l i n i d e n t i f y i n g t h e s y m m e t r i e s o f v i b r a t i o n a l modes w h i c h a r e c o u p l e d t o e l e c t r o n ic transitions. The s t a t e symmetry i s c a l c u l a t e d b y t h e d i r e c t p r o d u c t o f t h e e l e c t r o n i c t e r m w i t h t h e v i b r a t i o n a l t e r m (27) a n d c o m p a r e d w i t h t h e e x p e r i m e n t a l l y d e t e r m i n e d s t a t e symmetry. The a b i l i t y t o make symmetry a s s i g n m e n t s f o r e l e c t r o n i c a n d v i b r o n i c t r a n s i t i o n s i s a p r i m a r y advantage o f two-photon s p e c t r o s c o p y . The most s i m p l e t w o - p h o t o n e x p e r i m e n t s a r e p e r f o r m e d u s i n g o n l y one l a s e r f o r t h e e x c i t a t i o n p r o c e s s . I n t h i s c a s e t h e o n l y p o s s i b l e p o l a r i z a t i o n s a r e p a r a l l e l l i n e a r and c o r o t a t i n g c i r c u l a r . S i n c e t h r e e i n d e p e n d e n t p o l a r i z a t i o n measurements a r e r e q u i r e d t o u n i q u e l y s p e c i f y t h e symmetry o f t h e f i n a l s t a t e , t h e one l a s e r m e t h o d i s i n a d e q u a t e f o r t h i s p u r p o s e . W i t h two l a s e r s , t h e a d d i t i o n a l p o s s i b i l i t i e s o f p e r p e n d i c u l a r l i n e a r and c o n t r a - r o t a t i n g c i r c u l a r p e r m i t complete p o l a r i z a t i o n s t u d i e s . I t i s therefore n e c e s s a r y t o u t i l i z e two s e p a r a t e l y t u n a b l e l a s e r s i n o r d e r t o d e t e r m i n e m o l e c u l a r symmetry f r o m t w o - p h o t o n e x c i t a t i o n m e a s u r e ments . n

The N a p h t h a l e n e S p e c t r u m . The t w o - p h o t o n e x c i t a t i o n s p e c t r u m o f n a p h t h a l e n e s e r v e s t o d e m o n s t r a t e t h e c o n c e p t s d i s c u s s e d above. As a n e x a m p l e , t h e p a r t i a l e n e r g y l e v e l d i a g r a m o f F i g u r e 5 c a n be u s e d t o p r e d i c t d i f f e r e n c e s b a s e d on p a r i t y . The o n e - p h o t o n s p e c t r u m s h o u l d h a v e two bands c o r r e s p o n d i n g t o t h e A -> B ^ t r a n s i t i o n s , w h i l e t h e t w o - p h o t o n s p e c t r u m s h o u l d have one b a n d c o r r e s ponding t o the A •> Β transition. M i k a m i a n d I t o (30) h a v e s t u d i e d t h i s s y s t i m and found such a s i m p l e a n a l y s i s t o be p a r t i ally correct. T h a t i s , t h e A-^ B^ t r a n s i t i o n a p p e a r s v e r y w e a k l y , i f a t a l l , i n the, o n e - f h o t o n s p e c t r u m ; w h i l e t h e A •> Bp l g

u

g


wnvra

AND LYTLE

Two-Photon Excited Molecular Fluorescence

(235 r

lg

D

2u

J

(315 nm)

3u

\

—

1

•

—

Figure 5. Electronic energy level diagram for naphthalene


38

NEW

APPLICATIONS


t r a n s i t i o n appears v e r y weakly i n t h e two-photon spectrum. This a p r i o r i g e n e r a t e s l a r g e q u a l i t a t i v e d i f f e r e n c e s i n t h e shape o f t h e two s p e c t r a i n t h e s e w a v e l e n g t h r e g i o n s . On t h e o t h e r h a n d , t h e A ·> Β t r a n s i t i o n represents a f a i l ure o f such a simple a n a l y s i s . With one-photon e x c i t a t i o n t h i s t r a n s i t i o n i s symmetry f o r b i d d e n due t o o t h e r s e l e c t i o n r u l e s . As a result the absorption t o the Β s t a t e a p p e a r s a s weak s t r u c t u r e on t h e f a r s t r o n g e r B~ b a c k g r o u n d . ( S e e F i g u r e 6.) With twop h o t o n e x c i t a t i o n t h e " t r a n s i t i o n i s weak a n d a p p e a r s o n l y b e c a u s e of coupling t o b v i b r a t i o n s . (See F i g u r e 7·) Note t h e l a r g e q u a l i t a t i v e d i f f e r e n c e s b e t w e e n t h e s p e c t r a e v e n when t h e b a n d s a r e d e r i v e d f r o m t h e same e l e c t r o n i c t r a n s i t i o n . T h e r e a r e two reasons f o r t h i s dramatic change. F i r s t , t h e two-photon spectrum does n o t have a s t r o n g t r a n s i t i o n o f c o m p a r a b l e e n e r g y , w h i l e t h e one-photon s p e c t r u m does. T h i s f a c t i s r e s p o n s i b l e f o r t h e q u i t e d i f f e r e n t b a s e l i n e s . Second b r o n i c progression correspond two-photon e x c i t a t i o n they correspond t o b vibrations. The n a p h t h a l e n e example i l l u s t r a t e s t h a t t h e f e a t u r e s o f o n e and t w o - p h o t o n e x c i t a t i o n s p e c t r a d i f f e r b e c a u s e o f t h e change i n s e l e c t i o n r u l e s and u n p r e d i c t a b l e f e a t u r e s a r i s i n g from t h e p o s s i b i l i t y o f unique c o n f i g u r a t i o n i n t e r a c t i o n s . This supports t h e d r i v e t o u t i l i z e t w o - p h o t o n e x c i t a t i o n s p e c t r a a s a t o o l i n chem i c a l a n a l y s i s because a d d i t i o n a l i n f o r m a t i o n c a n be o b t a i n e d from t h e s p e c t r a and added degrees o f s e l e c t i v i t y a r e g r a n t e d . Q u a n t i t a t i v e A n a l y s i s I n Complex Samples V i a Two-Photon copy

Spectros

The a p p l i c a t i o n o f f l u o r i m e t r y as an a n a l y t i c a l t e c h n i q u e i s l i m i t e d i n many c a s e s b y t h e s p e c t r o s c o p i c p r o p e r t i e s o f t h e s a m p l e e n v i r o n m e n t . When a s i g n i f i c a n t f r a c t i o n o f t h e i n c i d e n t r a d i a t i o n i s absorbed by t h e m a t r i x , t h e measured f l u o r e s c e n c e i n t e n s i t y c e a s e s t o be a s i m p l e f u n c t i o n o f t h e f l u o r o p h o r c o n centration. This problem has been addressed by H o l l a n d , e t a l . ( 3 1 ) , who h a v e c o r r e c t e d t h e e m i s s i o n w i t h a s i m u l t a n e o u s l y m e a s u r e d v a l u e o f s o l u t i o n a b s o r b a n c e . S u c h a scheme was shown t o be v a l i d f o r o p t i c a l d e n s i t i e s £ 2. O f t e n , h o w e v e r , t h e a n a l y t e i s f o u n d i n a more h i g h l y a b s o r b i n g medium where c o n v e n t i o n a l t e c h n i q u e s a r e v e r y u n r e l i a b l e a n d s u c h c o r r e c t i o n p r o c e d u r e s c a n be d i f f i cult. The t w o - p h o t o n method i s q u a n t i t a t i v e i n a b s o r b i n g m e d i a b e c a u s e t h e r e i s n e g l i g i b l e a t t e n u a t i o n o f t h e beam i n t h i s p r o cess. F i r s t o f a l l , t h e i n c i d e n t beam i s t w i c e t h e w a v e l e n g t h o f the t r a n s i t i o n , thus t h e r a d i a t i o n i s not absorbed by t h e onephoton process. Second, t h e two-photon process i t s e l f i s o n l y weakly absorbing. F l u o r i m e t r i c A n a l y s i s i n Complex S a m p l e s . The r a t e o f f l u o r escence emission i s , by d e f i n i t i o n , equal t o t h e r a t e o f l i g h t a b s o r p t i o n t i m e s t h e f l u o r e s c e n c e quantum y i e l d .


3. wiRTH

AND LYTLE



39

40

NEW

F=

APPLICATIONS O F

LASERS TO

CHEMISTRY

I 0 A

or F = I (1-T)0

Eq.

9

where F i s t h e e m i s s i o n i n t e n s i t y , I . i s t h e a b s o r b e d e x c i t a t i o n i n t e n s i t y , I i s the i n c i d e n t e x c i t a t i o n i n t e n s i t y , Τ i s the sample t r a n s m i s s i o n and 0 i s t h e quantum y i e l d . A linear relation b e t w e e n e m i s s i o n i n t e n s i t y and c o n c e n t r a t i o n h o l d s o n l y when t h e sample a b s o r b a n c e , A, i s a p p r o x i m a t e l y e q u a l t o 1-T, b e c a u s e A, r a t h e r t h a n 1-T, i s p r o p o r t i o n a l t o c o n c e n t r a t i o n . The a p p r o x i m a t i o n A - 1-T b r e a k s down a t h i g h a b s o r b a n c e s , r e s u l t i n g i n r o l l o f f o f the c a l i b r a t i o n curve. T h i s s i t u a t i o n i s r e f e r r e d t o as the i n n e r f i l t e r e f f e c t . Under t h e s e c o n d i t i o n s , t h e i n t e n s i t y o f the i n c i d e n t r a d i a t i o n i s attenuated before reaching the center of t h e sample c e l l . When t h e r e i s mor d i m i n i s h e d a c c o r d i n g t o the t o t a l absorbance o f t h e sample. U s i n g Eq. 9, and t h e f a c t t h a t t h e r a t i o s o f t h e a b s o r b a n c e s e q u a l s t h e r a t i o o f t h e e m i s s i o n i n t e n s i t i e s f o r a b s o r b i n g samples (32), the r e l a t i o n s h i p between the absorbance o f the f l u o r o p h o r e , A , t h e t o t a l sample a b s o r b a n c e , A^, and t h e e m i s s i o n i n t e n s i t y can be d e r i v e d as F

Eq.

10

where Τ i s now t h e t o t a l sample t r a n s m i s s i o n . A c c o r d i n g t o Eq. 10, i f the t o t a l s o l u t i o n absorbance i s s m a l l , then Α - 1-T, and t h e s p e c i e s i n s o l u t i o n have independent responses. The e m i s s i o n i n t e n s i t y i n t h i s case i s p r o p o r t i o n a l t o c o n c e n t r a t i o n . At h i g h o p t i c a l d e n s i t i e s , t h e amount o f r a d i a t i o n e x c i t i n g t h e f l u o r o p h o r i s r e d u c e d by t h e r a t i o o f i t s a b s o r b a n c e t o t h e t o t a l s o l u t i o n value. The r e s u l t i n g i n n e r f i l t e r e f f e c t a l t e r s t h e i n c i d e n t i n t e n s i t y a t d i f f e r e n t d i s t a n c e s a l o n g t h e c e l l p a t h l e n g t h . Under t h e s e c i r c u m s t a n c e s , t h e f l u o r e s c e n c e i s no longer simply pro p o r t i o n a l t o c o n c e n t r a t i o n and, a t v e r y h i g h a b s o r b a n c e s , w i l l n o t e v e n be v i s i b l e a t t h e c e n t e r o f t h e s a m p l e . U n l i k e p u r e s u b s t a n c e s , d i l u t i n g the s o l u t i o n i s not g e n e r a l l y a p r a c t i c a l approach t o r e d u c i n g the problem. Often the s i g n a l would f a l l below the d e t e c t i o n l i m i t or the nature o f the f l u o r o p h o r c o u l d change b e c a u s e o f a s h i f t e d e q u i l i b r i u m . The i n n e r f i l t e r e f f e c t can be r e d u c e d by o b s e r v i n g t h e f l u o r e s c e n c e e m i s s i o n i n t h e f r o n t s u r f a c e c o n f i g u r a t i o n . T h i s method u l t i m a t e l y f a i l s a t h i g h c o n c e n t r a t i o n s because the p e n e t r a t i o n depth of the i n c i d e n t r a d i a t i o n i s dependent upon t h e sample a b s o r b a n c e . φ

A b s o r p t i o n I n t e r f e r e n c e s at E x c i t a t i o n Wavelengths. The e f f e c t o f a b s o r b i n g i n t e r f e r e n c e s was s t u d i e d by u s i n g a m o d e l s y s t e m c o n s i s t i n g o f p - t e r p h e n y l as t h e f l u o r o p h o r and b i p y r i d i n e


3.

wmTH

AND LYTLE

580


41

59 WAVELENGTH (nm)

Figure 7. Two-photon excitation spectrum of naphthalene

Figure 8. Spectral data for p-terphenyl and bipyridine. (a) Excitation and (b) emission spectrum of ip-terphenyl; (c) absorption spectrum of bipyridine.


42

NEW

APPLICATIONS OF

LASERS TO

CHEMISTRY

as t h e i n t e r f e r i n g c h r o m o p h o r e , i n t h e s o l v e n t c y c l o h e x a n e . The s p e c t r a l d a t a f o r t h i s s y s t e m a r e shown i n F i g u r e 8. The a b s o r p t i o n band o f b i p y r i d i n e o v e r l a p s t h a t o f p - t e r p h e n y l almost u n i f o r m i l y , while the emission i s i n a transparent s p e c t r a l region. A f i x e d amount o f p - t e r p h e n y l was e x c i t e d i n t h e p r e s e n c e o f a v a r y ing concentration of b i p y r i d i n e while the fluorescence of p - t e r p h e n y l was m o n i t o r e d . The r e s u l t s , w h i c h a r e p l o t t e d i n F i g u r e 9, show t h a t t h e o n e - p h o t o n e x c i t e d f l u o r e s c e n c e v a r i e s w i t h t h e s a m p l e a b s o r b a n c e r a t i o , as p r e d i c t e d . The f l u o r o p h o r c o n c e n t r a t i o n i n f o r m a t i o n i s l o s t upon o n e - p h o t o n e x c i t a t i o n when t h e c o n c e n t r a t i o n o f t h e chromophore i s unknown. I n t w o - p h o t o n e x c i t e d fluorescence, the inner f i l t e r e f f e c t i s nonexistent because the w a v e l e n g t h o f t h e i n c i d e n t r a d i a t i o n i s f a r removed from t h e s o l u t i o n a b s o r b a n c e and a n e g l i g i b l e amount o f power i s e x t r a c t e d by the n o n l i n e a r process. Therefore t h e i n t e n s i t y o f t h e beam i s n o t a f f e c t e d by the v a r i a t i o h i g h s o l u t i o n absorbance is retained. A b s o r p t i o n I n t e r f e r e n c e s a t E m i s s i o n W a v e l e n g t h s . A common problem e n c o u n t e r e d i n t h e f l u o r e s c e n c e a n a l y s i s o f complex samples i s the reabsorption of emission. T h i s o c c u r s when t h e e m i s s i o n band o f t h e a n a l y t e i s o v e r l a p p e d by t h e absorbance band o f a n o t h e r species i n solution. In r i g h t angle d e t e c t i o n t h e e m i s s i o n must p a s s t h r o u g h a c e l l p a t h l e n g t h o f 0.5 cm b e f o r e r e a c h i n g t h e de t e c t o r , t h u s t h e a b s o r b a n c e o f t h e s o l u t i o n c a n n o t e x c e e d 0.025 u n i t s a t the e m i s s i o n w a v e l e n g t h i n o r d e r t o keep the q u a n t i t a t i o n e r r o r b e l o w 5?. F o r a chromophore w i t h a m o l a r a b s o r p t i v i t y o f 1 0 ^ , i t s c o n c e n t r a t i o n must be b e l o w 2.5 yM t o k e e p t h e e m i s s i o n i n t e n s i t y r e p r e s e n t a t i v e o f t h e f l u o r o p h o r c o n c e n t r a t i o n . As i n t h e p r e v i o u s l y d i s c u s s e d i n t e r f e r e n c e c a s e , d i l u t i o n o f t h e sample w o u l d n o t n e c e s s a r i l y be a p r a c t i c a l a p p r o a c h . The r e a b s o r p t i o n p r o b l e m i s c i r c u m v e n t e d w i t h t w o - p h o t o n s p e c troscopy because i t i s p o s s i b l e t o u t i l i z e the f a c t t h a t the e x c i t a t i o n e f f i c i e n c y i s i n v e r s e l y p r o p o r t i o n a l t o the transverse area o f t h e beam. By f o c u s i n g t h e i n c i d e n t beam c l o s e t o t h e f r o n t surface of the c e l l , the path length of the emission i s decreased t h u s t o l e r a t i n g a h i g h e r chromophore c o n c e n t r a t i o n . The p a t h l e n g t h o b s e r v e d i n t h i s w o r k was e s t i m a t e d t o be 100 ym, a l l o w i n g t h e s o l u t i o n a b s o r b a n c e a t t h e e m i s s i o n w a v e l e n g t h t o be as h i g h as 3. The m o d e l s y s t e m u s e d i n t h i s s t u d y c o n s i s t e d o f t h e f l u o r o p h o r s b i s - m e t h y l s t y r y l b e n z e n e ( b i s M S B ) and 2,5-diphenyloxazole (PP0). The f l u o r e s c e n c e s p e c t r a o f F i g u r e 10 show t h a t t h e ab s o r b a n c e b a n d o f b i s M S B s t r o n g l y o v e r l a p s t h e e m i s s i o n b a n d o f ΡΡ0. F o r t h e e x p e r i m e n t , t h e c o n c e n t r a t i o n o f ΡΡ0 was h e l d c o n s t a n t , t h e c o n c e n t r a t i o n o f b i s M S B was v a r i e d , and t h e f l u o r e s c e n c e i n t e n s i t y o f ΡΡ0 was m o n i t o r e d f o l l o w i n g one- and t w o - p h o t o n e x c i t a t i o n . The e x p e r i m e n t a l r e s u l t s are p r e s e n t e d i n F i g u r e 11. I n the one-photon a n a l y s i s , f r o n t s u r f a c e d e t e c t i o n was i m p l e m e n t e d i n o r d e r t o e f f e c t


3.

wiRTH

AND LYTLE


Bipyridine

43

Absorbance

Figure 9. Fluorimetry data for the p-terphenyl determination, (a) Two-photon and (b) one-photon excitation.

250

300

350 Wave leng t h

400

450

500

( nm )

Figure 10. Spectral data for 2,5-diphenyloxazole (PPO) and bis-methylstyrylbenzene (bisMSB). (a) Excitation and (b) emission spectra of PPO; (c) excitation and (d) emission spectra of bisMSB.


44

N E W APPLICATIONS

0

1 bisMSB


2 Absorption

3 (350nm)

Figure 11. Fluorimetry data for the mixture analysis, (a) Onephoton and (b) two-photon excitation.


3.

wiRTH

AND LYTLE


45

a s h o r t p a t h l e n g t h , h o w e v e r , t h i s e m i s s i o n was s t i l l d i m i n i s h e d a t h i g h s a m p l e a b s o r b a n c e s due t o t h e r e a b s o r p t i o n p r o c e s s . The d i s t a n c e t h r o u g h w h i c h t h e f l u o r e s c e n c e t r a v e l e d was s u f f i c i e n t l y l a r g e t o r e s u l t i n q u a n t i t a t i o n e r r o r s . I n t h e two-photon case, the i n t e n s i t y remains constant, independent o f t h e i n t e r f e r e n c e c o n c e n t r a t i o n up t o s o l u t i o n a b s o r b a n c e s o f 3. I t s h o u l d be n o t e d t h a t t h e e f f e c t s o f e m i s s i o n r e a b s o r p t i o n can b e a l l e v i a t e d t o some e x t e n t i n o n e - p h o t o n s p e c t r o s c o p y b y c o n s t r u c t i n g a spectrometer w i t h very p r e c i s e f o c a l parameters. I t i s p o s s i b l e t o c o l l e c t fluorescence i n t e n s i t y from only t h e f i r s t 1Û0 ym o f t h e s a m p l e , b y u s i n g h i g h q u a l i t y o p t i c s . There a r e t w o l i m i t a t i o n s t o t h i s a p p r o a c h . F i r s t , 100 ym i s a b o u t t h e s m a l l e s t p r a c t i c a l d i s t a n c e t h a t c a n be a c h i e v e d , whereas t h e twop h o t o n method c a n g e t down t o < 10 ym. S e c o n d , t h e u s u a l f r o n t s u r f a c e i n t e r f e r e n c e s a r e s t i l l a b l e t o hamper t h e a n a l y s i s : a) t h e b a c k g r o u n d s c a t t e c e l l i s subject t o adsorptio s e l f may f l u o r e s c e . These p r o b l e m s a r e a v o i d e d w i t h t w o - p h o t o n e x c i t a t i o n because t h e i n c i d e n t l i g h t i s n o t focused through t h e s a m p l e c e l l i n t h e s p a t i a l r e g i o n where t h e f l u o r e s c e n c e i s o b served. F l u o r i m e t r i c A n a l y s e s i n A b s o r b i n g S o l v e n t s . An i m p o r t a n t s p e c i a l case o f an i n t e r f e r i n g chromophore i s t h e o p t i c a l l y dense s o l v e n t . W i t h one-photon e x c i t a t i o n , i t i s i m p e r a t i v e t h a t t h e s o l v e n t b e t r a n s p a r e n t where t h e s p e c i e s o f i n t e r e s t a b s o r b . When t h i s r e s t r i c t i o n precedes chemical c o n s i d e r a t i o n o f t h e solvent c h o i c e , t h e a n a l y s i s i s a p r i o r i l e s s t h a n t h e optimum. A s a w o r s t c a s e , t h e f l u o r o p h o r i s o f t e n i n , a n d c a n n o t b e removed f r o m , a n o p t i c a l l y dense m a t r i x . W i t h two-photon e x c i t a t i o n , t h e i n c i d e n t r a d i a t i o n i s twice t h e wavelength o f t h e solvent absorbance, thus i t e a s i l y penetrates into t h e matrix t o e x c i t e t h e species o f interest. The s p e c t r a l d a t a f o r a m o d e l s y s t e m , PPO i n a c e t o n e , a r e given i n F i g u r e 12. V i s u a l i n s p e c t i o n o f t h e s o l u t i o n s i n d i c a t e s t h a t v i r t u a l l y none o f t h e e x c i t i n g r a d i a t i o n r e a c h e s t h e c e n t e r o f t h e sample c e l l . As a r e s u l t , a f r o n t s u r f a c e c o n f i g u r a t i o n i s mandatory f o r one-photon e x c i t a t i o n . Even w i t h such an arrangement, s e n s i t i v i t y i s decreased by t h e numerical value o f t h e s o l v e n t , a c c o r d i n g t o Eq. 10. S i n c e two-photon e x c i t a t i o n i s n o t a t t e n u a t e d b y t h e s o l v e n t , a r i g h t a n g l e c o n f i g u r a t i o n may b e u s e d . F i g u r e 1 3 shows t h e f l u o r e s c e n c e c a l i b r a t i o n c u r v e s o b t a i n e d f o r t h e model systems d i s c u s s e d above. W i t h t h e l a s e r s o u r c e u s e d i n t h i s s t u d y , t h e d e t e c t i o n l i m i t (S/N = l ) f o r two-photon e x c i t a t i o n o f PPO i n a c e t o n e i s a p p r o x i m a t e l y 0.5 yM. T h i s r e s u l t i s p r i m a r i l y d e t e r m i n e d b y t h e p e a k power o f t h e s o u r c e , w h i c h i t s e l f c a n c e r t a i n l y b e i m p r o v e d b y a t l e a s t a n o r d e r o f m a g n i t u d e . The o n e - p h o t o n d e t e c t i o n l i m i t i s a p p r o x i m a t e l y t h e same a s t h a t f o r the two-photon case. Although t h e one-photon v a l u e can c o n c e i v a b l y be l o w e r e d , i t i s u l t i m a t e l y l i m i t e d b y t h e amount o f s c a t t e r e d


46

NEW

250

300

APPLICATIONS

350 W a v e l e n g th


400

450

(nm)

Figure 12. Spectral data for PPO in acetone, (a) Excitation and (b) emission spectra of PPO. The dashed curve represents the acetone cut off.


wiRTH

AND LYTLE


Figure 13. Fluorimetry data for the PPO in acetone determination, (a) Two-photon and (b) one-photon excitation.


48

NEW

APPLICATIONS O F LASERS T O C H E M I S T R Y

e x c i t a t i o n e n t e r i n g t h e e m i s s i o n monochromâtor. T h i s i s a n i n h e r e n t problem i n t h a t the e x c i t a t i o n and e m i s s i o n f r e q u e n c i e s are u s u a l l y c l o s e t o each other. N a t u r a l l y , t h ef r o n t surface arrangement maximizes t h e d i f f i c u l t y . T h e r e a r e no a p r i o r i r e a s o n s t o assume t h a t a n i n c r e a s e i n s o u r c e power w i l l i m p r o v e t h e d e t e c t i o n l i m i t , because the background noise i s i n c r e a s e d pro portionally. A g a i n , two-photon e x c i t a t i o n minimizes t h i s problem s i n c e the f r e q u e n c i e s are u s u a l l y f a r from each o t h e r and a r i g h t a n g l e o b s e r v a t i o n can be used. I t i s e v i d e n t f r o m F i g u r e 13 t h a t t h e o n e - p h o t o n c u r v e s e x h i b i t the well-known i n n e r f i l t e r r o l l - o f f a t h i g h concentrations. The l i n e a r i t y i n a n y f l u o r e s c e n c e c a l i b r a t i o n f a i l s a s s o o n a s t h e e m i t t e r a b s o r b a n c e becomes s u f f i c i e n t l y l a r g e a s t o b r e a k down t h e approximation that A - ( l - T ) . I n o p t i c a l l y dense s o l v e n t s t h i s r o l l o f f occurs a t c o n c e n t r a t i o n s s e v e r a l hundred times h i g h e r than i n t r a n s p a r e n t s o l v e n t s du s h o u l d be noted t h a t th i n c r e a s e d r o l l - o f f c o n c e n t r a t i o n because the lower l i m i t o f d e t e c t i o n i s s i m u l t a n e o u s l y r a i s e d b y t h e same f a c t o r . For two-photon e x c i t a t i o n t h e t o t a l a b s o r p t i o n i s s o s m a l l t h a t t h e r e i s no i n n e r f i l t e r e f f e c t o p e r a t i n g a n d , t h e r e f o r e , no c u r v a t u r e i n t h e c a l i b r a t i o n g r a p h . T h u s , f o r f l u o r o p h o r s i n o p t i c a l l y dense m a t r i c e s , t h i s method p r o d u c e s t h e l a r g e s t l i n e a r r a n g e . A c k n o w l e dgement s T h i s r e s e a r c h was s u p p o r t e d i n p a r t t h r o u g h f u n d s p r o v i d e d b y The N a t i o n a l S c i e n c e F o u n d a t i o n u n d e r G r a n t CHE77-2^312. M.J.W. g r a t e f u l l y acknowledges the American A s s o c i a t i o n o f U n i v e r s i t y Women f o r f e l l o w s h i p s u p p o r t . Literature Cited 1.

2. 3. 4. 5.

F o r a brief, b u t c o m p r e h e n s i b l e discussion of tensors, s e e F. Z e r n i k e a n d J. E. Midwinter, "Applied Nonlinear Optics", Wiley-Interscience, New Y o r k , 1973. B l o e m b e r g e n , N. in "Quantum Electronics: A Treatise", H. R a b i n and C. L. Tang, E d s . , A c a d e m i c Press, New Y o r k 1975. Maiman, Τ. Η., N a t u r e , (1960), 187, 493. F r a n k e n , P. A. et al., P h y s . Rev. Lett., (1961), 7, 118. K a i s e r , W. a n d Garrett, C. G., P h y s . Rev. Lett., (1961), 9,

455. 6. 7. 8. 9. 10. 11.

E c k h a r d t , G., et al., P h y s . Rev. Lett., (1962), 7, 229. B a s s , M., et al., P h y s . Rev. Lett., (1962), 8, 18. B a s s , M., et al., P h y s . Rev. Lett., (1962), 9 , 446. T e r h u n e , R. W., M a k e r , P. D., a n d S a v a g e , C. M. P h y s . R e v . Lett., (1962), 8, 404. C h a i o , R. Y., G a r m i r e , Ε . , a n d Townes, C. Η., P h y s . R e v . Lett., (1964), 12, 279. M a k e r , P. D. a n d T e r h u n e , R. W., P h y s . Rev., (1965), 137, 801.


3. 12. 13. 14. 15. 16. 17. 18.

19. 20. 21. 22. ron., 23. 24. 25. 26. 27. 28. 29. 30. 31. 32.

WIRTH

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LYTLE


49

M c C l a i n , W. Μ., A c c . Chem. R e s . , (1974), 7, 129. W i r t h , M. J. a n d Lytle, F. Ε . , Anal. Chem., (1977), 4 9 , 2054. Harris, J. M., C h r i s m a n , R. W., a n d Lytle, F. Ε . , Appl. P h y s . Lett., (1975), 2 6 , 16. Chan, C. K. a n d Sari, S. O., Appl. P h y s . Lett., (1974), 25, 403. Mahr, H. a n d Hirsch, M. D., Opt. Commun., (1975), 13, 96. Harris, J. Μ., G r a y , L. M., Pelletier, M. J., a n d Lytle, F. Ε . , M o l . Photochem., (1977), 8, 161. S t e i n m e t z , L. L . , R i c h a r d s o n , J. H., Wallin, B. W., a n d Book less, W. Α., presented at the Topical M e e t i n g for Picosecond Phenomena, Hilton Head, S o u t h Carolina, May 1978. K u h l , J., L a m b r i c h , R., a n d v o n d e r Linde, D., Appl. P h y s . Lett., (1977), 31, 657. J a i n , R. K. a n d Heritage, J. P. Appl. P h y s Lett., (1978) 32, 41. Ausschnitt, C. P. Jain, , Appl. P h y s Lett., ( 1 9 7 8 ) , 32, 727. Frigo, M. J., D a l e y , T., a n d Mahr, Η., I E E E J. Quant. Elect (1977), 1 3 , 101. G e o p p a r t - M a y e r , Μ., Ann. P h y s . (Leipzig), (1931), 9, 273. M c C l a i n , W. M. a n d Harris, R. Α., in "Excited States", Vol. 3 , (1977), E. C. L i m , E d . , A c a d e m i c Press, New Y o r k 1977. G e l b w a c h s , J. Α., Klein, C. F., a n d W e s s e l , J. Ε . , Appl. P h y s . Lett., (1977), 9 , 489. L e t o k h o v , V. S., Ann. Rev. P h y s . Chem., (1977), 2 8 , 1 3 3 . C o t t o n , F. Α., " C h e m i c a l Applications of Group T h e o r y " , WileyInterscience, New Y o r k 1971. Monson, P. R. a n d McClain, W. M., J. Chem. P h y s . , ( 1 9 7 0 ) , 5 3 , 29. M c C l a i n , W. M., J. Chem. P h y s . , (1971), 55, 2789. M i k a m i , N. a n d Ito, Μ., Chem. P h y s . Lett., (1975), 3 1 , 472. H o l l a n d , J. F., Teets, R. Ε . , Kelley, P. Μ., a n d T i m n i c k , Α., Anal. Chem., (1977), 4 9 , 706. M i c h a e l s o n , R. C. a n d L o u c k s , L. F., J. Chem. E d . , (1975), 5 2 , 652.

RECEIVED

August 25, 1978.


4 Laser-Excited Luminescence Spectrometry J. D. WINEFORDNER Department of Chemistry, University of Florida, Gainesville, F L 32611

Laser e x c i t e d atomi r e c e n t l y i n the d e t e c t i o n of u l t r a low concentrations of s p e c i e s . For example, atomic a b s o r p t i o n flame spectrometry (AAS) is l i m i t e d to ~10 atoms/cm . Mass spectrometry is g e n e r a l l y a lit tle l e s s s e n s i t i v e than AAS. Conventional atomic f l u o r e s c e n c e flame spectrometry (FAFS) i s capable of ~10 atoms/cm . Dop p l e r - f r e e , two photon atomic f l u o r e s c e n c e spectrometry (DAFS) i s capable of d e t e c t i n g ~10 species/cm but the absorption cross - s e c t i o n s must be l a r g e , n a t u r a l and collisional l i n e widths must be s m a l l , and r a d i a t i o n l i f e t i m e s must be s h o r t . Resonantly en hanced two-photon a b s o r p t i o n spectrometry (TPAAS) allows detec t i o n of ~10 atoms/cm , but the absorber must have an i n t e r m e d i ate s t a t e of the a p p r o p r i a t e energy to allow utilization of two counter-propagating or co-propagating l a s e r beams. Single-photon l a s e r e x c i t e d f l u o r e s c e n c e spectrometry (SAFS) has a l s o been used to detect ~ 1 0 Na atoms/cm . SAFS however, r e q u i r e s f r e quency-doubled dye l a s e r s f o r many t r a n s i t i o n s and i n some cases difficult frequency-mixing systems to o b t a i n good d e t e c t i o n lim its. L a s e r - e x c i t e d non-resonance AFS (LNRAFS) has been used to detect 10 atoms/cm , although d e t e c t i o n l i m i t s of ~10 atoms/cm have been p r e d i c t e d . In the LNRAFS case, the atom must have 3 l e v e l s with appropriate o p t i c a l and mixing p r o p e r t i e s . Finally, p h o t o i o n i z a t i o n d e t e c t i o n of a l a s e r - e x c i t e d gaseous system (PLE)L2>Ii of Cs has been found to y i e l d a d e t e c t i o n l i m i t of the order of 1 atom/cm^. The PLE system i s e x t r a o r d i n a r i l y s e n s i t i v e but i s plagued with problems d i f f i c u l t to circumvent i n a n a l y t i c a l spectrometry, such as the sampling method, i n c l u d i n g the means of producing atoms (or molecules), and the l a s e r source which must cover a wide-wavelength range. Photon c a t a l y s i s l i t which i n v o l v e s the use of a c t i v e n i t r o g e n produced by passing N2 through a microwave discharge, has been used to detect ~10^ atoms/cm^ of B i i n an Ar atmosphere. Photon c a t a l y s i s and mass spectrometry do not i n v o l v e the use of l a s e r e x c i t a t i o n but do 1,2

8

3

2,3

4

3

4

5

3

5

2

3

6

3

2

7

4

3

0-8412-0459-4/78/47-085-0 5 0$07.25/0 © 1978 American Chemical Society


3

4.

51

Laser-Excited Luminescence Spectrometry

wiNEFORDNER

8

r e s u l t i n good d e t e c t i o n l i m i t s , i . e . , order of or below 1 0 atoms/ cm . I t i s apparent that a c t i v e work i s c u r r e n t l y on-going i n the area of u l t r a - t r a c e d e t e c t i o n of atoms and molecules. The detec t i o n l i m i t s i n d i c a t e d above are t r u l y e x c i t i n g when one considers that a species with a molecular weight of 100 amu represents 1.67x10 g/cm assuming a d e t e c t i o n l i m i t of 1 atom/cm or 1.67 x l O ^ g/cm assuming a d e t e c t i o n l i m i t of 10° atoms/cm . I f the atoms are produced i n a t y p i c a l a n a l y t i c a l flame (the detec t i o n l i m i t of 1 cm" i s v a l i d only f o r a quartz v e s s e l ) , then the conversion f a c t o r of sample c o n c e n t r a t i o n ( i n pg/ml) a s p i r a t e d i n t o a chamber type laminar flame system to gas c o n c e n t r a t i o n (atoms/cm ) assuming a sample i n t r o d u c t i o n ( t r a n s p o r t ) r a t e of 3 cnr/min, an unburnt gas flow r a t e (room temperature) of 20 &/min, a flame temperature of 2500°K and a 10% n e b u l i z a t i o n e f f i c i e n c y i s ~10~9, i . e . atoms/cm corresponds t 3

2

2

3

- 1

3

3

Fundamental Considerations Pseudo-Continuum Laser E x c i t e d Atomic Fluorescence Spectrome t r y , AFS. B o u t i l i e r , et a l j X and others?> h» ^ ^ h a v e considered the atomic fluorescence radiance expressions f o r the case of "broad-band" (pseudo-continuum, i . e . , 6v^, where 6vg = spec t r a l h a l f - w i d t h of source of e x c i t a t i o n and 6v^ = absorption l i n e half-width) e x c i t a t i o n of atoms present i n a medium at low o p t i c a l d e n s i t i e s . The general expression f o r Bp, the fluorescence r a d i ance i s given by: ην

τ»

Y F Ε Ρ a

r ι

So36

η

m-i

ττ

-ζ ο — ι±

(1)

v

τη

χ.

or AU F

IT A

n

j'

m

"2

"Ι

(2)

s r

where: f l u o r e s c e n c e path length i n d i r e c t i o n of observation, m, fluorescence power e f f i c i e n c y ,

pF

absorbed W

=

s p e c t r a l i r r a d i a n c e from source of e x c i t a t i o n ,

2

m"",

hv^ = absorbed BA

dimensionless,

photon energy, J , 1

J

E i n s t e i n induced absorption c o e f f i c i e n t , s (Jm Hz


)

,

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APPLICATIONS

d e n s i t y o f a n a l y t e s p e c i e s i n lower


l e v e l of absorption

-3 transition, m

,

d e n s i t y of a n a l y t e s p e c i e s i n upper l e v e l of a b s o r p t i o n 3

transition, m δ

g

£'

,

s t a t i s t i c a l w e i g h t s o f l e v e l s £ and m, r e s p e c t i v e l y , d i -

m

mensionless, A.p = E i n s t e i n c o e f f i c i e n t o f s p o n t a n e o u s e m i s s i o n f o r f l u o r escence t r a n s i t i o n

s

\

n. = d e n s i t y o f a n a l y t

3

-3 transition, m

F o r a t h r e e - l e v e l atom o f t h e N a - t y p e , B_ i s g e n e r a l l y g i v e n Γ

by

A

4π

u£

h V

(3)

n

u£ T

A

T

£->u

+k u£

1+

,+k • uu u£ u u

v, So 1+-*- [1+ p ^ ' l u' V £u g

E

f

f

w h e r e a l l t e r m s a r e as d e f i n e d above e x c e p t t h a t t h e k s a r e p s e u d o - f i r s t order r a d i a t i o n l e s s d e a c t i v a t i o n rate constants ( s " " ) , Ε* i s a m o d i f i e d s a t u r a t i o n s p e c t r a l i r r a d i a n c e and i s g i v e n by 1

Z

u

c A , u' '£u

Y

f

f

B

(4)

, W m

f

u £ u £

w h e r e Y i £ i s t h e f l u o r e s c e n c e quantum e f f i c i e n c y and u , i s t h e u p p e r l e v e l i n v o l v e d i n t h e e x c i t a t i o n p r o c e s s , and £ i s t h e l o w e r l e v e l i n v o l v e d i n t h e a b s o r p t i o n p r o c e s s ( n o t e f o r some t r a n s i t i o n s u = u ) , and η i s t h e t o t a l d e n s i t y o f a n a l y t e atoms i n a l l e l e c t r o n i c s t a t e s . The s a t u r a t i o n s p e c t r a l i r r a d i a n c e , Ef ,i s '£u' g i v e n by u

f

t

+

NiU Vu

+ A

f

k

f

C

u £~ u u

k +k +A u£ u u 0

f

T

'£u

f

I V £

e

ι

k .

n

+

u

u£

'

u£

u

uu'

uX.


(5)

4.

wiNEFORDNER

53


For a t h r e e - l e v e l atom of the T£-type, Β

i s generally

given

Γ

by A u

r

h

v

u * ' \

E*

.

,

(6)

£-m v

£u

f

£ £

where a l l terms are as d e f i n e d above except f o r c A u£ -2 , W m , Υ Β u£ u£ T T

'£u

(7)

and the s u b s c r i p t s u, £ uppe (metastable) l e v e l , and the lower l e v e l ( f o r some t r a n s i t i o n s £ = £ ) , respectively. F i n a l l y , the s a t u r a t i o n s p e c t r a l i r r a d i a n c e , i s given by £u A +A + k +k u£ u£' u u£ (8) £ £ ' u £ ' u£' £u £u— + 8» ££ £ £ T

ol

f

, t 0

k

J

+ A

k

l + k

+ k

T

I t should be noted that e i t h e r the Na-type case or the T£type case r e v e r t s to the 2 - l e v e l case i f i n the former u -Hi and i n the l a t e r £'+£, i . e . , f

B_

Α

ir>£

£-m

«ΛΛ

(9) 1+

1+

£u £a

From the above expressions, p a r t i c u l a r l y ^n the l i m i t i n g c a ses of high source s p e c t r a l i r r a d i a n c e ( E >>E ) and i n the case of low source s p e c t r a l i r r a d i a n c e ( E

+ k

i 32 32 k

8

. 3

+

f2 -E e

g

21

1 2

/kf

3

which i s simply the 2 - l e v e l approximation f o r a 3 - l e v e l atom. (4) The s a t u r a t i o n s p e c t r a l i r r a d i a n c e , E , i . e . , the source s p e c t r a l i r r a d i a n c e where B-p becomes 50% the maximum pos s i b l e value, depends upon the frequency, v, of the t r a n s i t i o n ( A £ / B £ = δπην-Vc ) and upon the r a d i a t i o n a l and r a d i a t i o n l e s s r a t e constants ( r e f e r to equations 5 and 8). Therefore, t h e o r e t i c a l e s t i m a t i o n of E^ values from fundamental aspects i s d i f f i c u l t even though experimental measurement i s q u i t e easy. For a 2 - l e v e l atom, measure ment of Ef) values allows e s t i m a t i o n of quantum e f f i c i e n c i e s , Y. However, f o r 3 - l e v e l atoms, e s t i m a t i o n of quantum e f f i c i e n c i e s r e q u i r e s not only measurement of E^ but a l s o e x p l i c i t knowledge of c e r t a i n r a d i a t i o n a l and r a d i a t i o n l e s s r a t e constants. v

3

U

U

Narrow L i n e Laser E x c i t a t i o n Molecular Luminescence Spectro metry, MLS, B o u t i l i e r and W i n e f o r d n e r — have considered the molecular luminescence radiance expressions f o r the case of "narrow-line e x c i t a t i o n of molecules (6v |

(12)

J

(Q)>l'

(13a)

(13b)

where || i s the v i b r a t i o n a l overlap i n t e g r a l (FranckCondon f a c t o r ) between v i b r a t i o n a l l e v e l s i n the two e l e c t r o n i c s t a t e s involved i n the a b s o r p t i o n and luminescence processes (Q i s the v i b r a t i o n a l c o o r d i n a t e ) ; the Born-Oppenheimer approximator i s assumed to apply here (the overlap term i s d i m e n s i o n l e s s ) . For a three l e v e l molecule, B o u t i l i e r , et al.Jâ showed that


56

NEW

APPLICATIONS O F

LASERS T O

CHEMISTRY

the fluorescence radiance 3+1 f o r 1+3 e x c i t a t i o n or the forbidden process of 2+1 f o r 1+2 e x c i t a t i o n i s given by i=a, Σ A

.νυο,ιΛο,ιΛ *

1=0

1 +

—

(14)

/ε

E* 1 +

8„

V10, uj

/δν

Α

i=

Q

u'O.li

u'l

u'u

where a l l symbols have sents the r a d i a t i o n a l l other upper l e v e l ( i n case 1+3 e x c i t a t i o n , u = 3 and u = 2). For a three l e v e l molecule, B o u t i l i e r , et al.IS- showed that the luminescence radiance f o r 1+3 e x c i t a t i o n and 2+1 emission or f o r 1+2 e x c i t a t i o n and 3+1 emission i s given by i=oo

Σ A u 0,li i=0 tf

T

u 0,li Τ

(15) E

1+ 1 +

1 +

v

/ ? l u

uj,10

Ε

/6v

f

f

. u 0,li 1=0

u l

Λ

T

u u

A

10, uj where a l l terms have been defined above (note that u i s the r a d i a t i o n a l l y e x c i t e d upper s t a t e and u' i s the other upper s t a t e ; f o r 1+3 e x c i t a t i o n and 2+1 luminescence, u = 3 and u = 2. F i n a l l y , f o r the case of delayed fluorescence (1+3 e x c i t a t i o n , 3—>2 intersystem c r o s s i n g , 2—K3 intersystem c r o s s i n g , 3+1 emission) ?

DF

A

h V

n

.^ 30,li 30,li T 0

1+

+

^32 i=' Σ f 20,li 21 i=0 A

+ k

v - '3j,10 13

/δν + k

23jJ

Α

"l0,3j


• (eqn. cont.)

4.

wiNEFORDNER


57

(Eqn. continued from previous page) K

i=°° 4-k Σ A, i=0 3 0 , l i

K

23 32

+k 3132

(16)

.f

A 0

20,li

The s a t u r a t i o n s p e c t r a l i r r a d i a n c e Ε molecule i s given by

rE

+ k

21

+ k

23

for a 2-level 10,2j v

1 2

δν. A (17)

10,2 j

g

l

+

g

6v

A

•2j,10 J

2

and f o r a three l e v e l molecul Ε V

18J

13 J

3,i 10

j

(18)

?

'10,3 j

32

1 + — + go i=°° .f 20,li A

+ k

0

and f o r a three l e v e l molecule with 1+2 Ε 8n 18 J 3

10,2j

v

21

+ k

23

excitation

δν · A Α

l 2

2,1,10

}

(19) x

1 + — + go i=°° E

A

23

. 30,li i=0 n

+ k

31

+ k

32

The major conclusions r e s u l t i n g from the molecular lumines cence radiance expressions are l i s t e d below. (1) The radiance expressions f o r molecular luminescence are s i m i l a r to those f o r atomic f l u o r e s c e n c e and reduce to the atomic case i f ξ = 1. (2) For low source ( s p e c t r a l ) i r r a d i a n c e s , the f l u o r e s c e n c e radiance depends d i r e c t l y upon the source i r r a d i a n c e , the f l u o r e s c e n c e quantum e f f i c i e n c y , the emission t r a n s i t i o n p r o b a b i l i t y , and the t o t a l d e n s i t y of a n a l y t e . (3) For high source ( s p e c t r a l ) i r r a d i a n c e s , the fluorescence radiance depends d i r e c t l y upon the emission t r a n s i t i o n p r o b a b i l i t y , and the t o t a l analyte d e n s i t y but i s inde pendent of the source i r r a d i a n c e and the f l u o r e s c e n c e


58

NEW APPLICATIONS OF LASERS TO CHEMISTRY quantum e f f i c i e n c y . (4) For the 2 - l e v e l case under s a t u r a t i o n c o n d i t i o n s , abso l u t e n-p values can be measured by measuring under steady s t a t e c o n d i t i o n s and by knowing A g-, g , and £. (5) The s a t u r a t i o n i r r a d i a n c e E f o r a 3 - l e v e l molecule at room temperature i s 1CH to ΙΟ* x l e s s than f o r a 2 - l e v e l atom or molecule at any temperature or f o r a 3 - l e v e l atom or molecule at h i g h temperature, as i n flames. Because molecules have broad bandwidths compared to atoms, s a t u r a t i o n can be achieved e i t h e r by a high source s p e c t r a l i r r a d i a n c e over a narrow s p e c t r a l r e g i o n or a lower source s p e c t r a l i r r a d i a n c e over a much wider bandwidth, 1 >1 1 >1 0.03 1 0.4 0.3 0.6 >1 >1 3.9 5.7 >3.7 >4.5 4.7 >4.9 5 5.3 5.2 4.9 4.9 >4.3 >4.5 500

1000 200

30 10 100

50 6

0.05 0.1

0.02 0.2

0.2

1.7 3.7

3.3 3.3

3.6

32

28 28

36 28 28

28 37

Êxperimental d i f f i c u l t y was encountered i n o b t a i n i n g good l a s e r output at the Co l i n e s attempted.

°L0D = L i m i t of D e t e c t i o n , UL = Upper L i m i t of l i n e a r i t y , and LDR = l i n e a r dynamic range=UL/LOD.

^RF = resonance fluorescence; E-RF = e x c i t e d resonance f l u o r e s c e n c e ; S-DLF = Stokes d i r e c t l i n e fluorescence; AS-DLF = anti-Stokes d i r e c t l i n e f l u o r e s c e n c e ; TA-SLF = thermally a s s i s t e d stepwise l i n e f l u o r e s c e n c e ; E-AS-DLF = e x c i t e d anti-Stokes d i r e c t l i n e f l u o r e s c e n c e ; and E-S-SLF = e x c i t e d Stokes stepwise l i n e f l u o r e s c e n c e .

E x / F l = e x c i t a t i o n wavelength/fluorescence wavelength ( i f d i f f e r e n t than e x c i t a t i o n wavel e n g t h ) . The energy l e v e l s i n v o l v e d i n the t r a n s i t i o n and t h e i r gf and gA values can be found i n C. H. C o r l i s s and W. R. Bozman, "Experimental T r a n s i t i o n P r o b a b i l i t i e s f o r S p e c t r a l L i n e of Seventy Elements," NBS Monograph 53, 1962.

V

Tl

Sr Ti

Pb

Ni


Ag Al Ba Bi Ca Cd CoS Cr Cu Fe Ga In Li Mg Mn Mo Na Ni Pb Sr Ti Tl V

Element

0.43 123 ]_- See a l s o the chapter by B. R. Ware i n t h i s book). These measurements are the b a s i s o f techniques which are now e s t a b l i s h e d and have become known e i t h e doppler anemometry (6,8) Flygare (9) i l l u s t r a t e how u s e f u l such methods can be to chemistry. In other microscopic experiments, the f l u c t u a t i o n s of one component i n a r e a c t i n g mixture have been measured f l u o r i m e t r i c a l l y A u t o c o r r e l a t i o n of the measured fluorescence f l u c t u a t i o n s then enables the r a t e s of formation and l o s s o f that component to be measured, even though the r e a c t i o n mixture was at macroscopic equil i b r i u m . T h i s l a t t e r measurement i s the b a s i s of the new and e x c i t i n g f i e l d of fluorescence c o r r e l a t i o n spectroscopy (10,11,12). In many cases, i t i s more convenient to measure the power spectrum of measured f l u c t u a t i o n s than to determine the autocorrel a t i o n or c r o s s - c o r r e l a t i o n functions themselves (13,14). Whichever approach i s employed, the r e s u l t s are e s s e n t i a l l y the same, s i n c e the power spectrum and a u t o c o r r e l a t i o n f u n c t i o n of a waveform c o n s t i t u t e a F o u r i e r p a i r . That i s , one f u n c t i o n can be r e a d i l y obtained from the other merely by a p p l i c a t i o n of a F o u r i e r transformation. S i m i l a r l y , the c r o s s - c o r r e l a t i o n f u n c t i o n i s merely the F o u r i e r transform o f the cross-power spectrum of two waveforms. The novel approach to fluorescence l i f e t i m e measurement which i s o u t l i n e d below i s s i m i l a r i n concept to the procedures c i t e d above. Because i t , l i k e the others, i n v o l v e s the a n a l y s i s of f l u c t u a t i o n s induced i n a species of chemical i n t e r e s t , and because the f l u c t u a t i o n s are measured s p e c t r o s c o p i c a l l y , we have coined the acronym FAST ( F l u c t u a t i o n A n a l y s i s Spectroscopic Techniques) to c a t e g o r i z e them. F.A.S.T. Luminescence L i f e t i m e Measurement As implied e a r l i e r , luminescence l i f e t i m e s are o r d i n a r i l y determined by methods which, i n essence, are impulse-response f u n c t i o n determinations. A c c o r d i n g l y , i t would seem to be s t r a i g h t f o r w a r d to implement such techniques with a c o r r e l a t i o n approach. An instrument f o r such measurements i s i l l u s t r a t e d i n


122

NEW APPLICATIONS OF LASERS TO CHEMISTRY

Figure 2. As i l l u s t r a t e d i n Figure 2, the elements of a c o r r e l a t i o n l i f e t i m e f l u o r i m e t e r would be a randomly modulated e x c i t a t i o n source, e x c i t a t i o n and luminescence monochromators, a f a s t detect o r , and e i t h e r a c o r r e l a t i o n computer or spectrum analyzer. The s p e c i f i c a t i o n s f o r each one of these u n i t s w i l l be governed by the necessary e x c i t a t i o n and luminescence wavelengths and by the time range of the luminescence l i f e t i m e . In the simplest kind of instrument, the e x c i t a t i o n source would be a free-running flashlamp, the detector could be a f a s t p h o t o m u l t i p l i e r tube, and c o r r e l a t i o n could be c a r r i e d out using any one of a number of commercial hardware c o r r e l a t o r s . Such an approach was t e s t e d i n our laboratory f o r slowly decaying luminescence s i g n a l s and has revealed the p r a c t i c a l i t y of the t e c h n i que. In these i n i t i a l s t u d i e s , i t was found that i t i s important f o r the e x c i t a t i o n source to f l a s h randomly or f o r the duration between f l a s h e s to be considerabl process being observed of luminescence decay s i g n a l s with each other makes a l i f e t i m e determination r a t h e r d i f f i c u l t . C l e a r l y , t h i s p r e l i m i n a r y kind of device cannot be employed f o r the measurement of short luminescence l i f e t i m e s . Hardware c o r r e l a t o r s simply are not capable of s u f f i c i e n t l y r a p i d response to enable c a l c u l a t i o n of c o r r e l a t i o n functions on a nanosecond time s c a l e . To achieve nanosecond time r e s o l u t i o n with t h i s approach, somewhat d i f f e r e n t instrumentation i s r e q u i r e d . Nanosecond time r e s o l u t i o n requires that the e x c i t a t i o n source f l u c t u a t e or be modulated on a sub-nanosecond time s c a l e and at a f a i r l y large amplitude. In a d d i t i o n , f o r maximum s i g n a l to-noise r a t i o , the f l u c t u a t i o n s should occur continuously and should not be separated i n time as would the pulses from a f l a s h lamp. In p r e l i m i n a r y i n v e s t i g a t i o n s , s e v e r a l p o t e n t i a l sources were t e s t e d f o r t h i s a p p l i c a t i o n and a continuous-wave l a s e r was found to be most s u i t a b l e . In a d d i t i o n , because nanosecond-scale c o r r e l a t i o n was required, we found i t more expedient to employ spectrum a n a l y s i s than software or hardware c o r r e l a t i o n . The r e s u l t i n g instrument and i t s performance have been described i n a recent p u b l i c a t i o n (13), and w i l l only be b r i e f l y and q u a l i t a t i v e l y o u t l i n e d here. In the new instrument, l a s e r mode noise i s used as a pseudorandom fluorescence e x c i t a t i o n f u n c t i o n . Mode n o i s e i s j u s t the r a p i d f l u c t u a t i o n i n l a s e r output amplitude which r e s u l t s from "beating" (mixing) of the l a s e r o s c i l l a t i o n modes with each other. Because l a s e r modes occur at d i s c r e t e frequencies (wavelengths) (15), they produce v a r i a t i o n s i n the l a s e r ' s output which are a l s o at d i s c r e t e frequencies, as revealed by the comb-like f l u c t u a t i o n power spectrum of the l a s e r ' s output (16,17,18) . These d i s c r e t e frequency f l u c t u a t i o n s occur i n i n t e r v a l s of c/2£ (c = speed of l i g h t ; I = l a s e r c a v i t y length) up to frequencies as high as 4 GHz f o r an argon-ion l a s e r , [ i . e . , up to the Doppler width of the emission p r o f i l e of the a c t i v e medium (Ar i o n s ) ] . For a t y p i c a l


HiEFTjE

ET AL.

Measurement of Transient Chemical Events

System

o Inputs

-o

ο

ίο be

-o

o

Tested

-o

Outputs

Figure 1. Conceptual view of a system whose time response is electronic network, fluorescing molecule. corresponding response might then he, respectively, the RC time con stant, the chemical reaction rate, or the fluorescent lifetime.

Excitation Monochrom.

Random

•

Fluorescence Cell

Emission

Modulator

Monochromator cross-correlation < signal

PMTl High-Speed

Photodetector

Correlation Computer Spectrum Analyzer

Figure 2. Diagram of an instrument to measure luminescence lifetime using a randomly varying light source


124

NEW

APPLICATIONS O F LASERS T O

CHEMISTRY

rare-gas i o n l a s e r , with a 1 meter c a v i t y , c / 2 £ = 150 MHz. Essent i a l l y , the l a s e r output f l u c t u a t e s i n power at a l l these d i s c r e t e frequencies (150 MHz, 300 MHz, 450 MHz, etc.) simultaneously, thus causing f l u c t u a t i o n s i n e x c i t a t i o n o f the i l l u m i n a t e d sample at the same f r e q u e n c i e s . However, because the e x c i t e d s t a t e of a fluorophore e x h i b i t s a f i n i t e l i f e t i m e , f l u c t u a t i o n s i n the induced luminescence cannot occur at the highest frequencies present i n the v a r y i n g l a s e r output. Therefore, the power spectrum o f luminescence f l u c t u a t i o n s i s attenuated at h i g h e r f r e q u e n c i e s . From t h i s a t t e n u a t i o n , the luminescence l i f e t i m e can be c a l c u l a t e d . For example, luminescence from a fluorophore having an upper s t a t e l i f e t i m e o f 1 ns ( i . e . , a frequency response o f 1 GHz) would be able to f o l l o w the lower frequency l a s e r f l u c t u a t i o n s ( i . e . , at 150, 300, 450 MHz, etc.) but would not be able to f a i t h f u l l y follow the highest frequency f l u c t u a t i o n s h i g h e r frequencies woul luminescence v a r i a t i o n s . Mathematically, the luminescence l i f e t i m e can be found from the envelope o f the d i s c r e t e - f r e q u e n c y peaks i n the luminescence f l u c t u a t i o n power spectrum. A f t e r deconvolution, t h i s envelope i s L o r e n t z i a n , r e v e a l i n g the exponential time-domain p r o f i l e o f luminescence decay. Deconvolution i t s e l f i s s i m p l i f i e d i n t h i s approach, and merely i n v o l v e s a d i v i s i o n , s i n c e data are already i n the frequency (Fourier) domain Q2,3) . The advantages o f t h i s new approach are numerous. For one, the technique i s capable o f measuring luminescence l i f e t i m e s as short as those a c c e s s i b l e with a mode-locked l a s e r . In f a c t , the power spectrum measurement i n v o l v e d i n t h i s technique i m p l i c i t e l y c o r r e l a t e s the l a s e r f l u c t u a t i o n s and thereby emulates the l a s e r s performance when mode locked (19) . However, mode l o c k i n g i t s e l f i s not necessary, so that l a s e r o p e r a t i o n i s rendered both simpler and more r e l i a b l e . A l s o , t h i s method r e q u i r e s no large amplitude output p u l s e from the photodetector, thereby reducing the l i k e l i h o o d of s a t u r a t i o n . T h i s new method has drawbacks as w e l l . Most important o f these i s the need to perform a spectrum a n a l y s i s of the induced f l u o r e s c e n c e f l u c t u a t i o n s . This a n a l y s i s must be performed with the a i d o f a high frequency spectrum analyzer, whose cost i s subs t a n t i a l . From a p r a c t i c a l standpoint, i t would be f a r more a t t r a c t i v e to use less expensive instrumentation. One p o s s i b i l i t y would be to s t a t i o n fixed-frequency bandpass detectors (such as UHF t e l e v i s i o n tuners) at each o f the d i s c r e t e f l u c t u a t i o n f r e quencies. Of course, the exact l o c a t i o n of each o f these f r e quencies i s dependent on the p a r t i c u l a r l a s e r used, and the bandpass frequencies would have to be adjusted i f l a s e r sources were exchanged. Another l i m i t a t i o n i n the system as now c o n f i g u r e d i s the a c c e s s i b l e wavelength and time r e s o l u t i o n range. Although a r a r e gas i o n l a s e r emits only at a number of d i s c r e t e wavelengths, i t would be d e s i r a b l e to have a v a i l a b l e u l t r a v i o l e t 1


8.

HIEFTJE ET AL.

Measurement of Transient Chemical Events

125

r a d i a t i o n or r a d i a t i o n tunable over a broad wavelength range. In a d d i t i o n , mode noise from such a system extends only t o approxi mately 4 GHz, l i m i t i n g time r e s o l u t i o n t o approximately 0.1 ns. Presumably, both these l a t t e r objections could be overcome through use o f a continuous-wave dye l a s e r . With frequency doubling, such a l a s e r would be usable over most o f the wavelength range commonly employed f o r luminescence e x c i t a t i o n . Moreover, the l a s e r should e x h i b i t mode n o i s e t o frequencies as high as 100 GHz, making de t e c t o r speed the l i m i t i n g device i n the measurement o f u l t r a - s h o r t lifetimes. Literature Cited 1. Ippen, E. P., and Shank, L. V., Opt. Commun. (1976) 18, 27. 2. Bendat, J . S., and P i e r s o l A G. "Rando Data Analysi and Measurement Procedures" NY, 1971. 3. Bracewell, R., "The F o u r i e r Transform and I t s A p p l i c a t i o n " , McGraw-Hill, New York, NY, 1965. 4. E r n s t , R. R., J. Mag. Resonance (1970) 3, 10. 5. Gabler, R., Westhead, E. W., and Ford, N. C., Biophys. J. (1974) 14, 528. 6. She, C. Y., and Wall, L. S., J. Opt. Soc. Amer. (1975) 65, 69. 7. Brown, J. C., and Pusey, P. N., J. Phys. D. (1974) 7, L31. 8. LeBlond, J., and El Badawy, E. S., Appl. Opt. (1975) 14, 902. 9. Ware, B. R., and Flygare, W. Η., Chem. Phys. L e t t . (1971) 12, 81. 10. Magde, D., E l s o n , E., Webb, W. W., Phys. Rev. L e t t . (1972) 29, 705. 11. E l s o n , E. L., Magde, D., Biopolymers (1974) 13, 1. 12. Magde, D., Elson, E. L., Webb, W. W., Biopolymers (1974) 13, 29. 13. H i e f t j e , G. M., Haugen, G. R., and Ramsey, J. Μ., Appl. Phys. L e t t . (1977) 30, 463 14. Chu, Β., "Laser Light S c a t t e r i n g " , Academic Press, New York, NY, 1974. 15. Lengyel, Β. Α., " I n t r o d u c t i o n t o Laser Physics", John Wiley and Sons, New York, NY, 1966. 16. Casperson, L. W., Opt. Commun. (1975) 13, 213. 17. Bridges, T. J., and Rigrod, W. W., IEEE J. Quantum E l e c t r o n . (1965) QE-1, 303. 18. Sedel'nikov, V. Α., S i n i c h k i n , Y. P., and Tuchkin, V. V., Opt. Spectrosc. (1971) 31, 408. 19. Weber, H. P., and Danielmeyer, H. G., Phys. Rev. A (1970) 2, 2074. RECEIVED August 7, 1978.


9 Laser Applications in Photoelectrochemistry 1

1

S. P. PERONE , J. H. RICHARDSON, B. S. SHEPARD , J. ROSENTHAL , J. E. HARRAR, and S. M. GEORGE 1

General Chemistry Division, Lawrence Livermore, Laboratory, Livermore, CA 94550

Photoemission S t u d i e s . The U V - v i s i b l e i r r a d i a t i o n of an e l e c t r o d e / s o l u t i o n i n t e r f a c e can stimulate any of s e v e r a l i n t e r e s t i n g e l e c t r o d e processes and the e l e c t r o d e i s a e l e c t r o n s may occur, with the formation of solvated e l e c t r o n s and initiation of r e a c t i o n s with a v a i l a b l e scavengers. I f the s o l u t i o n absorbs the r a d i a t i o n and p h o t o l y s i s occurs, e l e c t r o a c t i v e p h o t o l y t i c intermediates and products may be detected at an i n d i c a t o r e l e c t r o d e by t h e i r e l e c t r o l y s i s currents under pot e n t i o s t a t i c c o n d i t i o n s . I f the e l e c t r o d e i s a semiconductor and absorbs r a d i a t i o n s u f f i c i e n t to promote e l e c t r o n s through the band gap, o x i d a t i o n or r e d u c t i o n process may be induced which would not occur i n the absence of r a d i a t i o n . I f a dye absorbed on a semiconductor e l e c t r o d e i s e x c i t e d by the i r r a d i a t i o n , the e x c i t e d state may undergo an o x i d a t i v e or r e d u c t i v e e l e c t r o n t r a n s f e r step which would not occur with the ground state s p e c i e s . These are examples of some of the more i n t e r e s t i n g r a d i a tion-induced e l e c t r o d e processes which have been studied. Much of the i n t e r e s t has been stimulated r e c e n t l y by the prospects f o r d i r e c t s o l a r - t o - e l e c t r i c a l energy or s o l a r - t o - c h e m i c a l energy conversion which might be p o s s i b l e with photoelectrochemical ce l i s . ( 1 - 3 ) The work reported here was designed to demonstrate the u t i l i t y of l a s e r sources for these photoelectrochemical s t u d i e s ; we have focused our a t t e n t i o n on those phenomena u n i quely r e l a t e d to the c h a r a c t e r i s t i c s of l a s e r i r r a d i a t i o n . In p a r t i c u l a r , we report here the r e s u l t s of our studies of l a s e r - i n d u c e d photoemission processes and l a s e r - induced p h o t o l y s i s . The former study was undertaken to i l l u s t r a t e the e f f o r t s of wavelength, source power, and i n t e n s i t y on photoem i s s i o n - r e l a t e d process; whereas the l a t t e r study was designed to i l l u s t r a t e the c a p a b i l i t i e s f o r photoelectrochemical quantum y i e l d measurements on t r a n s i e n t p h o t o l y t i c species made p o s s i b l e with a l a s e r source. 1Current address: Department of Chemistry, Purdue University, West Lafayette, IN 47907 This chapter not subject to U.S. copyright. Published 1978 American Chemical Society


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Photoemission of e l e c t r o n s from mercury e l e c t r o d e s i n t o e l e c t r o l y t e s o l u t i o n s has been studied e x t e n s i v e l y i n recent years.(4-10) O c c a s i o n a l l y other metals have been used as a source o f photoemitted electrons,(11) but the dropping mercury e l e c t r o d e (DME) i s g e n e r a l l y recognized to have d e s i r a b l e chara c t e r i s t i c s . (12.) I n t e r e s t i n photo-related currents has not only been i n c h a r a c t e r i z i n g the emission process but a l s o i n studying the r e a c t i o n s o f the r e s u l t i n g hydrated e l e c t r o n s with various s u i t able scavengers.(13-16) T h e o r e t i c a l studies have a l s o been i n i t i a t e d both with respect to the k i n e t i c s o f scavenging i t s e l f (17_-18) and t r a n s i e n t e f f e c t s i n the e l e c t r o c h e m i c a l detect i o n o f the photorelated phenomena.(21) Many previous experimental studies have been c a r r i e d out with continuous or choppe steady state (DC)(22) o under p o t e n t i o s t a t i c c o n d i t i o n s . Most o f the more recent work has used pulsed xenon flashlamps. ( 5 .6 .10 .13 .14.24.25 ) Detect i o n o f photo-related phenomena was by p o t e n t i o s t a t i c ( 1 0 , 2 4 ) or coulostatic(13,14,17) monitoring of the e l e c t r o d e process. Very l i t t l e work has involved the use o f l a s e r s as the r a d i a t o n source. The few laser-induced photoelectrochemistry studies which have appeared mostly used s o l i d - s t a t e l a s e r s (ruby or neodymium) with one or two l i n e s and a slow r e p e t i t i o n rate (sometimes s i n g l e - s h o t only) . ( 13 ,26-28) A recent report(J29) used the n i t r o g e n l a s e r at 337.1 nm. The work reported here had the general o b j e c t i v e o f studying the e f f e c t s o f very intense l a s e r sources on e l e c t r o d e photoemission processes. Thus, the s p e c i f i c goals of t h i s work were t h r e e f o l d : 1) to develop appropriate i l l u m i n a t i o n and measurement instrumentation f o r both pulsed and cw l a s e r sources i n conjunction with a conventional DME assembly; 2) to determine the e f f e c t o f source c h a r a c t e r i s t i c s on e l e c t r o d e photoemission processes i n the presence o f s u i t a b l e hydrated e l e c t r o n scavengers; 3) to evaluate the general u t i l i t y o f l a s e r sources f o r photoelectrochemical s t u d i e s . In order to achieve the above goals we have used both a pulsed n i t r o g e n pumped dye l a s e r and a cw argon i o n l a s e r . The dye l a s e r i s continuously tunable from 258 to 750 nm, the output c o n s i s t i n g o f 10 nsec pulses at a r e p e t i t i o n rate o f 1 to 50 Hz and peak powers o f s e v e r a l k i l o w a t t s . The argon ion l a s e r was a cw source, u s u a l l y chopped around 1 kHz. I t was operated on one o f four f i x e d wavelengths, with output powers up to 6 W. A conventional DME polarographic assembly was i l l u m i n a t e d under c o n t r o l l e d p o t e n t i a l conditions with c a p a b i l i t i e s f o r both polarographic steady state and t r a n s i e n t (boxcar i n t e g r a tor and o s c i l l o s c o p e ) current measurement c a p a b i l i t i e s . Scavengers used i n these studies included N2O, NO3", s e v e r a l d i v a l e n t


NEW

128

2 +

2 +

2+

APPLICATIONS OF LASERS TO CHEMISTRY

2 +

2 +

2 +

2 +

cations (e.g., C o , F e , Mn , N i , C u , C d and P b ) , and Co(NH ) . The photo-related current was studied as a f u n c t i o n of wavelength, e l e c t r o d e p o t e n t i a l , and i n t e n s i t y f o r a l l the s c a vengers, using both sources and a l l three d e t e c t i o n c a p a b i l i ties. D e f i n i t e scavenging of e l e c t r o n s was not observed with the metal c a t i o n s , i n contrast to NO3" and N2O. A h i g h l y non l i n e a r photo-effect was observed with the high peak-powered, pulsed l a s e r system. This e f f e c t was p a r t i c u l a r l y n o t i c e a b l e with the metal c a t i o n s . To our knowledge such a pronounced e f f e c t has not been observed before. Our work i l l u s t r a t e s some of the l i m i t a t i o n s as w e l l as advantages of using l a s e r sources with e l e c t r o c h e m i c a l d e t e c t i o n of photoemission c u r r e n t s . 3 +

3

6

Experimental Laser P h o t o l y s i s - Quantum Y i e l d Studies. E l e c t r o a c t i v e species can be q u a l i t a t i v e l y and q u a n t i t a t i v e l y c h a r a c t e r i z e d by chronoamperometric measurements at a microelectrode i n the p h o t o l y s i s cell.(30-33) I f the f a r a d a i c current f o r p h o t o l y t i c species i s d i f f u s i o n - c o n t r o l l e d and uncomplicated by k i n e t i c e f f e c t s , the C o t t r e l l equation a p p l i e s :

i

= nFAD

1 / 2

C°/(ut)

1 / 2

(1)

where i is the f a r a d a i c current at time t, η i s the number od e l e c t r o n s t r a n s f e r r e d , F i s the Faraday, A i s the e f f e c t i v e e l e c t r o d e area, and D i s the d i f f u s i o n c o e f f i c i e n t of the e l e c t r o a c t i v e s p e c i e s . Thus, a p l o t of i versus 1/ t should be l i n e a r with a slope p r o p o r t i o n a l to C°, the bulk concentra t i o n of e l e c t r o a c t i v e species i n s o l u t i o n . A t h e o r e t i c a l expression has been derived(30) d e s c r i b i n g the i n i t i a l concentration of intermediate, R, produced when a pulse of l i g h t i s passed through a s o l u t i o n containing a photor e a c t i v e s p e c i e s , 0,

Cj(b) = 4Q [aC°]exp[-abC°]

(2)

o

where C R i s the concentration of intermediate as a f u n c t i o n of pathlength (b), the quantum e f f i c i e n c y (Φ), the instantaneous i n i t i a l quanta of monochromatic l i g h t per u n i t area ( Q ) , the absorption c o e f f i c i e n t of the photoreactive species (a), and the concentration of the reactant before the f l a s h (Cq) . This r e l a t i o n s h i p assumes i d e a l conditons where C Q does not change Q


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during the f l a s h , the l i g h t pulse i s c o l l i m a t e d and monochromat i c , and no other absorbing species are produced during the flash. Replacement i n t h i s work of the p r e v i o u s l y used xenon flashlamp with a pulsed dye l a s e r e x c i t a t i o n source extends the v a l i d i t y and, consequently, the usefulness o f t h i s expression. The pulsed l a s e r s u p p l i e s c o l l i m a t e d , monochromatic l i g h t p u l ses o f high i n t e n s i t y and very short d u r a t i o n , and, t h e r e f o r e , allows measurements to be made at s e l e c t e d wavelengths and on shorter time s c a l e s . The present study involved l a s e r f l a s h p h o t o l y s i s e x p e r i ments using the F e ( l I I ) oxalate system. Both the photochemist r y (34-J37.) and the photoelectrochemistry(38 ,39) o f thé F e ( I I I ) oxalate system have been studied. These studies have shown by s p e c t r o s c o p i c and e l e c t r o c h e m i c a be photoreduced by bot a number of c o n f l i c t i n g r e a c t i o n mechanisms have been proposed f o r t h i s system, i t i s known that the f i n a l product i s F e ( l l ) . Quantum y i e l d s f o r the production o f F e ( I I ) have been reported f o r wavelengths l e s s than 600 nm.(40-42) In a d d i t i o n , f l a s h photoelectrochemical experiments have shown that the production of F e ( l l ) can be monitored at a p o t e n t i a l where the F e ( l l ) i s o x i d i z e d to F e ( l I I ) . Furthermore, when these o x i d a t i o n currents are d i f f u s i o n - c o n t r o l l e d and f o l l o w C o t t r e l l behavior, q u a n t i t a t i v e determinations o f the F e ( l l ) produced by the f l a s h can be made. (43) In the studies reported here the concentration of F e ( I I ) produced from laser-induced p h o t o l y s i s was determined from C o t t r e l l p l o t s . The l a s e r p h o t o l y s i s source used was a Xerox flash-lamp-pumped dye l a s e r with ~ 0.5 psec pulse width and - 1 j o u l e output energy i n the v i s i b l e r e g i o n . Because the l a s e r pulse i s monochromatic and c o l l i m a t e d , the r e l a t i o n s h i p between the concentration and the pathlength p r e d i c t e d by Equat i o n 2 was observed. In a d d i t i o n , working at a known pathlength and measuring both the photon f l u x and the F e ( I I ) i n i t i a l l y produced, quantum y i e l d s were determined at 442 and 457 nm. Reagents. A l l i n o r g a n i c s a l t s used i n t h i s work were reagent grade and were used without f u r t h e r p u r i f i c a t i o n : NaOH, N i C l , M n C l , C o C l , F e C l , F e N H ^ S O ^ · 12H 0, K C 0 4 , and P b C l (Baker), C u C l , C d C l and KC1 ( M a l l i n c k r o d t ) , Ο ο ( Ν Η ) 0 1 (Kodak) and NaNC>3 (MCB). Nitrous oxide (N 0) was obtained from Matheson and used without f u r t h e r p u r i f i c a t i o n . A l l s o l u t i o n s were made up i n l a b o r a t o r y deionized water which had been f u r t h e r p u r i f i e d by a Corning demineralizer and water s t i l l . F e r r i c oxalate s o l u t i o n s were made by d i s o l v i n g the appro p r i a t e amount o f FeNH4(S04) ·12H 0 i n an aqueous s o l u t i o n o f K C 04'H 0. The pH of the s o l u t i o n was adjusted by the a d d i t i o n of d i l u t e H S04 or KOH. A l l data reported here are for 0.853 χ 10~ M f e r r o i x a l a t e s o l u t i o n s at pH 6.0 and 0.25 M oxalate. The 2

2

2

2

2

2

2

2

2

2

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2

2

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F e ( l I I ) c o n c e n t r a t i o n was determined s p e c t r o - p h o t o m e t r i c a l l y with thiocyanate reagent.(44) Instrumentation f o r Photoemission Studies. Two l a s e r sys tems were used: 1) a n i t r o g e n pumped dye l a s e r , 2) an argon i o n l a s e r . A Molectron UV1000 n i t r o g e n l a s e r (1 MW peak power, 10 nanosecond pulse with 1-50 Hz r e p e t i t i o n rate) was used to t r a n s v e r s e l y pump a Molectron dye l a s e r operated i n the DL200 c o n f i g u r a t i o n . Doubling the fundamentals was accomplished by using angle-tuned KDP c r y s t a l s f o r second harmonic generation, with a Corning 7-54 f i l t e r to block the fundamental. The l a s e r was u s u a l l y operated at 30 Hz. T y p i c a l peak powers of funda mentals d e l i v e r e d to the e l e c t r o c h e m i c a l c e l l were 1-4 KW, c o r responding to a maximum power d e n s i t y o f ca. 0.5 MW/cm^. (The actual peak power d e l i v e r e d out of the l a s e r i s more than an order o f magnitude greater than t h i s but there are c o n s i d e r able losses i n s t e e r i n the DME.) Peak power 5% that o f the fundamental. Average powers at the c e l l were measured with a Scientech 362 power meter, and the r e l a t i v e power monitored p e r i o d i c a l l y with a Molectron J3 p y r o e l e c t r i c joulemeter. A Spectra-Physics 170-09 argon i o n l a s e r was used as the cw source. Four d i s c r e t e wavelengths were used: 514.5, 488.0, 457.9 and 351/364 nm. The cw output was modulated, u s u a l l y at 1 kHz, with a 50% duty cycle using an Ithaco 383A v a r i a b l e speed chopper. For the v i s i b l e l i n e s the average power d e l i vered to the c e l l was approximately 25% of the cw output power. For the uv l i n e s t h i s f i g u r e dropped to 8%. The l a s e r output was d i r e c t e d i n t o a sample chamber con t a i n i n g the DME. Figure 1 i s a schematic of the experimental apparatus. Figure 2 i s a p i c t u r e o f the e l e c t r o c h e m i c a l c e l l mounted i n the sample chamber. The c e l l i t s e l f i s transparent, the lower p o r t i o n c o n s i s t i n g o f a 1 χ 2 χ 4 cm S u p r a s i l curvette ( f l u r o r e s c e n c e type, P r e c i s i o n C e l l s ) and the upper p a r t Pyrex. The t o t a l volume i s approximately 40 ml. The c e l l s i t s i n a n e a r l y l i g h t t i g h t sample chamber s u i t a b l e f o r s p e c t r o s c o p i c s t u d i e s . A more d e t a i l e d d e s c r i p t i o n o f the s p e c t r o s c o p i c c h a r a c t e r i s t i c s o f the sample chamber has been published pre v i o u s l y . (45) The e l e c t r o c h e m i c a l c e l l c o n s i s t s of a three e l e c t r o d e system: a dropping mercury working e l e c t r o d e , a platinum wire counter e l e c t r o d e which can be p o s i t i o n e d very close to the DME, and a saturated calomel reference e l e c t r o d e (SCE) which measures the p o t e n t i a l o f the s o l u t i o n near the DME v i a a T e f l o n tube. The c e l l i s sealed with a T e f l o n cover. P r o v i s i o n i s made f o r bubbling v a r i o u s gases through the s o l u t i o n , flowing gases over the top o f the s o l u t i o n (to exclude oxygen) and emptying, r i n sing and f i l l i n g the c e l l without d i s t u r b i n g the o p t i c a l a l i g n ment. The e n t i r e e l e c t r o c h e m i c a l c e l l can be f i n e l y p o s i t i o n e d


PERONE ET AL.


XY recorder

Signal

Polarograph

Ar laser power supply +

Plasma

system DME Sample chamber Beam dump

Dye laser

2 laser

1P28

, N

Baffles and iris Lens Filter

Beam splitter

Trigger and reference High voltage supply

(Oscilloscope

Monochromator

8850 Boxcar integrator

t

Signal

Output

XY recorder

High voltage supply

Uigure 1. Schematic of the experimental apparatus



Figure 2. Electrochemical cell and polarograph interface mounted on sample chamber


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i n the three xyz d i r e c t i o n s , thus enabling the mercury drop to i n t e r s e c t the focused l a s e r beam i n l i n e with the r i g h t angle spectroscopic viewing assembling. A Princeton Applied Research (PAR) model 174A polarographic analyzer was used to c o n t r o l and scan the DME p o t e n t i a l , c o n t r o l the drop timer and monitor the c u r r e n t . A T e k t r o n i x 7904 o s c i l l o s c o p e with 7A15A and 7A19 p l u g - i n a m p l i f i e r s was used to observe the s i g n a l . The o s c i l l o s c o p e was p r i m a r i l y used f o r alignment, s i n g l e waveform monitoring and other d i a g n o s t i c purposes. The t r i g g e r was provided by the dye l a s e r pulse or modulated argon i o n l a s e r output. A PAR 162/163/164 boxcar i n t e g r a t o r was used f o r data ac q u i s i t i o n and averaging. The AC coupling i n the 164 gated i n t r e g r a t o r was modified to eliminate the l a r g e , slowly v a r y i n g DC component which was du variatio f th dro s i z e . The boxcar was use delaying the window ( t y p i c a l l y ps) y appropriat to c o i n c i d e with the s i g n a l maximum. Output from both the box car and the polarograph was displayed on x-y r e c o r d e r s . Procedures f o r Photoemission Studies. The c e l l was r i n s e d s e v e r a l times before the f i n a l s a l t s o l u t i o n was added. The s o l u t i o n was deoxygenated f o r a l e a s t 10 minutes, u s u a l l y with scrubbed (chromous c h l o r i d e and zinc amalgam) and water s a t u r ated argon. For the studies with N 0 the gas was allowed to bubble through f o r several minutes u n t i l a s u f f i c i e n t photoemis s i o n s i g n a l could be obtained. No attempt was made to determine the N 0 concentration. The m a j o r i t y of the other s o l u t i o n s were 3 mM i n the scavenging i o n . The supporting e l e c t r o l y t e s o l u t i o n was u s u a l l y 0.1 M KC1. To reduce polarographic maxima a d i l u t e s o l u t i o n of T r i t o n X-100 was added dropwise to the e l e c t r o c h e m i c a l c e l l u n t i l no f u r t h e r apparent r e d u c t i o n i n the maxima was noted. The polarograph was operated without any e l e c t r o n i c f i l t e r ing ( i . e . , no a d d i t i o n a l time c o n s t a n t s ) . The hangtime of the mercury drop was u s u a l l y 5 seconds; t y p i c a l s e n s i t i v i t i e s of the polarograph were 0.5 to 75 μA f u l l s c a l e . The polarograph drove the x-axis of both of the recorders, thus p e r m i t t i n g s i multaneous recording of both the DC polarograph and photoemis sion current vs p o t e n t i a l . The l a s e r beam was a l i g n e d with the DME both v i s u a l l y and i n s t r u m e n t a l l y ( i . e . , by monitoring the photo-related s i g n a l on the o s c i l l o s c o p e ) . P r e l i m i n a r y spectro s c o p i c scans were made by f i x i n g the p o t e n t i a l and scanning the monochromator, viewing the luminescence at r i g h t angles to the l a s e r beam. The luminescence was detected by an RCA 8850 photo m u l t i p l i e r tube, processed by the boxcar and recorder. Occa s i o n a l l y the emission wavelength was f i x e d and the p o t e n t i a l s canned. The n i t r o g e n l a s e r i s w e l l s h i e l d e d e l e c t r i c a l l y . In the present experiments the t r a n s i e n t current s i g n a l was monitored 2

2


134


many microseconds a f t e r the l a s e r pulse, and no e f f e c t a t t r i b u t able to the l a s e r discharge was detected. No e l e c t r i c a l i n t e r ference was n o t i c e a b l e from the cw argon ion l a s e r . Background s i g n a l s from blank s o l u t i o n s ( i . e . , e l e c t r o l y t e s o l u t i o n ) were measured f r e q u e n t l y as various conditons were changed (e.g., l a s e r i n t e n s i t y , wavelength, s e n s i t i v i t y , potential). Thus i t was s t r a i g h t f o r w a r d to compare any observed photo-related phenomena i n the presence and absence of scavenger. Instrumentation f o r Laser P h o t o l y s i s Studies. A Phase-R model 2100B fiashlamp-pumped tunable dye l a s e r (Phase-R Co., New Durham, N.H.) was used as the p h o t o l y s i s l i g h t source. When pumped with a model DL-18 c o a x i a l flashlamp, with a t r i a x adapter d i m i n i s h i n g the beam diameter to 12 mm, output pulses with energies as high as 1 - 5 joules and widths as narrow as 0.5 usee could be generate A commercial p y r o e l e c t r i Corp., Sunnyvale, CA) was used f o r l i g h t i n t e n s i t y measurements. The p h o t o l y s i s c e l l was constructed from 1/4-inch t h i c k p o l y a c r y l i c sheet cut to s i z e and bonded together with c h l o r o form. The c e l l held a t o t a l s o l u t i o n volume of approximately 7 ml. A quartz window was located i n the bottom of the c e l l through which the p h o t o l y s i s source was d i r e c t e d . The reference and counter e l e c t r o d e s were mounted permanently i n the c e l l w a l l to e l i m i n a t e any problems a s s o c i a t e d with reproducing t h e i r pos i t i o n s . Tubing connected the c e l l with a separate s o l u t i o n r e s e r v o i r , an a s p i r a t o r f o r s o l u t i o n removal, and a scrubbed nitrogen l i n e . The t h r e e - e l e c t r o d e monitoring system c o n s i s t e d of a hanging mercury drop working e l e c t r o d e (HMDE), a Pt counter e l e c trode, and a saturated calomel reference e l e c t r o d e (SCE). The mercury drop was suspended from a micrometer dispensing assembly (Metrohm E410 Hanging Mercury Drop E l e c t r o d e , Brinkman I n s t r u ments, Inc., Westbury, NY) f o r accurate c o n t r o l of drop s i z e . The e n t i r e HMDE assembly was p o s i t i o n e d above the c e l l v e r t i c a l l y and h o r i z o n t a l l y with a p r e c i s i o n of + 0.1 mm using a V e r t i c a l - T r a n s v e r s e Motion Mount ( E a l i n g Corp., Cambridge, MA). The counter e l e c t r o d e was constructed from copper metal covered with a t h i n platinum sheet and sealed i n the back w a l l of the cell. The reference e l e c t r o d e was made of a three-compartment c e l l with glass f r i t s s e p a r a t i n g the compartments. The f i r s t compartment contained the SCE, the second contained a 1 M KC1 s o l u t i o n , and the t h i r d contained a mixture of s o l v e n t , e l e c t r o l y t e , and b u f f e r . The t h i r d compartment was connected to a Luggin c a p i l l a r y mounted on the back w a l l of the c e l l with a Teflon f i t t i n g . The p o t e n t i o s t a t , described i n Ref. 46, had a c o n t r o l u n i t y - g a i n bandwidth o f 900 kHz and a monitoring bandwidth of 100 kHz. The computerized data a c q u i s i t i o n system has been described.21


9.

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135

Procedures f o r Laser P h o t o l y s i s Studies. The temperatures of the l a s e r dye and the water surrounding the t r i a x adapter were i n i t i a l l y e q u i l i b r a t e d to + 0.1°C. The absolute temperature v a r i e d between 18 and 20°C. These temperatures were continuously monitored throughout each s e r i e s o f experiments and adjusted i f necessary. The l a s e r was f i r e d at 17 kV, which corresponded to a charging energy o f 289 j o u l e s . The l a s e r dyes used were Coumarin 440 i n methanol ( λ = 442 nm) and Coumarin 460 i n ethanol ( λ = 457 nm), (Phase-R Co., New Durham, NH). F e ( l I I ) oxalate s o l u t i o n s were deaerated f o r at l e a s t t h i r t y minutes before s t a r t i n g each experiment. Following p h o t o l y s i s , the s o l u t i o n was a s p i r a t e d from the c e l l and new s o l u t i o n was obtained from the r e s e r v o i r . For each experiment, the c e l l containing the s o l u t i o n was o p t i c a l l y shielded fro ter, and a blank was ru any background curren shutter was then removed, the Fe(ox)3~^ s o l u t i o n was photolyzed, and F e ( l l ) o x i d a t i o n currents as a f u n c t i o n of time were monitored p o t e n t i o s t a t i c a l l y at -0.05V vs SCE. The current mea sured i n the blank was then subtracted. This procedure was r e peated a minimum o f three times at each set of c o n d i t i o n s , and r e s u l t i n g current-time curves were averaged to give the net result. Each averaged current-time curve was corrected f o r f a r a daic-induced charging current before f u r t h e r a n a l y s i s . The data were c o r r e c t e d f i r s t by the " d e r i v a t i v e method", based on the f o l l o w i n g r e l a t i o n s h i p : 0

0

d i

T

where i p i s the Faradaic c u r r e n t , i ^ i s the t o t a l current, R i s the uncompensated c e l l r e s i s t a n c e , Cnj_, i s the c a p a c i tance o f the working e l e c t r o d e double l a y e r , and ( d i ^ / d t ) i s the time d e r i v a t i v e of the t o t a l current. The c e l l time con stant ( R C J ) L ) J b determined experimentally as o u t l i n e d p r e v i o u s l y . ( 3 1 ) I f data show C o t t r e l l behavior f o l l o w i n g t h i s c o r r e c t i o n , they can be assumed to be d i f f u s i o n - c o n t r o l l e d . The d e r i v a t i v e method tends to introduce a large amount o f noise i n t o the c a l c u l a t e d Faradaic c u r r e n t . I f C o t t r e l l behav i o r i s observed, however, the raw data can be c o r r e c t e d by a tabulated t h e o r e t i c a l c o r r e c t i o n f a c t o r which does not introduce noise.(47) As with the d e r i v a t i v e c o r r e c t i o n method, knowledge of the c e l l time constant i s r e q u i r e d . I t has been pointed out(31) that the " e f f e c t i v e " RC ( R C f f ) a f t e r l i g h t i r r a d i a t i o n may d i f f e r s i g n i f i c a n t l y from the experimentally determined u

c

a

n

e

U

e


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RC because only part of the s p h e r i c a l e l e c t r o d e i s exposed to photolyzed s o l u t i o n (~ 50-75%). Thus, the e f f e c t i v e c e l l time constant may be less than that measured experimentally, where the t o t a l surface i s i n v o l v e d . F o r t u n a t e l y , when Faradaic currents are d i f f u s i o n - l i m i t e d , the value of R C f f can be e s timated from the raw data, as theory p r e d i c t s that ( ^ - - ) 0.85 RC.(31) Thus, the e f f e c t i v e RC can be determined from the p o t e n t i o s t a t i c current-time curve f o l l o w i n g f l a s h i r r a d i a t i o n by observing the time at which the current goes through a maxi mum. I t was t h i s procedure that was followed i n the work r e ported here. e

t

=

ma

Results of Photoemission

K

Studies

To evaluate the e f f e c t f lase characteristic e l e c t r o d e photoemissio eous s o l u t i o n s were used l y t e only (KC1 or NaOH). The second contained i n e r t e l e c t r o l y t e and a w e l l c h a r a c t e r i z e d scavenger (N 0 or NO3""). The t h i r d contained i n e r t e l e c t r o l y t e and one of s e v e r a l d i v a l e n t metal ions ( F e , N i , M n , C o , C u , P b , or C d . The f i r s t two types o f s o l u t i o n s provided f o r d i r e c t comparison with pre vious studies using more conventional i l l u m i n a t i o n sources. The t h i r d type o f s o l u t i o n provided e l e c t r o a c t i v e species for which d i s t i n c t s e n s i t i v i t y to source c h a r a c t e r i s t i c s was observed. In a l l o f the d i s c u s s i o n s below we w i l l use the general terms "photo-related" currents or "photocurrents" to describe any current s i g n a l s which are dependent on e l e c t r o d e i l l u m i n a t i o n , regardless o f whether photoemission o f e l e c t r o n s i s known to occur. The term "photoemission-related" currents w i l l be used whenever the s p e c i f i c phenomenon o f e l e c t r o n photoemission i s to be considered. The behavior o f N 0 as a scavenger i s w e l l known, and i t s photoelectrochemical c h a r a c t e r i s t i c s have been p r e v i o u s l y described.(4,9J3.14.16.27.29) I t r e a c t s very r a p i d l y with hydrated e l e c t r o n s to form molecular n i t r o g e n and hydroxyl r a d i c a l . The l a t t e r species i s r e d u c i b l e over the e n t i r e mer cury e l e c t r o d e p o t e n t i a l range. Thus, e l e c t r o d e photoemission i n the presence o f N 0 y i e l d s a net cathodic current at a l l p o t e n t i a l s negative to the photoemission t h r e s h o l d value f o r the p a r t i c u l a r wavelength of r a d i a t i o n . When Ν 0 ~ i s the scavenger the i n i t i a l product i s NO3*" , which reacts r a p i d l y with water (τχ/ < 15 με) to form N0 . The N0 i s e a s i l y reduced to n i t r i t e ion at p o t e n t i a l s negative of about -0.9 V vs SCE. Thus, cathodic photoemission-related currents are seen i n the presence o f ΝΟβ" with s u b s t a n t i a l en hancement at s u f f i c i e n t l y negative p o t e n t i a l s . The behavior o f c e r t a i n d i v a l e n t t r a n s i t i o n metal ions as hy drated e l e c t r o n scavengers has been reported previously.(22.24.25) 2

2 +

2 +

2+

2 +

2 +

2 +

2 +

2

2

2

3

2

2


2

9.

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137

The i n i t i a l product postulated i s the s h o r t - l i v e d univalent c a tion. In the absence o f an o x i d i z a b l e species the univalent c a t i o n i s r e o x i d i z e d to the 2 state at the e l e c t r o d e or by r e a c t i o n with the solvent. A photoelectrochemical study(£4) with N i ^ as the scavenger described the e f f e c t s o f t h i s type of mechanism and observed photocurrents. The f o l l o w i n g mechanism was suggested to e x p l a i n the current pulses that were observed with pulsed i r r a d i a t i o n o f a DME i n the polarographic plateau region:(24) +

+

e"\ . + Ni (aq)

Ni

+

+ 2

-> N i

+

(4)

+ H 0 -* 2

(6) H

Η

(ads)

ν + H 0 + e (ads) 2 o

-> H + OH 2

(7)

Thus, i t was suggested that the observed photocurrents were photoemission-related and that r e a c t i o n s (2) and (3) i n the d i f f u s i o n l a y e r would r e s u l t i n enhanced currents. Current enhancement occurs because not only are photoemitted e l e c t r o n s scavenged, but a l s o the ultimate product, H atoms, i s reducible f u r t h e r . Moreover, there i s no net d e p l e t i o n of N i ^ , as i t i s regenerated by r e a c t i o n o f N i with solvent. Although no studies with other d i v a l e n t metal ion scavengers have been r e ported, i t i s l i k e l y that the N i 2 e l e c t r o d e process provides a model system. The magnitudes of photoemission-related currents depend on several f a c t o r s . F i r s t l y , the quantum e f f i c i e n c y o f the photo emission event i t s e l f increases with negative p o t e n t i a l . Se condly, i f the scavenging r e a c t i o n r e s u l t s i n an e l e c t r o i n a c t i v e product, e l e c t r o n s are permanently removed from the e l e c t r o d e , and the net cathodic current w i l l i n c r e a s e , up to a p o i n t , with the scavenging rate constant. T h i r d l y , i f the scavenging reac t i o n r e s u l t s i n an e l e c t r o r e d u c i b l e s p e c i e s , the net cathodic current w i l l be enhanced f u r t h e r . The fundamental aspects of e l e c t r o d e photoemission and subsequent scavenging processes have been discussed e l s e where ( ^ ^ J J ^ ^ J ^ ) and w i l l not be repeated here. However, i t should be emphasized that the i n i t i a l e l e c t r o n emission event and subsequent scavenging of that e l e c t r o n must be complete i n less than about 1 a f t e r l i g h t absorption by the e l e c t r o d e . A l s o , these events do not u s u a l l y extend beyond 50-100 Â from the electrode surface. +

+

+



138

The studies reported here were conducted with the two l a s e r sources described i n the Experimental s e c t i o n . Various uv and v i s i b l e wavelengths were used, and the beam c h a r a c t e r i s t i c s were monitored and documented f o r each of the studies reported below. In each case, photocurrents were monitored i n three d i f f e r e n t ways: 1) conventional polarographic output; 2) an o s c i l l i s c o p e d i s p l a y of t r a n s i e n t or modulated s i g n a l s ; and 3) boxcar averaging of t r a n s i e n t or modulated currents synchronized with e i t h e r l a s e r source. N0 2

and NO3"

Solutions

Results with Chopped CW Laser Source. Figure 3 i s a t y p i cal o s c i l l o s c o p e d i s p l a y of both the 1 kHz modulated l a s e r r a d i a t i o n and the AC coupled synchronou photoemissio t observed at -1.6 V vs In the absense of r a d i a t i o Furthermore, while there was some small amount of photorelated current i n the absence of scavenger, there was a tremendous enhancement of current a t t r i b u t a b l e to photoemission a f t e r a d d i t i o n of N 0 or NO3". Figure 4 i s a DC polarogram i l l u s t r a t i n g the large change i n DC current upon i r r a d i a t i o n . F i g u r e 5 i l l u s t r a t e s the AC coupled synchronously detected boxcar averaged s i g n a l . A small photorelated current (probably thermal i n o r i g i n , v i d e i n f r a ) i s seen even i n the blank i n the v i c i n i t y of -0.2 V; however, the photorelated current a t t r i b u t a b l e to photoemission and scavenging e a s i l y dominates Figure 5B. Figure 5C i l l u s t r a t e s the complete photoemission s i g n a l as a f u n c t i o n of DME p o t e n t i a l . The decrease i n photoemission current at more negative p o t e n t i a l s was c o n s i s t e n t l y observed, c o i n c i d i n g with the p o t e n t i a l at which solvent r e d u c t i o n commenced. We i n v e s t i g a t e d the dependence of photorelated current on both wavelength and l i g h t i n t e n s i t y using both N 0 and NO3" scavengers. Figure 6 i l l u s t r a t e s the f u n c t i o n a l dependence of the photoemission current (ipg) ° DME p o t e n t i a l : the theorect i c a l l y expected(7.8.16.23) l i n e a r i t y of ( i p g ) ' ^ with respect to p o t e n t i a l i s observed. The uv output i s not s t r i c t l y monochomatic (351/364 nm or 3.53/3.41 ev), which p o s s i b l y accounts for the d i f f e r e n t slope observed. A l s o , N03~ (the i n i t i a l product of N03~ scavenging) can be r e o x i d i z e d p o s i t i v e of -1.1 V (vs SCE),(.2£} which would tend to lower the apparent photoemission c u r r e n t . A l l of the data represented by Figure 6 were obtained from boxcar signal-averaged p l o t s such as Figure 5C. D i f f e r e n t absolute v e r t i c a l scales apply to each wavelength, but the r e l a t i v e dependence on wavelength and p o t e n t i a l is apparent . F i g u r e 7 i l l u s t r a t e s the dependence of the photorelated Current on l a s e r i n t e n s i t y . Once again, r e l a t i v e currents were obtained from the boxcar signal-averaged output as i n Figure 5C. In Figure 7 the photoemission current was monitored at a f i x e d 2

2

n

0

2

2


9. PÉRONÉ ET AL.


139

B)

A

(C)

Figure 3. Oscilloscope display of synchronous DC-coupled photoemission current with chopped cw-Kr hser (407 nm, 150 mW). (A) Reference signal, light on (5 V/division); (B) reference signal, light off; (C) polarographic signal, light off. Solution was 3 mM NaN0 in 0.1M KCl, Ε = -1.70 V (vs. SCE). +

3


NEW

ι

(A)

APPLICATIONS OF

LASERS TO

CHEMISTRY

Γ

, , ι 11 11 I I 11 "

{ 1

u

ρττττ Ί

Γ (Β)

Λ λ

A À

trrrrrrr_L 0

-0.2

-0.4

J

-0.6

I

-0.8

I

-1.0

I

-1.2

L

-1.4

V (vs. SCE)

Figure 4. DC polarogram for chopped cw-laser irradiation with and without NO ~ scavenger, λ — 457.9 nm, laser output power = 0.5 W. (A) 0.1M KCl, 2.5 μΑ/division; (B) 3 mM NaN0 in 0.1M KCl, 2.5 μΑ/division. s

3


PERONE ET AL.

0.0


-0.2

-0.4

-0.6

-0.8

-1.0

-1.2

-1.4

V (vs.SCE)

0.0

-0.2

-0.4

-0.6

-0.8

-1.0

-1.2

-1.4

V (vs. S C E )

V (vs.SCE)

Figure 5. Boxcar-averaged photo-related current for chopped cw-laser irradiation with and without N0 ~ scavenger, ACcoupled, synchronously-detected, λ = 457.9 nm, laser output power = 0.5 W. (A) 0.1M KCl, amplified 7.7X; (Β) 3 mM NaN0 in 0.1M KCl, amplified 1.95X; (C) S mM NaN0 in 0.1 M KCl, .77χ. 3

3

3


NEW

APPLICATIONS OF

LASERS TO

•

CHEMISTRY

D

•

° •

»

"I

Δ Δ

A Ο •

°

03

α •

ι

03

«

cc

3

Δ

Ο

°

Δ

°

Δ

°

ο

Δ

•

• Δ

ο ο

Δ •

Ο Δ

•

ο

Δ • •

J -0.4

I

-0.6

-0.8

Δ

I -1.0

ο

Δ Ο

I -1.2 V (vs.

L -1.4

-1.6

-1.8

-2.0

SCE)

0A

Figure 6. (i ) vs. DME potential for chopped cw-hser source, supporting electrolyte was 0.1M KCl in all cases. (Φ) N 0 351/ 364 nm (3.53/3.41 eV); ({J) 3 nM NaNO , 457.9 nm (2.71 eV); (A)3mM NaN0 , 488.0 nm (2.54 eV); (O) 3 mM NaNO , 514.5 nm (2.41 eV). PE

2

s

3

s



PERONE ET AL.

ΙΟ"

10°

1

Ί0

1

Watts

Figure 7. Dependence of photo-related current on cw-hser intensity, 3 mM NaN0 in 0.1M KCl, λ = 514.5 nm, rehtive photoemission current vs. hser output. (Ο) Ε = 1.75 V; (Φ) Ε = -0.3 V. 3


144

NEW

APPLICATIONS OF

LASERS TO

CHEMISTRY

p o t e n t i a l while the l a s e r power was increased. Beyond 4 W (or ca. 2 W peak power a c t u a l l y focused onto the mercury drop) satu r a t i o n occurs, corresponding to a maximum quantum e f f i c i e n c y f o r photoemision of ca. 0.1% before s a t u r a t i o n . Figure 7 also i l l u s t r a t e s the dependence on r e l a t i v e i n t e n s i t y at two d i f f e r e n t potentials. In t h i s case n e u t r a l d e n s i t y f i l t e r s ( c a l i b r a t e d for high energy pulsed l a s e r s ) were used to decrease the r e l a t i v e i n t e n s i t y at a p o t e n t i a l where photoemission occurs (-1.75 V) and a l s o at a p o t e n t i a l where the photo-related current may be due to thermal heating and p e r t u r b a t i o n of the double l a y e r (-0.3 V). In each case a l i n e a r dependence on l a s e r i n t e n s i t y was observed (as i n d i c a t e d by u n i t y slope on a l o g - l o g p l o t ) . Results with Pulsed Dye Laser Source. Figure 8 i l l u s t r a t e s the temporal response of the photoemission current observed with the pulsed l a s e r r e f l e c t s the c e l l tim width (10 ns) and scavenging time constant ( 1 με) are c o n s i d erably l e s s than the s e v e r a l hundred microseconds observed f o r the current s i g n a l . Data taken with the high peak power pulsed l a s e r were much more ambiguous than those obtained with the cw or modulated a r gon ion l a s e r . For example there was a much more s i g n i f i c a n t t r a n s i e n t photorelated current detectable at more negative po t e n t i a l s , even i n the absence of scavenger. This phenomenon i s i l l u s t r a t e d i n Figure 9A. With the a d d i t i o n of scavenger and attenuation of the l a s e r power (accomplished by adding n e u t r a l density f i l t e r s ) * , a n o t i c e a b l e increase i n t r a n s i e n t current at p o t e n t i a l s near ~ -1.6 V and the appearance of t r a n s i e n t photo-related currents at l e s s negative p o t e n t i a l s were observed (Figure 9B). However, unattenuated l a s e r r a d i a t i o n led to a tremendous enhancement i n photo-related current, extending f a r p o s i t i v e of the photoemission threshold (Figure 9C). No n o t i c e able threshold could be observed i n a DC polarogram. However, t h i s i s not s u r p r i s i n g because of the low average power of the pulsed l a s e r (ca. 0.6 mW). The phenomenon represented i n F i g u r e 9C was accompanied by v i s u a l l y observable d i s r u p t i o n (streaming) i n the v i c i n i t y of the mercury drop. This streaming would con tinue b r i e f l y a f t e r the l i g h t was blocked before once again be coming quiescent. This phenomenon could only be observed with l i g h t at 520 nm, the most intense and t i g h t l y focused wavelength p o s s i b l e . A s l i g h t attenuation i n power, e i t h e r by neutal den s i t y f i l t e r s or tuning, eliminated t h i s unusual streaming pheno menon, i n d i c a t i n g a thermal or non-linear o r i g i n . Figure 10 i l l u s t r a t e s the dependence of t r a n s i e n t photo-re l a t e d currents on pulsed l a s e r i n t e n s i t y . The r e s i d u a l back ground current shows an e s s e n t i a l l y l i n e a r dependence on l a s e r i n t e n s i t y whereas the data f o r NaN0 does not. The t o t a l photoemission current at -1.80 V (vs SCE) shows a non-linear behavior, and represents a sum of at l e a s t two components: 1) the background c o n t r i b u t i o n which may r e f l e c t thermal e f f e c t s , 3


PERONE ET AL.


Time

Figure 8. Oscilloscope disphy of AC-coupled photoemission current synchronized to laser pulse, λ = 580 nm, Ε = —1.5 V, 5 mM ΝαΝ0 in 0.1M NaOH, 200 mV/division.


3

NEW

146

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i m p u r i t i e s , multiphoton events, and e " ( ) - e (aq) a n n i h i l a t i o n react i o n s ; and 2) the simple scavenging of hydrated e l e c t r o n s by n i t r a t e ions. The photo-related current at -1.20 V (vs SCE) has a nearly l i n e a r but large slope on a l o g - l o g p l o t , i n d i c a t ing a very high but simple non-linear dependence on l a s e r power. Figure 11 i l l u s t r a t e s the dependence of the t r a n s i e n t photoemission current on p o t e n t i a l using N2O scavenger and v a r i ous wavelengths. Once again, data were obtained from boxcar averaged traces such as Figure 9B and a d i f f e r e n t absolute vert i c a l s c a l e was used f o r each wavelength. Several i n t e r e s t i n g points are represented i n t h i s f i g u r e . F i r s t l y , the currents observed at fundamental ( v i s i b l e ) frequencies show a nearly l i n e a r r i s e continuously over a very wide p o t e n t i a l range, as expected f o r photoemission current. Currents of the two uv wavelengths do not, but sho distinctl saturated e f f e c t w i t h i n h a l f a v o l t fro s h i f t i n threshold i s c o n s i s t e n photo gy u n t i l reaching the uv l i n e s . The thresholds f o r 260 nm and 305 nm are not at p o s i t i v e p o t e n t i a l s (vs SCE) as they should be f o r photoemission. This e f f e c t i s a l s o n o t i c e a b l e but l e s s pronounced with the argon ion l a s e r data (Figure 6). Good agreement e x i s t s between the two sets of data ( f o r cw and p u l sed l a s e r sources) when comparing the data f o r v i s i b l e wavelengths; s l i g h t d i f f e r e n c e s may be a t t r i b u t e d to i n t e n s i t y d i f f e r e n c e s and the p r e v i o u s l y mentioned e f f e c t s using the high peak power l a s e r . In general, the enhancement of current over the background observed with the pulsed dye l a s e r was only a f a c t o r of 2 to 7, whereas the enhancement observed with the argon ion l a s e r was i n excess of a f a c t o r of 100. a q

T r a n s i t i o n metal c a t i o n s . The f o l l o w i n g t r a n s i t i o n metal ions were i n v e s t i g a t e d , u s u a l l y with both the chopped cw argon ion l a s e r and the pulsed dye l a s e r : Mn , F e , Co*, N i , C u , C u , P b , C d and Co(NH3)5^ . The half-wave p o t e n t i a l s (E1/2) f o r the uncomplexed ions i n 0.1 M KCl were -1.45 V, -1.30V, -1.25 V, -1.10 V, -0.20 V, -0.45 V, and -0.60 V, r e s p e c t i v e l y . The two reduct i o n p o t e n t i a l s for C o ( N H ) were -0.35 V, and -1.35 V. Results with Chopped CW Laser Source. The various metal c a t i o n s may be categorized according to whether or not t h e i r reduction wave occurs w e l l before, approximately c o i n c i d e n t with or w e l l a f t e r the expected photoemission t h r e s h o l d . For example, C u , P b and C d r e d u c t i o n p o t e n t i a l s a l l f a l l more p o s i t i v e than the photoemission threshold f o r 514 nm r a d i a t i o n . A l a r g e DC component of the photo-related s i g n a l i s observed. This e f f e c t appeared to be more s i g n i f i c a n t f o r t h i s group of metal cations which were more e a s i l y reduced, since s i m i l a r r e s u l t s were 2+

2 +

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2 +

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+

3 +

3

2 +

2 +

6

2 +



Figure 9A. Boxcar-signal-averaged, AC-coupled, synchronously detected photo-related currents using pulsed laser as radiation source. Supporting electrolyte was 0.1 M KCl, λ = 520 nm. No scavenger, no neutral densityfilter,amplified 7.7X.

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Figure 11. Dependence of transient photoemission-related cur rent on DME potential and wavelength. Supporting electrolyte was 0.1M KCl, scavenger was N 0. (Φ) 260 nm (4.77 eV); (A) 305 nm (4.06 eV); ({J) 386 nm (3.21 eV); (A) 406 nm (3.05 eV); (O ) 520 nm (2.38 eV); (U) 610 nm (2.03 eV). 2


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NEW APPLICATIONS OF LASERS TO CHEMISTRY 2 +

2 +

obtained for C u and P b . Because copper i s s e v e r a l orders of magnitude l e s s soluble i n mercury than i s cadmium,(48) i t appears that the fate o f the r e d u c t i o n product i s i r r e l e v a n t . I t i s not c l e a r why the photo-related current s i g n a l i n Figure 12 i s not completely modulated. This may be due to slow p h y s i c a l or chemical steps involved i n the photocurrent process a c t i n g as a dampening f a c t o r . No polarographic maximum was observed f o r C d , but a maximum was observed for C u . A d i l u t e s o l u t i o n o f T r i t o n X-100 was added u n t i l no f u r t h e r reduction i n the maximum was obtained. However, except for a small s h i f t i n the apparent threshold, the a d d i t i o n o f maximum suppressor had no e f f e c t on the photo-related current observed with chopped cw l a s e r r a d i a t i o n (Figure 13). The unique p o t e n t i a l dependence o f photo-re lated current i l l u s t r a t e d i n Figure 13 was g e n e r a l l y observed f o r a l l the d i v a l e n t c a t i o the r i s e i n photo-relate polarographic current at Έ>\/2' Figure 14 i l l u s t r a t e s a s i m i l a r phenomenon observed with C0CI2, which has a reduction p o t e n t i a l j u s t s l i g h t l y more ne gative than any expected photoemision. S i m i l a r r e s u l t s were obtained for FeCl2, which a l s o have r e d u c t i o n p o t e n t i a l s appro ximately coincident with the photoemission t h r e s h o l d . Finally, r e s u l t s s i m i l a r to C0CI2 were also obtained for M n , which has a reduction p o t e n t i a l much more negative than the photoemis sion threshold expected for 457.9 nm (ca. -0.9 V, see Figure 6 ) . Q u a l i t a t i v e l y a l l ' the photorelated"~currents observed with chopped cw l a s e r i r r a d i a t i o n for the various metal cations were s i m i l a r , commencing at the onset o f c a t i o n reduction and slowly tapering o f f as the DME p o t e n t i a l became more negative. It is important to note that the onset o f photo-related current i s i n no way c o r r e l a t e d with the photoemission threshold — sometimes commencing e a r l i e r , sometimes l a t e r . Any p o s s i b l e photoemission was obscured by t h i s other phenomenon. However, t h i s photorel a t e d current d i d not have an unusual dependence on l a s e r power i n t e n s i t y . The slope i s nearly 1.0, i n marked contrast to the r e s u l t s obtained with the pulsed l a s e r (Figures 10 and 15). Results with Pulsed Laser Source. Data taken with the pulsed dye l a s e r a l s o provided no d e f i n i t e evidence f o r a d d i t i o n a l photoemision r e l a t e d current beyond that observed with the blank. With the exception o f r a d i a t i o n at 520 nm, the only d i s t i n c t i o n between the blank and the s o l u t i o n c o n t a i n i n g the metal c a t i o n (when t r a n s i e n t photo-related currents were moni tored) corresponded to a d i s c o n t i n u i t y near the r e d u c t i o n poten t i a l wave, mostly seen when the reduction occured i n a region where photocurrents were observed. For example a d i s c o n t i n u i t y was observed with M n (E^/2 ~1·45 V) but not with a C u so l u t i o n (E1/2 0.20 V) when i r r a d i a t i n g with 406 nm. The only exception to t h i s was ΟοίΝΗβ)^" ", which had r e d u c t i o n waves at 2 +

2 +

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=

2 +

=

1


PERONE ET AL.


(B)

(C)

Figure 12. Oscilloscope display of DC-coupled, photo related current synchronized to square-wave-modulated argon-ion laser, λ = 514.5 nm, laser output power = 0.5 W, 3 mM CdCl in 0.1M KCl, Ε = -0.7 V (vs. SCE). (A) Light on; (B) light off; (C) ground; lower trace is 1 kHz chopped argon-ion laser output. Curves (A), (B), and (C) were at the same vertical sensitivity and dc offset. 2



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Figure 13. Boxcar-averaged, photo-related current for chopped cw laser. AC-coupled, synchronously detected, laser output power = 2 W, 3 mM CuCl in 0.1M KCl, λ = 514.5 nm, same vertical sensitivity. (A) Without Triton X-100; (B) with Triton X-100. 2


PERONE ET AL.


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Figure 15A. Rehtive photo-related current (% S/S ) vs. relative pulsed laser intensity (l/l ), λ = 520 nm, 0.1 M KCl was the sup porting electrolyte. 3 mM CoCl , (Φ) Ε = —1.75 V, (Ο) Ε = -1.41 V. 0

0

2


PERONE ET AL.


l/l

0

Figure 15B. Relative photo-related current (% S/S ) vs. relative pulsed laser intensity (1/I ), λ = 520 nm, 0.1M KCl was the sup porting electrolyte. 10 mM CoCl , (%) Ε = -1.8 V, (Ο) Ε = -1.35 V, (Π) Ε « - 0 . 9 V. 0

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-0.35 V and -1.35 V (vs SCE) but i n no way d i f f e r e d from the blank i n i t s t r a n s i e n t response to the l a s e r pulse. Of a l l the metal ions which had reduction waves i n the photocurrent region, Co(NH3)5 alone d i d not e x h i b i t a polarographic maximum. The e x c e p t i o n a l behavior observed with 520 nm r a d i a t i o n included non-linear photocurrent dependence on l a s e r i n t e n s i t y and streaming phenomena ( v i d a supra). F i g u r e 15 represents the dependence of the t r a n s i e n t photo-related current on pulsed l a s e r i n t e n s i t y at 520 nm f o r two d i f f e r e n t s o l u t i o n s . The r e s u l t s are s i m i l a r : a nearly l i n e a r dependence on i n t e n s i t y at more negative p o t e n t i a l s , i n c r e a s i n g to a much higher-order pro cess at less negative p o t e n t i a l s . In the case of C o (Figure 15B) t h i s higher order dependence i s observed even before the reduction wave. I n t e r e s t i n g l y enough t h i s e f f e c t was n e a r l y absent i n the ΟοίΝΗβ)^" " s o l u t i o n . This r e s u l t can probably be a t t r i b u t e d to absorptio c o e f f i c i e n t at 520 nm i t e n s i t y dependent. 3+

2 +

1

D i s c u s s i o n of Photoemission Studies This study i l l u s t r a t e s the s i g n i f i c a n t e f f e c t of compres sing a given amount of energy i n t o a narrow pulse. Even under the most favorable circumstances, b a r e l y d i s c e r n i b l e photoemis s i o n currents were obtained with the cw argon l a s e r when i t s output power was reduced to the average output power of the pulsed l a s e r (-0.6 mW). At higher average powers the photoemis sion currents seen with the cw argon l a s e r f a r exceeded those seen with the pulsed l a s e r . On the other hand, even at 1.5 W the unusual photorelated currents observed with the pulsed l a s e r were not observed with the cw argon l a s e r . The e f f e c t of p o l a r i z a t i o n on photoemission currents i s s t i l l somewhat ambiguous.(8,18,28) In these studies the argon ion l a s e r was v e r t i c a l l y p o l a r i z e d , but the pulsed l a s e r was e s s e n t i a l l y unpolarized. T u n a b i l i t y i n the pulsed dye l a s e r proved to be a substan t i a l asset. Our r e s u l t s are the f i r s t s e r i e s of current vs po t e n t i a l curves obtained at s e v e r a l narrow bandwidth wavelengths. The absence of a s h i f t i n photoemission threshold p r o p o r t i o n a l to the change i n photon energy i n the uv i s noteworthy (Figure 11). This has been suggested before.(49) and can even be detec ted i n e a r l i e r work.(4,) There are s e v e r a l p o s s i b i l i t i e s which might c o n t r i b u t e to t h i s observation. Two f a c t o r s are a lower concentration of scavenger and a lower concentration of support ing e l e c t r o l y t e i n our work. The former would tend to decrease the observed photoemission current, due to an increased probabi l i t y of the hydrated e l e c t r o n r e t u r n i n g to the e l e c t r o d e . T h i s might lead to a more negative measured t h r e s h o l d . The l a t t e r would tend to increase the mean distance from the e l e c t r o d e where the e l e c t r o n s are s o l v a t e d . The l a r g e r t h i s d i s t a n c e , of


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course, the harder i t i s f o r e l e c t r o n s to r e t u r n to the e l e c trode and hence the l a r g e r the photo-related c u r r e n t . This e f f e c t would tend to s h i f t the t h r e s h o l d to l e s s negative potent i a l s . (S_ 50) A l s o , a t h i r d f a c t o r to be considered i s that the uv data arose from doubling fundamental l a s e r frequencies; with the accompanying loss i n photon f l u x there would be a decrease i n photoemission current, and hence a more negative measured threshold. Another i n t e r e s t i n g p o i n t , a l s o not completely explained, is the r e l a t i v e l y large background photo-related s i g n a l observed with the pulsed l a s e r . Q u a l i t a t i v e l y our blanks resemble those p r e v i o u s l y obtained using conventional sources ;(6,10) the n u l l point was observed at about -0.6 v o l t s (vs SCE). Some r e s i d u a l photo-related current may have r e s u l t e d from i m p u r i t i e s i n the water or s a l t s used. No p a r t i c u l a r e f f o r t such as treatment wi th SO3" / uv i r r a d i a t i o n , ( 5 a possible contribution current noted with the pulsed l a s e r , both i n comparison to other workers using conventional r a d i a t i o n sources and our own concurrent work with the argon ion l a s e r , makes i t u n l i k e l y that imp u r i t i e s were p l a y i n g a s i g n i f i c a n t r o l e . I t i s f a r more l i k e l y that e~"(aq) - e""(aq) a n n i h i l a t i o n r e a c t i o n s are s i g n i f i c a n t i n pulsed l a s e r s t u d i e s . The r e l a t i v e l y high f l u x would create a correspondingly l a r g e r l o c a l concentration o f e"(aq). The slope i n Figure 10 i s c e r t a i n l y greater than 1.0, although a slope o f 2.0 would be expected for a s t r i c t l y bimolecular e""(aq) - e~(aq) which accounted f o r a l l o f the photorelated current.(29) Thermal p e r t u r b a t i o n i s undoubtedly r e s p o n s i b l e for a s u b s t a n t i a l p o r t i o n o f the photo-related current observed here.(5,6,8,13) This i s p a r t i c u l a r l y true of the r e s u l t s with the pulsed dye l a s e r , where wavelength c o n s i d e r a t i o n s e l i m i n a t e photoemission as a cause. Furthermore, compared to other l a s e r studies done using a mercury pool electrode;(26,28) one might e a s i l y e n v i s i o n a greater thermal e f f e c t here using the much smaller DME. I t i s even conceivable that the focused pulsed l a s e r led to d i s r u p t i o n o f the mercury or c a t a l y s i s of hydrogen reduction; such a p o s s i b i l i t y has been suggested(27) and i s c o n s i s t e n t with our observations of a streaming from the mercury s u r f a c e . On the other hand, the l i m i t f o r d e s t r u c t i o n of the double l a y e r has been estimated to be i n excess o f 10 MW cm"" , (29) and our power f l u x e s were considerably l e s s than t h a t . Two-photon emission has not been c o n c l u s i v e l y shown.(18,27) Our photon i n t e n s i t y vs current data does not unequivocally suggest a two-photon process to e x p l a i n anomalous observations with 520 nm pulsed l a s e r i r r a d i a t i o n . In f a c t , the observed higher order processes are c o n s i s t e n t with what has been suggested might occur under large thermal excursions.(27) Photo-related currents observed i n the presence o f t r a n s i t i o n metal ions pose d i f f i c u l t questions. Contrary to the conclusion of other workers, (,22 . 24) the observed photo-related currents do 3i

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not appear to be a s s o c i a t e d p r i m a r i l y with photoemission. A l though t r a n s i e n t photo-related currents are observed on the reduct i o n plateau of each metal i o n ( j u s t as reported e a r l i e r (24))> these s i g n a l s do not d i f f e r appreciably from those observed with blank e l e c t r o l y t e s o l u t i o n . Moreover, the onset of photo-related currents i s t i e d to the onset of polarographic reduction c u r r e n t s , but i s not s p e c i f i c a l l y r e l a t e d to the photoemission p o t e n t i a l (vide supra). Thus, although photoemission and scavenging reactions must be occuring with t a n s i t i o n metal ion s o l u t i o n s , the net e f f e c t on observed currents i s r e l a t i v e l y small compared to the primary phenomenon g i v i n g r i s e to photo-related currents. Our explanation f o r the observed photo-related currents i n duced with the CW l a s e r i n the presence of metal ion reduction i s simply that the chopped l a s e r source perturbs the Nernstian e q u i l i b r i u m which e x i s t s at the e l e c t r o d e surface i n a manner s i m i l a r to that impose p e r t u r b a t i o n may be therma i s the generation of current pulses which go through a maximum near the R\/2> y i e l d i n g a p l o t of photo-related current vs pot e n t i a l which looks very s i m i l a r to a d i f f e r e n t i a l pulse p o l a r o gram (Figures 13,14). As t h i s r e s u l t was h i g h l y dependent on l a s e r power, these observations provide another example of the p o t e n t i a l of l a s e r i r r a d i a t i o n to provide more d e t a i l e d informat i o n about the e l e c t r o d e - s o l u t i o n i n t e r f a c e . In t h i s regard i t i s noteworthy that a d d i t i o n of T r i t o n X-100 d i d not appreciably change the shape or magnitude of the photo-related current, but d i d s h i f t the apparent threshold f o r C u (Figure 15). T r i t o n X-100, being a r e l a t i v e l y large organic molecule, would be expected to have c e r t a i n i n s u l a t i n g p r o p e r t i e s , both e l e c t r o c h e m i c a l l y and from a d i f f u s i o n standpoint.(8.50) However, i t s t i l l i s not easy to c o r r e l a t e t h i s e f f e c t with a molecular model. 2 +

Results and D i s c u s s i o n f o r Laser P h o t o l y s i s Studies Determination of F e ( I I ) Concentration from F l a s h Photoreduction To a c c u r a t e l y determine the F e ( I I ) concentration produced from the photoreduction of F e ( I I I ) oxalate, the measured F e ( I I ) oxidat i o n currents must be corrected f o r faradaic-induced charging current.(31.47) Because raw data corrected by the " D e r i v a t i v e method" (see Experimental) showed C o t t r e l l behavior, the currents were subsequently c o r r e c t e d according to the " c o r r e c t i o n f a c t o r " procedure o u t l i n e d above. C o t t r e l l p l o t s f o r the two currenttime curves are shown i n Figure 16. The c o r r e c t e d data c l e a r l y show more i d e a l C o t t r e l l behavior. In order to c a l c u l a t e C° from the slope of the C o t t r e l l p l o t , the e f f e c t i v e e l e c t r o d e area and the d i f f u s i o n c o e f f i c i e n t of the e l e c t r o - a c t i v e species must be known. The t o t a l area of the working e l e c t r o d e was determined by c o l l e c t i n g ten d r o p l e t s , weighing, and b a c k - c a l c u l a t i n g from the known density of mercury, assuming s p h e r i c a l drops. Since the l i g h t i s c o l l i m a t e d , the e f f e c t i v e working area of the e l e c t r o d e ,


PERONE ET AL.


Figure 16. Cottrell plot of current-time data for Fe(II) oxida tion after laser flash irradiaion of ferrie oxalate solution. E = -0.5 V vs. SCE; RC = 706 psec; λ = 437 nm; b = 1 mm; [Fe(Ox)t] = 0.853 χ 10 U. eff

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or the area i r r a d i a t e d , was assumed to be 50% of the t o t a l area or 1.07 χ lO-^cm^. χ d i f f u s i o n c o e f f i c i e n t of F e ( l l ) was assumed to equal that of F e ( l I I ) , which was experimentally deter mined to be 7.09 χ 10~^ cm /sec by p o t e n t i a l - s t e p chronoamperometry. The observation of d i f f u s i o n - c o n t r o l l e d currents over a time period of about 5 msec i s s i g n i f i c a n t i n view of e a r l i e r F e ( l l l ) oxalate studies conducted i n t h i s laboratory.(38,39) The e a r l i e r studies were based on chronoamperometric data obtained from timedelay p o t e n t i o s t a t i c e l e c t r o l y s i s . Time-delay a n a l y s i s ( 3 8 ) em ploys f a s t e l e c t r o a n a l y t i c a l sampling of the photolyzed s o l u t i o n at v a r i o u s times a f t e r the f l a s h . By v a r y i n g the sampling time f o r consecutive experiments, the time-dependent behavior of the e l e c t r o a c t i v e species can be followed. The F e ( l l ) o x i d a t i o n cur rent was found to change with time, passing through a minimum at about 10 msec and then i n c r e a s i n Present r e s u l t s i n d i c a t c i e s produced by the f l a s h does not change with time. These findings r a i s e v a l i d questions about the p r e v i o u s l y proposed mechanism of the r e a c t i o n . The present data imply that the F e ( l l ) concentration may not be changing with time, and that the f i n a l product concentration i s being measured. Even i f there i s a r e a c t i o n i n v o l v i n g conversion of one o x i d i z a b l e form to another, the net current should be p r o p o r t i o n a l to f i n a l product as long as C o t t r e l l behavior i s observed at a l l times. I d e n t i c a l r e s u l t s were obtained i n a p r e l i m i n a r y study of the F e ( l I I ) oxalate system using a xenon f l a s h lamp e x c i t a t i o n source. F e ( I I ) o x i d a t i o n currents were found to be d i f f u s i o n - c o n t r o l l e d over the e n t i r e time range (.3-250 msec). These r e s u l t s are s i g n i f i c a n t because they r e i n f o r c e the f i n d i n g s of the laser-induced p h o t o l y s i s study and e l i m i n a t e the p o s s i b i l i t y that the " d i s c r e pancy" with the e a r l i e r studies i s a wavelength-dependent pheno menon. An explanation of the c o n f l i c t i n g r e s u l t s i s not obvious. The monitoring techniques are d i f f e r e n t - continuous versus timedelay, and the e a r l i e r workers were not aware of the f a r a d a i c - i n duced charging current c o n t r i b u t i o n s to the t o t a l c u r r e n t . I t i s p o s s i b l e that the apparent changes i n F e ( I I ) c o n c e n t r a t i o n noted e a r l i e r may have been due to o p e n - c i r c u i t d e p l e t i o n of F e ( l l ) due to double-layer charging a f t e r the f l a s h . η θ

2

Concentration of F e ( l l ) vs.

Pathlength

B i r k and Perone(30) have developed the t h e o r e t i c a l r e l a t i o n ship (Equation 2) d e s c r i b i n g the pathlength dependence of the i n i t i a l concentration of intermediate produced by f l a s h photo l y s i s , based on a d e r i v a t i o n s i m i l a r to that by Hercules f o r o p t i c a l flourescence.(51) Because of the complexity of t h i s r e l a t i o n s h i p and the necessary assumptions made i n i t s d e r i v a t i o n , B i r k points out that i t i s more p r a c t i c a l to examine a


9.

PERONE ET

AL.


163

r a t i o of CJj[ measurements made at two path lengths, b and b + Ab. From Equation 2, an expression r e l a t i n g the change i n i n t e r mediate c o n c e n t r a t i o n to the change i n pathlength f o l l o w s

C

R

( b )

= exp(a'AbC°)

( 8 )

C° (b+Ab)

where a" i s the e f f e c t i v e absorption c o e f f i c i e n t . The mono chromatic, c o l l i m a t e d , pulsed l a s e r source makes the v e r i f i c a t i o n of Equation 8 f e a s i b l e for the F e ( I I I ) oxalate system. The concentration f F e ( l l ) produced b th photoreductio of F e ( l I I ) oxalate at 44 three pathlengths. As pathlengt Fe(ll) c e n t r a t i o n decreases as expected. Table I.

Dependence of F e ( l l ) Concentration on Pathlength i n P h o t o l y s i s C e l l All

Data Set

1 2 3

4

concentrations, χ

Pathlength, b(mm)

10 M

CR(b)(\44 ) 2

1 5 10

CR(b)(X45 )

1.58 1.41 1.02

7

.93 .65 .47

Table II compares the data with the t h e o r e t i c a l r e l a t i o n s h i p expressed i n Equation 8. L i t e r a t u r e values(52) of 0:442 and 0:457 were used to c a l c u l a t e the p r e d i c t e d concentration r a t i o s . These values were 0:442 = 13.8, 0:457 9.0 M ^ c m (note: α = 2.3 ε ) . As can be seen there i s considerable d e v i a t i o n from the p r e d i c tions . =

-1


NEW

164 Table I I .

\(nm)

442

CHEMISTRY

Comparison o f F e ( I I ) Concentration Dependence to Theory. C a l c u l a t i o n of

Measured concentration r a t i o s from Table I

Predicted r a t i o using

o^Ccm^M 1)

= = 1.12 1.12 = 1.38 = 1.54

1.05 1.05 1.06 1.11

.33 X 3. 3.33 χ 10 7,.58 X 10 5,.64 X 10

4

C1 = 1.43 1.43 C i //CC9 = C / C = 1.38 C1/C3 = 1.9

1.03 1.03 1.04

.52 X 10 10.52 χ 10 7 .58 X 10

4

d/C C!/C C /C C1/C3 2

2

457


9

3

2

2

3

Average a " values:

5.52

χ 1 0 (\=442nm)

9.01

χ 1 0 (\=457nm)

4

4

4

4

The d e v i a t i o n s from Equation 8 can be explained by two e f f e c t s . F i r s t , the intense l a s e r pulse may cause some heating of the s o l u t i o n . This changes the r e f r a c t i v e index of the l i quid and the l a s e r beam i s dispersed by a "negative l e n s " e f f e c t . (53) This decreases the amount of l i g h t a v a i l a b l e for p h o t o l y s i s at the Hg drop, and, t h e r e f o r e , a smaller concentra t i o n o f F e ( I I ) i s produced than that p r e d i c t e d . This beam d i s p e r s i o n increases with increased pathlength, which explains the more pronounced d e v i a t i o n s i n F e ( I I ) c o n c e n t r a t i o n f o r l a r g e r changes i n b. The second p o s s i b l e reason for poor agreement i n Table I I i s due to a t r a n s i e n t inner f i l t e r e f f e c t ; i . e . , the amount o f a v a i l a b l e i n c i d e n t l i g h t at the mercury drop i s being reduced by a s t r o n g l y absorbing intermediate s p e c i e s . This e f f e c t would a l s o be l a r g e r over longer pathlengths. This l a t t e r e f f e c t i s probably the most s i g n i f i c a n t . From the concentration r a t i o s i n Table I I , i t i s p o s s i b l e to c a l c u l a t e an e f f e c t i v e absorption c o e f f i c i e n t a', which accounts f o r the absorption o f a l l absorbing species produced from the reactant as w e l l as the reactant i t s e l f . I t may a l s o account f o r the "negative l e n s " e f f e c t . Average values o f a' for the two wavelengths are reported i n Table I I . I t i s not s u r p r i s i n g that these absorption c o e f f i c i e n t s are l a r g e r than the experimentally measured a values for F e ( l l l ) , because previous workers have reported a p h o t o l y t i c intermediate which i s more s t r o n g l y absorbing than the s t a r t i n g m a t e r i a l i n the wavelength region o f interest.(40.41) The c a l c u l a t e d a' values w i l l be o f subsequent use i n determining Φ 4 4 2 ^ Φ457 from Equation 2. a n
uus* When the frequencies (jt^ and (jDg are i n c i d e n t on the sample, there w i l l be a simultaneous generation o f a s i g n a l a t ^ = 2 ω - ^ which w i l l have a frequency lower than the pump frequency. This i s known as CSRS (coherent Stokes Raman s c a t t e r i n g , ' s c i s s o r s ) . Stimulated Raman s c a t t e r i n g i s a l s o a kind o f coherent Raman scattering. I t may be viewed as a case o f spontaneous Raman 2

st

e

3

2

2

Γ

R

2

r

2

2

χ

a

0

a

lf

2

1


lf

10.

HUDSON

173

Coherent Anti-Stokes Raman Scattering

s c a t t e r i n g where the s i g n a l generated a t a frequency d i f f e r e n t from the s i n g l e i n c i d e n t frequency i n t e r a c t s w i t h the a p p l i e d frequency and i s a m p l i f i e d i n a c e r t a i n propagation d i r e c t i o n . I t shows a threshold behavior not observed w i t h three-wave mixing. Coherence o f the CARS S i g n a l The most important experimental aspect o f a CARS experiment i s that the s i g n a l i s generated as a coherent beam. For l i q u i d samples t h i s beam i s d i r e c t e d a t an angle w i t h r e s p e c t to the incident and ω beams. This beam may therefore be s p a t i a l l y f i l t e r e d from a l l other r a d i a t i o n from the sample and a l l o f the s i g n a l d i r e c t e d to the d e t e c t o r . No f i l t e r i n g monochromator i s needed to analyze the r a d i a t i o n . This r e s u l t s i n a greater c o l l e c t i o n e f f i c i e n c y compared to the spontaneous Raman e f f e c t It a l s o means that the r e s o l u t i o determined by monochromato of monochromaticity o f the l a s e r e x c i t a t i o n beams. Furthermore, since the f l u o r e s c e n c e from a sample i s i s o t r o p i c , the s p a t i a l f i l t e r i n g used to i s o l a t e the CARS beam reduces the sample fluorescence by a f a c t o r o f about 1 0 . The anti-Stokes nature o f the CARS s i g n a l and the Stokes nature o f the fluorescence can be used to remove the fluorescence by o p t i c a l f i l t e r i n g . A l s o the CARS s i g n a l i s much stronger than the corresponding spontaneous Raman s i g n a l . O v e r a l l , t h i s leads to a r e j e c t i o n o f fluorescence r e l a t i v e to spontaneous Raman s c a t t e r i n g by a f a c t o r o f 1 0 . 2

4

9

In a spontaneous Raman experiment an i n c i d e n t l a s e r beam with frequency i s d i r e c t e d i n t o a sample and the s c a t t e r e d r a d i a t i o n i s resolved i n t o i t s frequency components i n c l u d i n g those a t uo = ± uo · The plus s i g n corresponds to anti-Stokes s c a t t e r i n g and the minus s i g n to Stokes s c a t t e r i n g . The spon taneous Raman e f f e c t i s i n e l a s t i c l i g h t s c a t t e r i n g : both energy and momentum are exchanged between the sample and the r a d i a t i o n field. The emitted r a d i a t i o n i s i s o t r o p i c a l l y d i s t r i b u t e d and a l l frequency components are present a t the same time. They must be analyzed w i t h a l a r g e monochromator which determines the s p e c t r a l r e s o l u t i o n and the s i g n a l c o l l e c t i o n e f f i c i e n c y . The r a d i a t i o n which i s detected has no phase r e l a t i o n s h i p w i t h the i n c i d e n t radiation. An important aspect o f spontaneous Raman s c a t t e r i n g i s that i t s d e s c r i p t i o n i n terms o f the s e m i c l a s s i c a l approximation ( i . e . , a c l a s s i c a l f i e l d i n t e r a c t i n g w i t h a quantum mechanical medium) r e q u i r e s e x t r a assumptions i n v o l v i n g f l u c t u a t i o n s o f the medium (a normal mode dependent p o l a r i z a b i l i t y i n a m i c r o s c o p i c region) or i n the r a d i a t i o n f i e l d (zero-point f l u c t u a t i o n s ) . These assumptions are discussed elsewhere (I). The incoherent nature o f Raman s c a t t e r i n g i s r e l a t e d to these assumptions. A f u l l y quantum mechanical d e s c r i p t i o n o f Raman s c a t t e r i n g includes a spontaneous emission process which cannot be treated by the s e m i c l a s s i c a l theory without some a d d i t i o n a l assumptions. 2

P


174

NEW

APPLICATIONS OF

L A S E R S TO

CHEMISTRY

CARS (and i t s r e l a t i v e s ) can be described i n a f u l l y semic l a s s i c a l fashion without any a d d i t i o n a l assumptions. A l l that i s required i s the d e f i n i t i o n of a medium p o l a r i z a t i o n w r i t t e n as a power s e r i e s expansion i n the strength of the applied e l e c t r i c fields : P(r,t) =

(

( r , t ) + P 3>

( r , t ) + P
L " (Ms> *·· ôja, i . e . , a l l combinations of the form η ^ imy^. The induced p o l a r i z a t i o n at a given frequency (e.g., 2 ^ - uog) w i l l r e s u l t i n an emitted r a d i a t i o n f i e l d . Each volume element i n the sample (at p o s i t i o n r ) w i l l produce t h i s f i e l d w i t h a p a r t i c u l a r phase. The key f a c t o r at t h i s l e v e l of d e s c r i p t i o n i s that the r a d i a t e d f i e l d s may add i n phase for p a r t i c u l a r propaga t i o n d i r e c t i o n s . The t o t a l f i e l d w i l l be large f o r t h i s d i r e c t i o n . T h i s coherence also occurs for the f i r s t order p o l a r i z a t i o n , Σ} * (£>*0· The p o l a r i z a t i o n induced by a s i n g l e a p p l i e d frequency, ω, which i s p r o p o r t i o n a l to Ε(ω) only has components at frequency uo. The r e s u l t i n g t o t a l propagating e l e c t r i c f i e l d i s p a r t i c u l a r l y strong i n a d i r e c t i o n c o l i n e a r with the propagation d i r e c t i o n because the phase of the r a d i a t i o n produced i n one volume element i s i n phase w i t h the r a d i a t i o n of a l l other volume elements i n that d i r e c t i o n . For the f i r s t order ( l i n e a r ) p o l a r i z a t i o n t h i s e f f e c t gives r i s e to the o r d i n a r y r e f r a c t i v e index. By analogy with the higher order phenomenon, t h i s could be d e s i g nated coherent Rayleigh s c a t t e r i n g . This l i n e a r ( f i r s t order) case i s important because i t i l l u s t r a t e s the r e l a t i o n s h i p between the three aspects of the term "coherent as i t a p p l i e s to experiments l i k e CARS. F i r s t , there i s the c o l l i m a t i o n aspect of the s i g n a l . The measured r a d i a t i o n i s r e s t r i c t e d to a p a r t i c u l a r d i r e c t i o n . Second, there i s the f a c t that the r a d i a t i o n i n the detected beam has a phase r e l a t i o n to the applied r a d i a t i o n . This i s due to the f a c t that the p o l a r i z a t i o n i n a p a r t i c u l a r volume element depends on E ( r , t ) i n that element. T h i r d , there i s the f a c t that the detected r a d i a t i o n i s the square of the sum of the f i e l d s from many volume elements rather than the sum of the squares of the f i e l d s from 1

3

2

2

2

1

11


10.

HUDSON

175


each element. The detected r a d i a t i o n i s the square o f the coherent s u p e r p o s i t i o n o f the r a d i a t i o n from a l l s c a t t e r i n g elements. The r e l a t i o n between the c o l l i m a t i o n o f the CARS s i g n a l and the extended nature o f the s c a t t e r i n g volume can be viewed as an expression o f the u n c e r t a i n t y r e l a t i o n f o r the r a d i a t i o n . The u n c e r t a i n t y i n the r a d i a t i o n wave vector k i s small since i t s frequency and d i r e c t i o n are defined. Therefore, the volume element g i v i n g r i s e to the r a d i a t i o n must be extended i n space. There i s another sense i n which the term coherent i s o f t e n used. In t h i s sense the term r e f e r s to i n t e r f e r e n c e e f f e c t s between d i f f e r e n t resonances o f the same molecule. Resonance Raman s c a t t e r i n g shows t h i s type o f coherence i n d i s t i n c t i o n to fluorescence. The coherence o f CARS and coherent Rayleigh s c a t t e r i n g i s s p a t i a l coherence r e s u l t i n g from i n t e r f e r e n c e i n v o l v i n g d i f f e r e n t molecules. O e f f e c t s observed i n CAR d i f f e r e n t resonances on d i f f e r e n t molecules and thus the d i s t i n c t i o n between these two uses o f the term i s not u s u a l l y made. The coherence o f the CARS s i g n a l i s a property o f the r a d i a tion f i e l d . I t has a d i r e c t analogy i n the l i n e a r phenomenon which gives r i s e to the r e f r a c t i v e index. I t i s not r e l a t e d to any c o l l e c t i v e e x c i t a t i o n s of the m a t e r i a l induced by the r a d i a t i o n f i e l d . The important f a c t o r i s whether the r a d i a t i o n generated i n one volume element i s i n phase w i t h the r a d i a t i o n generated a t a d i f f e r e n t volume element. One way to i n t e r p r e t the s p e c i a l s i t u a t i o n associated with the forward propagation o f r a d i a t i o n i n the l i n e a r p o l a r i z a t i o n case i s based on momentum conservation f o r the "absorbed and "reemitted" photon. Reemission i n the forward d i r e c t i o n f o r a photon o f the same frequency as the e x c i t a t i o n frequency r e s u l t s i n no net momentum t r a n s f e r to the sample. This c o n d i t i o n (Ak = 0) i s the c o n d i t i o n which determines the d i r e c t i o n f o r the propagation o f the r a d i a t i o n i n a three-wave mixing experiment. In t h i s case the Ak = 0 c o n d i t i o n becomes 11

2k

- ÎS2 - kg = 0

(3)

where k = [n(u))(ju/c"]Ê where η i s the r e f r a c t i v e index. This i s known as the phase matching c o n d i t i o n . Before d i s c u s s i n g t h i s aspect o f CARS, we turn to a b r i e f d i s c u s s i o n o f i t s quantum mechanical d e s c r i p t i o n . A general expression f o r the quantum mechanical s c a t t e r i n g p r o b a b i l i t y f o r any process described by the t r a n s i t i o n operator Τ between i n i t i a l (I) and f i n a l (F) states o f the matter i s

S

= Σ Σ pJ ô(E N)

T

t

-E - A S

rad

)

F I


(4)

176

new

applications

of

lasers

t o

chemistry

where I and F r e f e r to the e n t i r e N - p a r t i c l e system, p j W i the N - p a r t i c l e d e n s i t y matrix element f o r the i n i t i a l s t a t e and the d e l t a f u n c t i o n imposes the r e s t r i c t i o n that the change i n energy of the matter (Ep - E-j-) must be balanced by a change i n energy o f the r a d i a t i o n f i e l d ( A E j ) . The s t a t e s I and F r e f e r only to the matter; the photon-state i n t e g r a t i o n has been performed and the r e s u l t i n g f a c t o r s are incorporated i n the d e f i n i t i o n of the operator Τ along w i t h the appropriate f u n c t i o n o f the matter c o o r d i n a t e s . The summations are over a l l p o s s i b l e i n i t i a l and f i n a l s t a t e s c o n s i s t e n t w i t h energy conservation and an i n i t i a l state p o p u l a t i o n . T h i s d e n s i t y matrix weighted summation i s e q u i v a l e n t to an ensemble average. The s c a t t e r i n g p r o b a b i l i t y S i s t h e r e f o r e a f u n c t i o n o f the thermodynamic q u a n t i t i e s which determine the d e n s i t y matrix elements and o f A E d which i s measured. The s p a t i a l and photo Τ depend on the p a r t i c u l a s p a t i a l p a r t o f the operator i s the instantaneous e l e c t r i c d i p o l e moment while f o r Raman s c a t t e r i n g , i t i s the instantaneous p o l a r i z a b i l i t y . For these cases, and f o r CARS, t h i s operator may be represented as a sum over i n d i v i d u a l operators each c o n t a i n i n g the coordinates o f the p a r t i c l e s i n i n d i v i d u a l molecules. Thus, s

r a (

r a

S

= Σ Σ p^ ESδ(Ε -Ç -ΔΕ

N)

,).

(5)

ρ

We w i l l assume that the N - p a r t i c l e d e n s i t y matrix element can be represented as a product o f 1 - p a r t i c l e d e n s i t y matrix elements p i ^ f ° the i n i t i a l s t a t e o f molecule a . We t h e r e f o r e exclude c o l l e c t i v e e x c i t a t i o n s o f the matter. For spontaneous Raman s c a t t e r i n g , the experiment c o n s i s t s o f measurement of the production o f photons with frequency «j due to the presence o f photons with frequency . In t h i s case, there must be a change i n the energy o f the r a d i a t i o n f i e l d (unless U>L (JU2) n d t h e r e f o r e there must be a change i n the energy o f the matter. This w i l l g e n e r a l l y be due to the e x c i t a t i o n o f one molecule i n the sample. The f i n a l s t a t e depends on which mole cule i s e x c i t e d . For a given p a i r o f i n i t i a l and f i n a l s t a t e s , only one molecule and t h e r e f o r e one t w i l l c o n t r i b u t e . In t h i s case we have r

a

2

=

a

a

(6)

where i and f r e f e r to the i n i t i a l and f i n a l s t a t e s o f molecule α w i t h energies Cf and e^. Note that S i s the sum over a l l molecules o f the ensemble average o f the t r a n s i t i o n p r o b a b i l i t y for each molecule. There are no cross terms i n v o l v i n g d i f f e r e n t molecules. Raman s c a t t e r i n g i s an incoherent process. γ


10.

HUDSON

177


For CARS, the change i n the energy of the r a d i a t i o n f i e l d i s zero. This i s because the operator Τ contains creation operators f o r and u)2 photons and d e s t r u c t i o n operators f o r two UDi photons. Since ^ = 2 ^ - ω , the n e t energy change A E d = + u)a " 2 ^ i s zero. This means that the f i n a l s t a t e can be the same as the i n i t i a l s t a t e . In t h i s case we have 2

r a

ce ρ 1 = Ι Σ Σ p J \ i | t | i >| αϊ α α' α (

2

.

(7)

1

±

The o v e r a l l t r a n s i t i o n p r o b a b i l i t y i s seen to be the absolute square o f the ensemble operator t The i n t e n s i t i n v o l v i n g p a i r s o f molecules i n c l u d i n g those which are very f a r a p a r t . The s c a t t e r i n g i s s p a t i a l l y coherent. The c o n d i t i o n that the i n i t i a l s t a t e i s the same as the f i n a l s t a t e i n a CARS experiment means that there i s no change i n the momentum o f the m a t e r i a l system. This means that there i s a l s o no change i n the momentum o f the r a d i a t i o n f i e l d . Since the momentum i s t h i s leads to equation (-3) which determines the d i r e c t i o n o f the emitted beam w i t h frequency . This phase matching c o n d i t i o n can be used to d e r i v e an expression f o r the optimum angle between the i n c i d e n t and ω beams. The v e c t o r diagram corresponding to equation (3) i s shown i n f i g u r e ( 1 ) . The length o f each v e c t o r i s determined by the a s s o c i a t e d frequency and the medium r e f r a c t i v e index at that frequency, k = n(oj)yj/c. The v e c t o r s w i l l only sum to zero i f the angle between the two i n c i d e n t beams has the value θ given by a

#

2

0

c

o

s

9

=

4nfng + n f ^ | - nj«g 4η η α) ω 1

2

1

(

8

)

2

where n^ i s the r e f r a c t i v e index a t frequency α)ΐ· For normal solvents which have a small d i s p e r s i o n and f o r small frequency s h i f t s Δ = υΰχ - UD we may assume that the r e f r a c t i v e index i s a l i n e a r f u n c t i o n of the frequency so that 2

«3 = % n

_

+ δ c

(9)

For CARS, ^ > w > and δ i s p o s i t i v e f o r m a t e r i a l s whose r e f r a c t i v e index increases with frequency. Using the n o t a t i o n «) = («!, U) + Δ = 0*3 > n d oo - Δ = (Jus and n e g l e c t i n g terms second order i n δ and f o u r t h order or higher i n 0 we have 2

a

O


178

NEW

APPLICATIONS OF LASERS TO CHEMISTRY

For oo corresponding to 5000 Â and A/2TTC = 1000 cm"" the phasematching angles f o r water, benzene and carbon d i s u l f i d e are 0 . 5 ° , 1° and 1.3°, r e s p e c t i v e l y . For d i l u t e gases the d i s p e r s i o n of the r e f r a c t i v e index i s very small s i n c e η « 1 f o r a l l frequencies and hence the phase matching angle becomes zero. The quantum mechanical d e s c r i p t i o n of the l i n e a r o p t i c a l phenomena can be based on s i m i l a r c o n s i d e r a t i o n s . I f one photon w i t h frequency ^ i s destroyed and one w i t h frequency tug i s detected, then, i f Φ ω , the i n i t i a l and f i n a l states must d i f f e r and momentum cannot be conserved by the r a d i a t i o n f i e l d alone. However, i f υϋχ U)2 there w i l l be a coherent e f f e c t c o r responding to ^ = kg. T h i the medium. There w i l (Rayleigh s c a t t e r i n g ) . The i n t e n s i t y of the forward or propagat ing beam i n an i s o t r o p i c sample compared to the incoherent side scattered r a d i a t i o n gives an idea o f the r e l a t i v e magnitude of these two types o f phenomena. There i s therefore a common b a s i s f o r the d e s c r i p t i o n of coherent l i n e a r and coherent n o n l i n e a r o p t i c a l phenomena. In each case there i s a p o s s i b i l i t y of no net change i n the s t a t e o f the matter because the process being monitored r e s u l t s i n no net change i n the energy or momentum o f the r a d i a t i o n f i e l d . The simple quantum formalism o u t l i n e d above shows that t h i s c o r responds to s c a t t e r i n g from an extended r e g i o n i n space. Spon taneous Raman s c a t t e r i n g i s d i f f e r e n t i n that i t involves the incoherent summation of processes from i n d i v i d u a l molecules. There are a number of other s i m i l a r i t i e s between CARS and normal forward propagation. For i n s t a n c e , n e i t h e r process r e s u l t s i n a frequency d i s t r i b u t i o n f o r the detected r a d i a t i o n under monochromatic e x c i t a t i o n c o n d i t i o n s . This i s i n c o n t r a s t to Rayleigh and Raman s c a t t e r i n g where e x c i t a t i o n o f the low frequency t r a n s l a t i o n a l and r o t a t i o n a l degrees o f freedom of the matter leads to a continuous i n t e n s i t y d i s t r i b u t i o n . The d e s c r i p t i o n o f these incoherent phenomena r e q u i r e s the i n t r o d u c t i o n of some f l u c t u a t i o n i n e i t h e r the matter or the r a d i a t i o n f i e l d . In the case o f Rayleigh s c a t t e r i n g , i t i s usual to consider that the p o l a r i z a b i l i t y i n a small r e g i o n o f space i s modulated by d e n s i t y f l u c t u a t i o n s . The frequency spread o f the scattered l i g h t i s a measure o f the time dependence of these f l u c t u a t i o n s . In the case of coherent phenomena, such f l u c t u a t i o n s must be averaged over the e n t i r e s c a t t e r i n g volume which i s macroscopic i n s i z e . For homogeneous media there are no macroscopic v a r i a t i o n s i n the p r o p e r t i e s o f the sample and thus no i n e l a s t i c processes which are not compensated by r a d i a t i o n f i e l d changes. When a l i n e a r l y p o l a r i z e d beam of l i g h t t r a v e r s e s a symmetric i s o t r o p i c medium i t emerges with i t s p o l a r i z a t i o n u n a f f e c t e d . 1

2

=


10.

HUDSON

Coherent

Anti-Stokes

Raman

179

Scattering

[ L i q u i d s and gases formed by asymmetric ( c h i r a l ) molecules are s a i d to be asymmetric i s o t r o p i c m a t e r i a l s . They have r o t a t i o n symmetry but l a c k i n v e r s i o n symmetry. Such m a t e r i a l s w i l l not be considered.] I f a CARS experiment i s performed w i t h and ω beams having p a r a l l e l p o l a r i z a t i o n , i t i s found that the beam i s p o l a r i z e d p a r a l l e l to the e x c i t a t i o n beams. For the case where the and ω beams are p o l a r i z e d perpendicular to each other i t i s found that the ^ beam i s p o l a r i z e d p a r a l l e l to the ω beam. In these two cases the ^ beam i s completely p o l a r i z e d . This d i f f e r s from the spontaneous Raman case where the detected s i g n a l may be d e p o l a r i z e d . The i n f o r m a t i o n obtained from a Raman p o l a r i z a t i o n r a t i o measurement may be obtained i n a CARS experiment by a comparison o f the i n t e n s i t y observed f o r the two i n c i d e n t p o l a r i z a t i o n conditions just described. 2

2

2

The Frequency Dependenc We now t u r n t o the theory o f the dependence o f the CARS s i g n a l on the values o f the frequencies and ω . T h i s i s most e f f i c i e n t l y done by u s i n g a standard diagrammatic representa t i o n (£). F i r s t , consider the spontaneous Raman e f f e c t . This process i s defined by the two diagrams i n f i g u r e (2) and t h e i r a s s o c i a t e d p e r t u r b a t i o n theory expressions. These two terms represent s c a t t e r i n g amplitudes. I t i s convenient to d e f i n e the t o t a l s c a t t e r i n g amplitude α as a f u n c t i o n o f the s i n g l e i n c i d e n t frequency by using the f a c t that a t a Raman peak uu (Wi - ou · 2

=s

2

P

f

α(ωχ)

s i ^ i

Hi

" *\

+

(11)

)

In t h i s expression (ju = υοχ - υυ and the M^j are e l e c t r i c d i p o l e matrix elements. The frequency denominators are given as r e a l quantities. I n c l u s i o n o f damping (and t h e r e f o r e absorption) may be done by r e p l a c i n g the intermediate s t a t e frequencies by complex v a l u e s . T h i s i s discussed i n greater d e t a i l below. From equation (6) we see that the Raman i n t e n s i t y i s r e l a t e d to the ensemble average o f the absolute square o f aiuux). r

S

RAM

2

a

s

(

« l a S ) I »

1

2

)

The i n t e n s i t y o f the Raman s c a t t e r i n g process i s l a r g e when the i n c i d e n t frequency i s near one o r more o f the intermediate s t a t e e x c i t a t i o n frequencies, i . e . , « (ju . T h i s i s known as resonance enhancement. The corresponding s i t u a t i o n f o r CARS i s shown i n f i g u r e ( 3 ) . Twelve diagrams and twelve amplitude c o n t r i b u t i o n s can be con s t r u c t e d subject to the c o n d i t i o n s that two photons o f frequency are destroyed, one new photon o f frequency ω i s created and a

2



Figure 1. The phase-matching vector diagram for a CARS experiment. The length of each κ vector is nw/c where the refractive index is measured at frequency W at the subscripts on κ correspond to ω ω Or 0)3. 1}

2>

Figure 2. Time-ordered perturba tion diagrams describing Raman ( UJ . These terms therefore c o n t r i b u t e to a background s i g n a l i The t h i r d group o a l s o c o n t r i b u t e o n l y a background s i g n a l to the CARS spectrum so long as 2 ^ i s not near any sharp two-photon e l e c t r o n i c t r a n s i t i o n s . These terms are s a i d to give r i s e to an e l e c t r o n i c back ground s i g n a l i n the sense that no v i b r a t i o n a l e x c i t a t i o n s o f the ground state are involved as intermediate states i n the s c a t t e r ing process. The r e p r e s e n t a t i o n o f one o f these terms on the b a s i s o f an energy l e v e l diagram i s given i n f i g u r e (4b). The s c a t t e r i n g amplitude f o r the CARS process i s the sum o f the 12 terms o f f i g u r e - ( 3 ) . I f t h i s amplitude i s designated Μυϋχ ,uo ) we see from equation (7) that the s i g n a l i n t e n s i t y i s given by the square o f the ensemble average o f Α Ο ι ^ , υ ^ ) . 1

r

=

r

r

2

r

1

r

r

-1

r

r

2

2

2

S

CARS

=

!« ^,t« )»| A

s

(14)

3

I t i s important that the ensemble average be performed before the square i s taken. The quantum mechanical d e s c r i p t i o n o f CARS o u t l i n e d above does not use the concept o f the t h i r d order s u s c e p t i b i l i t y which i s the s t a r t i n g p o i n t o f the s e m i c l a s s i c a l theory. This i s the e s s e n t i a l d i f f e r e n c e between these two treatments. A t t h i s p o i n t , however, we r e v e r t to the s e m i c l a s s i c a l d e s c r i p t i o n p r i m a r i l y because there i s an e s t a b l i s h e d procedure (1,10) f o r i n t r o d u c t i o n o f the resonance damping f a c t o r s i n t h i s formalism. The expression corresponding to equation (14) i n the s e m i c l a s s i c a l p i c t u r e contains the f a c t that the i n t e n s i t y o f the detected s i g n a l i s p r o p o r t i o n a l to the square o f the ensemble averaged i . e . , I ((χ ))| . The r e s u l t s o f t h i s averaging procedure depend on the nature o f the p o l a r i z a t i o n o f the and u) beams and the thermodynamic parameters o f the sample. E x p l i c i t expression w i l l o n l y be given below f o r s p e c i a l cases. The general r e s u l t s are given elsewhere (I). The b a s i c equation used to analyze CARS experiments may be w r i t t e n i n the form 3

2

2


10.


HUDSON

|χ«

I = | B +^(ô -i 2

r

= Β

2

1

Y r

r |

+ 2Β/?δ (δ Η ?Γ 2

Γ

185

s

+ ^(δ .+

1

2

Ύ

2 Ύ

(15)

1

.Γ

The background term Β i s the sum o f the CSRS and e l e c t r o n i c back ground c o n t r i b u t i o n s to plus a l l o f the CARS terms except f o r the one a s s o c i a t e d w i t h the resonance due to s t a t e r . Β i s only a slowly v a r y i n g f u n c t i o n o f οΟχ and CDs and i s u s u a l l y dominated by i t s r e a l p a r t . Imaginary c o n t r i b u t i o n s to Β can come from twophoton resonances (near 2 ^ ) . For the r e s t o f t h i s a r t i c l e we w i l l assume that Β i s a r e a l constant. The q u a n t i t y δ i s ci) - Δ U) - ( o ^ - Ute) · I t i s δ which i s v a r i e d i n order to o b t a i n a CARS spectrum. The damping f a c t o r y has been introduced f o r the Raman resonance. This i s the l i n e w i d t h o f the Raman resonance under i n v e s t i g a t i o n . /? contains the matrix elements and onephoton resonance denominator reference we w i l l give a case namely that i n which a molecule has a s e r i e s o f v i b r o n i c t r a n s i t i o n s ( p o s s i b l y to s e v e r a l d i f f e r e n t e l e c t r o n i c s t a t e s ) a l l with the same m o l e c u l e - f i x e d p o l a r i z a t i o n (along the molecule X - a x i s ) . We assume that a l l o f the other e l e c t r o n i c t r a n s i t i o n s with d i f f e r e n t p o l a r i z a t i o n s are a t too h i g h an energy to be important. Then (1) Γ

=

P

r

Γ

r

(ρδ'! /20tf )a((ju1)a(ufe)

(16)

4

where ρ i s the number d e n s i t y , ζ i s a l o c a l f i e l d c o r r e c t i o n f a c t o r o f t e n approximated by ζ = (n +2)/3 and the f a c t o r o f 20 comes from the o r i e n t a t i o n averaging. The imaginary damping f a c t o r s i y and i y are r e l a t e d to the l i f e t i m e s o f the i n t e r mediate s t a t e s a and b. These w i l l be discussed i n d e t a i l below. This expression i s v a l i d f o r the case ε · ε = 1 ( p a r a l l e l p o l a r i zation). I t should be m u l t i p l i e d by 1/3 f o r ê *£ = 0 (perpendicular polarization). From equation (16.) we see that i f uu - υϋχ » y and iSk - Wq » v f o r a l l intermediate s t a t e s a and b, then /p i s r e a l . This f a c t was used i n the expansion o f equation (15). Further more, under these c o n d i t i o n s αΧυ^) ^α(υυ^) so /p i s p r o p o r t i o n a l t o the t o t a l Raman cross s e c t i o n f o r t h i s t r a n s i t i o n [see equation (12)]. The s p e c t r a l shape described by equation (15) shows the i n t e r f e r e n c e behavior c h a r a c t e r i s t i c o f coherent phenomena. The 2

a

b

χ

2

1

a

s

a

b


NEW

186

APPLICATIONS O F LASERS TO

CHEMISTRY

t h i r d term i s a L o r e n t z i a n peak centered a t § = 0 with a h a l f width a t h a l f height o f y and a peak i n t e n s i t y o f /? /γ?. The f i r s t term i s a constant. The second term r e s u l t s from the coherent i n t e r a c t i o n o f the f i e l d s generated by the two kinds o f c o n t r i b u t i o n s to the t o t a l t h i r d order s u s c e p t i b i l i t y . I t has a c h a r a c t e r i s t i c d i s p e r s i o n shape being zero a t δ = 0, p o s i t i v e f o r Δ < ω and negative f o r Δ > uu · The kinds o f curves obtained i n CARS s p e c t r a are i l l u s t r a t e d i n references (2) and (4). For large values o f /?/γΒ the curve i s dominated by the L o r e n t z i a n . The d i s p e r s i o n shape cross term can be revealed on l o g a r i t h m i c p l o t s . For /?/γΒ « 1 both c o n t r i b u t i o n s to the spectrum are important. For /?/γΒ « 0.1 the curve i s dominated by the d i s p e r s i o n term. For /?/γΒ « 1 the p o s i t i v e and negative maxima are a t u) ± γ. r

2

T

Γ

r

Ρ

P

I f there are two o i t i s necessary to sum values o f r and i n c l u d e the cross terms i n the expansion o f the absolute square. A n a l y t i c a l Problems A s s o c i a t e d with the Background C o n t r i b u t i o n The major problem w i t h the use o f CARS as a general a n a l y t i c a l or s p e c t r o s c o p i c technique i s the high background l e v e l which a r i s e s from the e l e c t r o n i c c o n t r i b u t i o n to χ . For the strong Raman l i n e s o f pure l i q u i d s /?/γΒ = 10 to 100 (/?/γΒ « 100 for the 992 cm" mode o f benzene). Neat l i q u i d s t h e r e f o r e present no problem s i n c e the peak i n t e n s i t y r e l a t i v e to the background l e v e l i s (/ρ/γΒ) or 10 to 1 0 . However, f o r d i l u t e s o l u t i o n s the background l e v e l i s dominated by the c o n t r i b u t i o n from the s o l v e n t while the Raman c o n t r i b u t i o n of i n t e r e s t i s p r o p o r t i o n a l to the s o l u t e c o n c e n t r a t i o n . As a s o l u t i o n i s d i l u t e d the shape o f the CARS spectrum changes from one dominated by the L o r e n t z i a n peak to one dominated by the d i s p e r s i o n cross term. In the l i m i t o f great d i l u t i o n the spectrum equals the background a t Λ U) with p o s i t i v e and negative peaks on e i t h e r side o f the resonance. By i t s e l f the change i n s p e c t r a l shape due to a high back ground l e v e l i s not a p a r t i c u l a r problem although i t complicates the i n t e r p r e t a t i o n o f the s p e c t r a . The major problem i s the decreased s i g n a l to noise r a t i o . The noise i n a spontaneous Raman spectrum i s u s u a l l y determined by the shot noise o f photon count ing s t a t i s t i c s and i s t h e r e f o r e roughly the square r o o t o f the t o t a l s i g n a l . For the CARS case the t y p i c a l photon f l u x l e v e l s i n the s i g n a l beam provided by modern l a s e r s i s on the order o f 10 photons s e c " . At these s i g n a l l e v e l s the shot noise c o n t r i b u t i o n to the t o t a l noise i s n e g l i g i b l e i n comparison to the shot to shot f l u c t u a t i o n s o f the l a s e r . A complete d i s c u s s i o n o f the o r i g i n o f noise i n a CARS spectrum i s beyond the scope o f t h i s article. Some o f the important c o n s i d e r a t i o n s have been o u t l i n e d elsewhere (jL) where references are g i v e n . The most important 3

1

2

2

4

=

r

10

1


10.

HUDSON


187

aspect o f the noise i n a CARS spectrum i s that s i n c e i t i s due to l a s e r f l u c t u a t i o n s i t can be l a r g e l y removed by use o f a reference arm. For d i l u t e s o l u t i o n s t u d i e s , the best reference m a t e r i a l i s the s o l v e n t . On the whole, however, i t does not appear that CARS o f f e r s any a n a l y t i c a l advantages r e l a t i v e to spontaneous Raman s c a t t e r i n g f o r d i l u t e s o l u t i o n a p p l i c a t i o n s unless the sample i s fluorescent. Resonance CARS One o f the most u s e f u l and i n t e r e s t i n g features o f spontane ous Raman s c a t t e r i n g i s e l e c t r o n i c resonance enhancement. When the e x c i t a t i o n frequency i s near a strong e l e c t r o n i c t r a n s i t i o n the Raman s c a t t e r i n g amplitude cc(uo) becomes very l a r g e . The increase i n the Raman cross s e c t i o n r e l a t i v e to e x c i t a t i o n a t lower frequencies can s i b l e to study d i l u t e s o l u t i o n t h i s process i s s e l e c t i v e i n two r e s p e c t s . F i r s t , only the Raman t r a n s i t i o n s o f the molecule o r chromophore which has the e l e c t r o n i c t r a n s i t i o n become enhanced. Second, only c e r t a i n v i b r a tions o f the molecule are enhanced so that the r e s u l t i n g Raman spectrum may be much simpler than that observed w i t h e x c i t a t i o n away from the e l e c t r o n i c t r a n s i t i o n . The Raman t r a n s i t i o n s which are enhanced are g e n e r a l l y those which appear i n the high r e s o l u t i o n v i b r o n i c absorption and fluorescence spectrum. These, i n turn, are the v i b r a t i o n s corresponding to normal modes which are coupled to the e l e c t r o n i c t r a n s i t i o n . E x c i t a t i o n o f these modes accompanies the e l e c t r o n i c t r a n s i t i o n because the e x c i t e d s t a t e geometry i s d i s p l a c e d along these normal mode d i r e c t i o n s (leading to enhanced Franck-Condon f a c t o r s ) or because t h e i r e x c i t a t i o n mixes other e l e c t r o n i c s t a t e s with the s t a t e o f i n t e r e s t r e s u l t ing i n v i b r o n i c i n t e n s i t y borrowing (Herzberg-Teller c o u p l i n g ) . Resonance Raman s c a t t e r i n g i s therefore u s e f u l i n the i n t e r p r e t a t i o n o f h i g h r e s o l u t i o n molecular e l e c t r o n i c s p e c t r a , and, more g e n e r a l l y , i n the study o f the geometries o f e x c i t e d e l e c t r o n i c states. An important complication i n the use o f resonance Raman s c a t t e r i n g i n such s t u d i e s i s that fluorescence i s a l s o a very u s e f u l t o o l f o r the study o f e x c i t e d e l e c t r o n i c s t a t e s . As a r e s u l t , the molecules f o r which we have the most d e t a i l e d p i c t u r e o f t h e i r e x c i t e d s t a t e s tend to be f l u o r e s c e n t . Furthermore, the low temperature c o n d i t i o n s which lead to w e l l resolved e l e c t r o n i c absorption s p e c t r a , and thus w e l l resolved resonance Raman e x c i t a t i o n s p e c t r a , tend to favor f l u o r e s c e n c e . CARS e x h i b i t s e x a c t l y the same resonance enhancement mechan isms as the spontaneous Raman e f f e c t but i s not s u b j e c t to the complications o f f l u o r e s c e n c e . Therefore, resonance CARS appears to be a promising technique i n molecular spectroscopy. From equation (16) we see that when υοχ i s near a resonance uu o r i s near a resonance the q u a n t i t y /f> becomes complex. a


188

NEW


CHEMISTRY

In f a c t , the Raman amplitude α(ω) a l s o becomes complex under these c o n d i t i o n s but since the Raman s i g n a l i s p r o p o r t i o n a l to the absolute square o f α(υϋ) t h i s has no e f f e c t on the Raman i n t e n s i t y . For CARS, the complex nature o f /p must be included i n the expan s i o n o f equation (15). L e t t i n g

R = R + i l

(17)

we now have 2BRÔ !x

( 3 J

+

R

r

T%+fi)

2

+ I

+

2

- 2ΒΙγ

Γ

(

(egH^f)

1

8

)

R and I can be e i t h e r p o s i t i v e or negative. Under c e r t a i n con d i t i o n s R can be very smal great v a r i e t y o f lineshape and I > 0 the L o r e n t z i a n peak centered a t 6 = 0 w i l l be negative i f R + I - 2BIy i s n e g a t i v e . R and I are p r o p o r t i o n a l to the solute c o n c e n t r a t i o n while Β i s p r o p o r t i o n a l to the solvent con c e n t r a t i o n ( f o r d i l u t e s o l u t i o n s ) so i f I ^ 0 there w i l l , i n p r i n c i p l e , be some s o l u t e c o n c e n t r a t i o n a t which the "peak" becomes "negative", i . e . , below the asymptotic background l e v e l r a t h e r than above i t . Another c h a r a c t e r i s t i c feature o f resonance CARS s p e c t r a i s that R can be negative so the sense o f the d i s p e r s i o n shaped cross term i s r e v e r s e d . From equation (16) we see that t h i s happens when, f o r i n s t a n c e , ci^ > u) but < f o r two important v i b r o n i c resonances. The complicating feature o f resonance CARS spectroscopy i s that since the v i b r a t i o n a l resonances may be e i t h e r " p o s i t i v e " or "negative" i t i s d i f f i c u l t to i d e n t i f y a feature as a v i b r a t i o n a l resonance. Furthermore, the shape o f a resonance CARS spectrum depends on the e x c i t a t i o n wavelength, the c o n c e n t r a t i o n and the p o s i t i o n o f the e l e c t r o n i c resonance which i n turn may depend on the solvent and the sample temperature. However, i f CARS s p e c t r a are obtained a t various values o f the c o n c e n t r a t i o n so that R and I are v a r i e d r e l a t i v e to Β i t i s p o s s i b l e to use equation (18) to decompose the s p e c t r a i n a unique f a s h i o n to o b t a i n the resonance frequencies ω · From equation (18) we see that i f Β = 0 i t i s not p o s s i b l e to determine R and I s e p a r a t e l y since the only remaining term i s R + I . However, i n the presence of a f i n i t e background (Β Φ 0) both the s i g n and magnitude o f R and I can be determined. When Β = 0 the absolute square o f χ reduces to the absolute square o f /Ρίδρ-ίγ,.)"" and from equation (16) we see that |/?| = (constant) |α(υθ! ) | |a((*k ) | · Thus, the CARS i n t e n s i t y f o r Β = 0 i s p r o p o r t i o n a l to the Raman i n t e n s i t y measured a t two e x c i t a t i o n f r e q u e n c i e s . For d i l u t e s o l u t i o n s i t i s always p o s s i b l e to increase Β r e l a t i v e to /? by f u r t h e r d i l u t i o n and then R and I can be determined. The independent determination of R and I as a r

2

2

r

a

ρ

2

2

3

1

2

2

2


10.

HUDSON

189


function o f provides much more i n f o r m a t i o n than the determina t i o n o f R + I provided by Raman s c a t t e r i n g . T h i s new informa t i o n can be obtained because o f the coherent nature o f the s c a t t e r i n g process. Under resonance c o n d i t i o n s , the most important terms i n equation (16) are the resonant terms. Thus, 2

2

« ~ 20tf \ \

- i

Y

) I \ (^-«fe-iy,

a

/

to a good approximation. A r e s o l v e d e l e c t r o n i c a b s o r p t i o n spec trum provides values o f oo and | ( Μ ) ^ | . The spectrum can be f i t to a model f o r the e x c i t e d e l e c t r o n i c s t a t e such as a d i s placed harmonic o s c i l l a t o r model. The f a c t o r s ( M ) can then be c a l c u l a t e d from t h i s model the normal modes which t h i s model and to make some assumption concerning the correspon dence between the ground and e x c i t e d e l e c t r o n i c s t a t e normal modes.) In order to c a l c u l a t e the r e a l and imaginary parts o f /? from t h i s model i t i s necessary to s p e c i f y the values o f y f o r each v i b r o n i c resonance. One procedure i s to a d j u s t γ so that the a b s o r p t i o n spectrum i s reproduced. However, the v i b r o n i c bandwidths o f an a b s o r p t i o n spectrum are normally determined by inhomogeneous broadening, i . e . , the s o l u t e molecules are i n a d i s t r i b u t i o n o f environments w i t h each environment having a s l i g h t l y d i f f e r e n t e l e c t r o n i c spectrum. The observed l i n e w i d t h i s t h e r e f o r e not the homogeneous l i n e w i d t h γ]· but r a t h e r the sum o f the homogeneous and inhomogeneous widths r = vjj + y j . I t i s not immediately c l e a r which l i n e w i d t h should be used i n equation (19). T h i s problem a l s o occurs i n the theory o f resonance Raman s c a t t e r i n g . A recent a n a l y s i s f o r t h i s case has been given by Penner and Siebrand (11). T h e i r model i s as f o l l o w s . They assume that α ( ω ) i s given by 2

a

χ

Ε

x

r a

a

&

1

a

, Μ r aΜa i.

α(α>) - Σ ( a

J

ν

(20)

(j^-tM-ΐγ

η

where γ i s the same f o r each v i b r o n i c resonance and i s the homo geneous l i n e w i d t h r e l a t e d to the v i b r o n i c s t a t e l i f e t i m e . They next assume t h a t

u) = Ob + ηω,' a

21

( )

where (JUQ i s the v i b r a t i o n l e s s e l e c t r o n i c e x c i t a t i o n energy. T h i s , o f course, can be checked by comparison w i t h the a b s o r p t i o n spectrum and i s not an e s s e n t i a l f e a t u r e o f t h e i r model. They next assume that the inhomogeneous broadening r e s u l t s i n a


NEW

190


CHEMISTRY

L o r e n t z i a n d i s t r i b u t i o n o f values o f ω with a d i s t r i b u t i o n width γ . The assumption that t h i s d i s t r i b u t i o n i s L o r e n t z i a n g r e a t l y s i m p l i f i e s the subsequent c a l c u l a t i o n but i s probably not c r i t i c a l . The t o t a l Raman i n t e n s i t y as a f u n c t i o n of ω i s then obtained as the average o f |α(υυ)| over the L o r e n t z i a n d i s t r i b u t i o n [equation (12)]. The r e s u l t o f t h i s c a l c u l a t i o n i s that the resonance Raman e x c i t a t i o n p r o f i l e depends on both the homogeneous and inhomogeneous l i n e w i d t h i n such a way that each must be known i n order to c a l c u l a t e the e x c i t a t i o n p r o f i l e . Penner and Siebrand propose that the homogeneous l i n e w i d t h can be extracted from the e x c i t a t i o n p r o f i l e and the a b s o r p t i o n spectrum. The relevance o f t h i s c a l c u l a t i o n to the i n t e r p r e t a t i o n o f resonance CARS s p e c t r a i s that a p p l i c a t i o n o f the same model leads to a d i f f e r e n t r e s u l t . The reason f o r the d i f f e r e n c e i s that i n the Raman s c a t t e r i n g case the q u a n t i t y being averaged i s |cc(u))| so that the dampin product have opposite s i g n s averaged i s αΟϋχ )α(υ^ ) [equations (14) and ( 1 6 ) ] . In t h i s case the signs o f the damping terms i n the two f a c t o r s of the product are the same (both n e g a t i v e ) . This moves one of the poles o f the contour i n t e g r a t i o n r e l a t e d to the average over the L o r e n t z i a n d i s t r i b u t i o n . The r e s u l t i s that the ensemble averaged value o f i s the same as equation (19) but with y replaced by T = v + y . Thus, i n the CARS case the damping f a c t o r i s to be i n t e r p r e t e d as the t o t a l l i n e w i d t h o f the v i b r o n i c t r a n s i t i o n as observed i n the a b s o r p t i o n spectrum. This d i f f e r e n c e between resonance CARS and resonance Raman s c a t t e r i n g r e s u l t s from the d i f f e r e n c e between a coherent and an incoherent phenomenon and the a s s o c i a t e d d i f f e r e n c e i n the ensemble average used i n t h e i r d e s c r i p t i o n . Although the s p e c i f i c form o f the f i n a l r e s u l t s depends on the assumption o f a L o r e n t z i a n inhomogeneous d i s t r i b u t i o n , i t i s g e n e r a l l y true that CARS and Raman s c a t t e r i n g w i l l lead to d i f f e r e n t e x p r e s s i o n s . There i s one f u r t h e r aspect to t h i s problem. The expression for CSRS d i f f e r s from that f o r CARS [equation (19)] i n that the second sum has energy denominators o f the form -tt)2 i y · This means that the average over the inhomogeneous d i s t r i b u t i o n i s very s i m i l a r to the spontaneous Raman case s i n c e there are d i f f e r e n t signs f o r the two energy denominators. The r e s u l t i n g expression i s the same as f o r the Raman case i n that terms appear which depend i n d i v i d u a l l y on the homogeneous l i n e w i d t h . T h i s d i f f e r e n c e between CARS and CSRS depends on the p r e s c r i p t i o n f o r i n t r o d u c t i o n o f the signs o f the damping constants i n the semic l a s s i c a l theory (10). T h i s p r e s c r i p t i o n i s based on the r e q u i r e ment that the t h i r d order s u s c e p t i b i l i t y must be an a n a l y t i c f u n c t i o n of the a p p l i e d f i e l d s which are a t frequencies υοχ and w for CARS and and f o r CSRS. The quantum theory o f these t h i r d order s u s c e p t i b i l i t i e s (Hans C. Andersen, p r i v a t e communica tion) d i f f e r s from the s e m i c l a s s i c a l theory with regard to the signs o f the damping constants i n the CSRS expression. The 0

1

2

2

a

a

a

a

+

b

2


10.

HUDSON


191

quantum theory p r e d i c t s the same negative signs f o r CSRS as f o r CARS. The i d e n t i c a l form o f the two expressions a r i s e s from the general feature o f the quantum theory that states that the response o f the matter must be c a l c u l a t e d as i f a l l f i e l d s are present i n c l u d i n g the generated f i e l d (12). Thus, f o r CARS and CSRS the same expression i s obtained since the same three f i e l d s are present. This subtle d i f f e r e n c e has an e a s i l y measurable f i r s t order e f f e c t on resonance CSRS s p e c t r a since i t changes the values o f R and I c a l c u l a t e d from equation (20). I t also affects the inhomogeneous average as discussed above such that i n both CARS and CSRS the t o t a l l i n e w i d t h should be used f o r γ. The d i f f e r e n c e between the s e m i c l a s s i c a l and quantum theories of the t h i r d - o r d e r s u s c e p t i b i l i t y has not y e t been c o n c l u s i v e l y compared to an experimental r e s u l t . I t seems l i k e l y that the s e m i c l a s s i c a l theory w i l l b t t high l a s e f i e l d inten sities. Conclusions At the present stage o f the development o f CARS s e v e r a l con c l u s i o n s can be made concerning the kinds o f a p p l i c a t i o n s where i t w i l l be a u s e f u l method w i t h advantages over the spontaneous Raman effect. F i r s t , remote sensing a p p l i c a t i o n s can be very p r o f i t a b l y done by CARS, i . e . , d e t e c t i o n o f the s i g n a l a t a large distance from the sample. Second, h i g h r e s o l u t i o n gas phase, low tempera ture o r molecular beam a p p l i c a t i o n s are c l e a r l y favored by the f a c t that the r e s o l u t i o n i s determined by the l a s e r l i n e w i d t h . For s o l u t i o n studies there are obvious advantages i f the sample i s fluorescent. There may a l s o be advantages when very small e f f e c t s are o f i n t e r e s t such as c h i r o o p t i c a l e f f e c t s . Such a p p l i c a t i o n s r e q u i r e f u r t h e r o p t i c a l engineering and l a s e r development. In the case o f resonance enhancement new information i s a v a i l a b l e from a CARS experiment p a r t i c u l a r l y i n r e l a t i o n to the mechanism o f resonance enhancement and i t s r e l a t i o n to r e s o l v e d e l e c t r o n i c spectra.



192

Literature

Cited

1.

Andersen, Hans C. and Hudson, Bruce S., in "Molecular Spectroscopy, V o l . 5," Barrow, R. F., Long, D. A. and Sheridan, J . , e d i t o r s , pp. 142-201, The Chemical S o c i e t y , London, 1978.

2.

T o l l e s , W. Μ., N i b l e r , J . W., McDonald, J . R. and Harvey, A. B., Appl. Spectroscopy (1977) 20, 253.

3.

N i b l e r , J . W. and Harvey, A. B., in " A n a l y t i c a l Raman Spectroscopy," K i e f e r , W., e d i t o r , W i l e y - I n t e r s c i e n c e , New York (in press).

4.

N i b l e r , J . W., Shaub W A. B., in " V i b r a t i o n a Durig, J. R., e d i t o r

5.

Hudson, B. S., Ann. Rev. Biophys. Bioeng. (1977) 6, 135.

6.

F l y t z a n i s , C., in "Quantum E l e c t r o n i c s : A T r e a t i s e , " Rabin, H. and Tang, C. L., E d i t o r s , pp. 9-207, Academic, New York, 1975.

7.

T o l l e s , W. D. and Turner, R. D., Appl. Spectroscopy (1977) 31, 96.

8.

Terhune, R. W. and Maker, P. D., i n "Lasers: A S e r i e s o f Advances," Levine, Α. Κ., e d i t o r , pp. 295-372, Dekker, New York, 1968.

9.

P e t i c o l a s , W. L., Ann. Rev. Phys. Chem. (1967) 18, 233.

M.

McDonald

J

R

and Harvey

10.

Butcher, P. N., "Nonlinear O p t i c a l Phenomena," Ohio State U n i v e r s i t y Engineering P u b l i c a t i o n s , Columbus, Ohio, 1965.

11.

Penner, A. P. and Siebrand, W., Chem. Phys. L e t t . 39, 11.

12.

Feynman, R. P., "Quantum Electrodynamics," p. 4, Benjamin, Reading, Massachusetts, 1962.

(1976)

RECEIVED September 8, 1978.


11 Spectroscopy by Inverse Raman Scattering EDWARD S. YEUNG

Ames Laboratory—USDOE and Department of Chemistry, Iowa State University, Ames, IA 50011

Spectroscopy by i n v e r s e Raman s c a t t e r i n g (SIRS) is one of many new techniques in the development o f h i g demonstrated experimentally by Jones and S t o i c h e f f i n 1964 (1), its growth was somewhat l i m i t e d by c e r t a i n t e c h n i c a l difficulties. However, some o f the p r o p e r t i e s inherent to SIRS show good poten tial f o r a p p l i c a t i o n s in chemistry, ranging from high r e s o l u t i o n spectroscopy to chemical a n a l y s i s . T h e o r e t i c a l Considerations From a conceptual p o i n t of view, SIRS i n v o l v e s p h y s i c a l processes t h a t may not be immediately obvious. The best way to a p p r e c i a t e the p h y s i c a l processes t h a t l e a d t o SIRS is to r e l a t e it to normal Raman s c a t t e r i n g (RS). In F i g u r e 1 ( a ) , we show the molecular t r a n s i t i o n s i n v o l v e d in RS, Molecules are e x c i t e d by light at the frequency w to some intermediate s t a t e , which can be L

real or vitual, and a r r i v e a t the Raman-active l e v e l with the emission o f a photon o f frequency w , the Stokes frequency. S

Energy c o n s e r v a t i o n simply r e q u i r e s that w -w be equal to the L

s e p a r a t i o n o f the Raman l e v e l s .

S

The conversion from

to

is

an extremely i n e f f i c i e n t process, and i s of the order of 10-8 f the more f a v o r a b l e s i t u a t i o n s . T h i s , i n p a r t , has prevented the development o f RS as a t r a c e a n a l y t i c a l technique i n t y p i c a l s i t u a t i o n s , where the i n t e r a c t i o n i s non-resonant. The conversion e f f i c i e n c y can be enhanced t o the order o f 75$ (2.) by using stimu l a t e d Raman s c a t t e r i n g (SRS). F i g u r e l ( b ) shows the t r a n s i t i o n scheme f o r SRS, which i s nothing more than i n c r e a s i n g the e x c i t a t i o n u n t i l s t i m u l a t e d emission becomes important, thus f u r t h e r im proving the t r a n s i t i o n p r o b a b i l i t y . Because of the requirement f o r s t i m u l a t e d emission, there i s a t h r e s h o l d i n SRS. SIRS, as shown i n F i g u r e l ( c ) , i s intermediate between the f i r s t two cases. Both photons, a t ω and at are s u p p l i e d t o the sample. The o

τ

0-8412-0459-4/78/47-085-193$05.50/0 © 1978 American Chemical Society


r

NEW

194

intense r a d i a t i o n at ω

α

APPLICATIONS OF

L A S E R S TO

CHEMISTRY

guarantees that the system i s i n the

D

stimulated emission regime. Such stimulated emission, however, must be accompanied by the absorption of photons at ω^. One t h e r e f o r e simply monitors the d e p l e t i o n of the l a s e r beam at i n the presence of the intense l a s e r at ω . σ

The measurement of

D

absorption r a t h e r than emission leads to the term " i n v e r s e Raman", The quantum mechanical d e s c r i p t i o n of the SIRS process i s quite s t r a i g h t f o r w a r d . Following the treatment of Placzek (3.), one f i n d s that the p r o b a b i l i t y of a Raman t r a n s i t i o n can be ex pressed as |a|

2

α M (W L

g

+ 1)|S|

2

(l)

where S i s the normal Rama the photon number d e n s i t i e s at the e x c i t a t i o n and the Stokes f r e quencies, r e s p e c t i v e l y . In RS, W) Excitation, (**rs&) spontaneous scattering, stimulated scattering, and (z^&>) absorption.

Figure 2. Experimental arrangement for SIRS using two pulsed lasers


CHEMISTRY

11.

YEUNG

Inverse Raman Scattering

201

l a s e r p u l s e . On the other hand, i t i s extremely d i f f i c u l t t o o b t a i n q u a n t i t a t i v e i n f o r m a t i o n because o f the nature o f photo graphic response. Tsunoda (20) used a p h o t o e l e c t r i c d e t e c t i o n scheme t o f o l l o w the events i n SIRS. However, the broad s p e c t r a l output o f the dye f l u o r e s c e n c e used as t h e "continuum" s t i l l does not a l l o w t r u l y q u a n t i t a t i v e r e s u l t s t o be obtained. I f one modifies one o f the schemes u s i n g dye l a s e r s by s p e c t r a l l y nar rowing t h e output a t ω^, one should be able t o d e t e c t t h e ab s o r p t i o n p h o t o e l e c t r i c a l l y by u s i n g p a r t o f the dye l a s e r output as a r e f e r e n c e i n t e n s i t y . S t i l l , one might f i n d i t d i f f i c u l t t o determine P because o f the p u l s e d nature o f the two beams. A g

v i d i c o n can be used i n s t e a d o f the photographic p l a t e t o r e c o r d SIRS over a wide range o f Raman s h i f t s . By u s i n g a double-beam arrangement (e.g. imaging t h e r e f e r e n c e p o r t i o n o f t h e dye l a s e r on a d i f f e r e n t p a r t o q u i t e a b i t b e t t e r tha The major problem with laser-pumped p u l s e d dye l a s e r s as a source o f the "continuum" i s the u n r e l i a b l e mode output o f these systems. The o p t i c a l elements i n the dye l a s e r c a v i t y ( c e l l windows, m i r r o r s ) c r e a t e standing waves t h a t a r e wavelength dependent. These i n t e r f e r e n c e e f f e c t s can cause lower dye l a s e r output a t s e l e c t e d wavelengths which can be confused w i t h i n v e r s e Raman a b s o r p t i o n . L y t l e (21) suggested t h a t index matching o f the solvent and o f f - a n g l e dye c e l l geometry can be used t o m i n i mize these e f f e c t s . One can a l s o use a double-beam arrangement to account f o r l a s e r inhomogeneities. A t o t a l l y d i f f e r e n t concept f o r SIRS i s based on a Qswitched ruby l a s e r and a cw dye l a s e r (22). The o p t i c a l a r rangement i s shown i n F i g u r e k. A g i a n t - p u l s e ruby l a s e r , R, i s d i r e c t e d by M2 i n t o the sample c e l l , C, and then i n t o a f a c t o r y - c a l i b r a t e d b a l l i s t i c thermopile, B, so t h a t t h e energy of each l a s e r pulse can be measured. The l o s s e s o f t h e ruby l a s e r from the c e l l windows and the two m i r r o r s , M2, a r e em p i r i c a l l y determined so t h a t t h e energy i n the c e l l i s obtained. Since t h e power i s always lower than the s t i m u l a t e d Raman t h r e s h o l d f o r the sample, d e p l e t i o n w i t h i n the c e l l i s n e g l e c t e d . The ruby l a s e r i s plane p o l a r i z e d , and can be arranged t o have p a r a l l e l o r perpendicular p o l a r i z a t i o n t o the dye l a s e r . A com m e r c i a l cw dye l a s e r , D, having a s p e c t r a l output o f l e s s than 1

1 cm i s beam-expanded t e n times so t h a t i t i s completely covered by the l a r g e r ruby l a s e r beam but s t i l l samples a major p o r t i o n o f the ruby l a s e r . The dye l a s e r i s d i r e c t e d i n t o the sample c e l l by Ml a f t e r p a s s i n g through an o p t i c a l delay l i n e , ODL, and e v e n t u a l l y enters a double g r a t i n g monochromator, S. The o p t i c a l delay l i n e i s used t o i s o l a t e t h e dye l a s e r from the ruby l a s e r pulse during the i n v e r s e Raman event. Otherwise, t h e disturbance can cause l a r g e f l u c t u a t i o n s i n the dye l a s e r i n t e n s i t y . The dye l a s e r i s c o n t r o l l e d by a Pockels c e l l , PC, and i s switched on j u s t s h o r t l y b e f o r e the ruby l a s e r p u l s e . T h i s i s


NEW

202

APPLICATIONS OF

LASERS TO

-M9 M8 NITROBENZENE

RUBY

DYE

CU GU S>

L

MONOCHROMATOR

Figure 3. Experimental arrangement for SIRS for small Raman shifts

Ml

k M2

4=5''

PC

r

M2 Ml

Figure 4. Experimental arrangment for SIRS based on one pulsed and one cw laser


CHEMISTRY

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YEUNG

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necessary to avoid s a t u r a t i n g the p h o t o m u l t i p l i e r tube, P, which can handle l a r g e c u r r e n t s , but o n l y f o r short times. This way, good s i g n a l - t o - n o i s e r a t i o s can be achieved. The monochromator i s set at the dye l a s e r frequency, ω , with a s p e c t r a l s l i t width τ

of 2 nm so t h a t most o f the s t r a y ruby r a d i a t i o n i s r e j e c t e d . The a c t u a l tuning o f SIRS i s determined by the dye l a s e r and not by the spectrometer, which i s j u s t roughly synchronized with the tuning. The events are i n i t i a t e d by the f i r i n g of the ruby l a s e r , and a t r i g g e r generator, T, i n t u r n c o n t r o l s the Pockels c e l l and the o s c i l l o s c o p e , 0. The i n v e r s e Raman event i s recorded i n a s i n g l e t r a c e on the o s c i l l o s c o p e (500 MHz bandwidth). A t y p i c a l s i g n a l t r a c e i s shown i n F i g u r e 5· The dot at the upper l e f t - h a n d corner represents the z e r o - i n t e n s i t y l e v e l , i . e . when the dye l a s e r i s o f f . The e a r l y p a r t o f the t r a c e gives the dye l a s e r i n t e n s i t y befor f i r s t decrease i n i n t e n s i t and i s the inverse Raman a b s o r p t i o n . The second, much l a r g e r , decrease i n i n t e n s i t y i s from the disturbance of the ruby p u l s e i n the dye l a s e r c a v i t y , and has no s i g n i f i c a n c e i n SIRS. The degree o f absorption can be d e r i v e d d i r e c t l y from t h i s t r a c e . The pulsed + cw arrangement i n SIRs g i v e s more r e a d i l y q u a n t i t a t i v e i n f o r m a t i o n about the process. I t a l s o can provide s p e c t r a l information at higher r e s o l u t i o n . However, one does not have the advantage of o b t a i n i n g a l a r g e p o r t i o n of the Raman spectrum, as i s t y p i c a l of using more "continuum" type sources for ω . L What are some other o p t i c a l arrangements that may be d e v e l oped f o r SIRS i n the future? For pulsed type measurements, one i s f a c e d with the d i f f i c u l t y of spanning the complete s p e c t r a l r e g i o n o f Raman fundamental t r a n s i t i o n s u s i n g a s i n g l e dye, e i t h e r i n f l u o r e s c e n c e or i n a dye l a s e r . A p o s s i b l e a l t e r n a t i v e i s a laser-pumped o p t i c a l parametric o s c i l l a t o r (0P0). Such an arrangement may be s i m i l a r to t h a t i n F i g u r e 2 where the dye l a s e r i s r e p l a c e d by an 0P0. A s i n g l e n o n l i n e a r c r y s t a l pumped by the second harmonic of the ruby l a s e r should be able t o span the r e g i o n 100 cm"" to 1+500 cm" f o r Raman t r a n s i t i o n s . One ex pects to be able to minimize the e f f e c t s o f standing wave i n t e r ferences mentioned above i n dye l a s e r systems. The s p e c t r a l out put o f the 0P0 i s broad enough 10 cm" ) t o allow i n d i v i d u a l Raman l i n e s t o be recorded p h o t o g r a p h i c a l l y , and can be f u r t h e r narrowed t o allow higher r e s o l u t i o n s t u d i e s . The question t h a t remains i s whether or not SIRS can be done i n a cw mode. This q u i c k l y reduces t o a question about the power d e n s i t i e s a v a i l a b l e i n common cw l a s e r s . In argon i o n systems, one can have the order of 100 W i n a s i n g l e l i n e i n s i d e the c a v i t y . By f o c u s i n g down to a spot s i z e of 8y, one w i l l achieve τ

1

1

1

a P

Q

of 50 MW/cm

with a Rayleigh range of 0.U

mm.

For pure


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Figure 5. Inverse Raman absorption as recorded on an oscilloscope. Horizontal scale = 50 nsec/division.


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l i q u i d s o f strong Raman s c a t t e r e r s , Eq. (6) p r e d i c t s an i n v e r s e Raman absorption o f 0.1%. Such an experiment i s c e r t a i n l y pos s i b l e , b u t i s not s t r a i g h t f o r w a r d . In f a c t , we note t h a t s i n c e the power d e n s i t y i s i n v e r s e l y p r o p o r t i o n a l t o the area o f t h e beam waist and the Rayleigh range i s d i r e c t l y p r o p o r t i o n a l t o the same, there i s no advantage i n i n c r e a s i n g the degree o f focusing. Applications A n a l y s i s o f F l u o r e s c i n g samples. One o f the most a t t r a c t i v e features o f SIRS i s the p o s s i b i l i t y o f r e j e c t i n g f l u o r e s c e n c e i n the r e c o r d i n g o f a Raman spectrum. The f i r s t mechanism f o r t h i s d i s c r i m i n a t i o n comes from the f a c t t h a t one i s monitoring a t the anti-Stokes s i d e o f th band absorption i n t h i Stokes f l u o r e s c e n c e . The second, and the most important, mecha nism i s from the s p a t i a l d i s t r i b u t i o n o f the two processes. Fluorescence i s over l+π s t e r a d i a n s whereas i n SIRS only the l a s e r (the order o f m i l l i r a d i a n s i n divergence) i s detected. In p r i n c i p l e , t h e r e f o r e , one can use s p a t i a l f i l t e r i n g t o d i s c r i m i n a t e against f l u o r e s c e n c e . An a c t u a l a p p l i c a t i o n o f t h i s nature has been reported by Lau e t a l . (23). There, they observed SIRS f o r rhodamine Β and rhodamine 6G i n ethanol s o l u t i o n . Normal Raman spectroscopy on these u s i n g v i s i b l e l i g h t sources i s v i r t u a l l y impossible be cause o f the high fluorescence background. Using SIRS, they have 1

been able t o r e c o r d the Raman spectrum over a 1000 cm range with good s i g n a l - t o noise r a t i o . Because o f the proximity o f the e l e c t r o n i c absorption bands, the spectrum i s r e s o n a n t l y enhanced and i s very pronounced. S i m i l a r degrees o f d i s c r i m i n a t i o n against fluorescence can be expected i n mixtures where species other than the analyte f l u o r e s c e . The only r e a l d i f f i c u l t y i n the case o f f l u o r e s c i n g samples i s t h a t they can show s i g n i f i c a n t absorption a t ω and a t ω . I n the f i r s t case, an e l e c t r o n i c L b absorption may be mistaken f o r SIRS. The remedy f o r t h i s i s t o a l s o r e c o r d ω i n the absence o f the photons a t ω t o o b t a i n a L b r e f e r e n c e . I n the second case, the e x c i t i n g power w i l l be de p l e t e d and the q u a n t i t a t i v e information i s l o s t . Trace Determinations. Even though SIRS can g i v e an enhance ment over normal Raman s c a t t e r i n g o f the order o f 10^, i t does not mean that the l i m i t o f d e t e c t i o n w i l l be improved by that same amount. Photon-counting i n normal RS i s i n h e r e n t l y more s e n s i t i v e than the a b s o r p t i o n measurement i n SIRS. I n p r i n c i p l e , one can use extremely long pathlengths t o compensate f o r the low concentration o f the a n a l y t e . However, maintaining the high power d e n s i t i e s over such long paths and keeping the two l a s e r s τ

0


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s p a t i a l l y overlapping may "be t e c h n i c a l l y d i f f i c u l t . In g e n e r a l , one can expect t o be able t o use SIRS f o r the determination o f minor components i n mixtures, but probably not f o r u l t r a t r a c e analysis. We can use Eq. (6) t o estimate the d e t e c t i o n l i m i t s t h a t can be achieved i n SIRS f o r t y p i c a l Raman s c a t t e r e r s . For a strong Raman s c a t t e r e r , e.g. C C l ^ d e s c r i b e d above, one can expect a l i m i t o f d e t e c t i o n of 0.1$ c o n c e n t r a t i o n i n s o l u t i o n f o r a 10 cm

2 c e l l and a power d e n s i t y , Ρ , of 15 MW/cm , i f the detector i s b s e n s i t i v e enough to r e c o r d a 0.1% a b s o r p t i o n . For atmospheric

2 pressure gases, 3 MW/cm i s s u f f i c i e n t t o produce the same amount of a b s o r p t i o n . Lower concentration f o r by u s i n g longer absorptio l a s e r powers. The l i m i t on the u s e f u l l a s e r power i s determined by the e f f e c t o f s e l f - f o c u s i n g (2k), which destroys the q u a n t i t a t i v e nature o f SIRS, the breakdown o f the m a t e r i a l (25.), and the onset o f other n o n l i n e a r processes. A study o f SIRS i n a mixture, p a r t i c u l a r l y the minor compo nent, was r e p o r t e d by Gadow et a l . ( l U ) . They t r i e d u s i n g the mode-locked ruby l a s e r i n an arrangement s i m i l a r t o McQuillan et a l . {26) and found t h a t the d e t e c t i o n l i m i t i s 15-20 mole per cent f o r the minor component i n t o l u e n e , which i s roughly the same as t h a t found i n SRS (27). Much b e t t e r performance was achieved i n a o n e - c e l l arrangement. The d e t e c t a b i l i t y of the minor component i n toluene s o l u t i o n was found to be 5 mM f o r p y r i d i n e and 1 M f o r a n i l i n e . The d i f f e r e n c e i n these can be a t t r i b u t e d t o the d i f f e r e n c e s i n g i n each case. The authors noted that the d e t e c t a b i l i t y i s p a r t l y l i m i t e d by the decrease i n Pg as the major component reaches t h r e s h o l d f o r SRS. They sug gest t h a t by i n t r o d u c i n g a s e l e c t i v e absorber to prevent SRS i n the major component, a f a c t o r of seven improvement can be ob tained. An obvious way t o t r y t o improve the d e t e c t i o n l i m i t i n SIRS i s t o have the sample i n s i d e the dye l a s e r c a v i t y . I n conven t i o n a l a b s o r p t i o n spectroscopy, the i n t r a c a v i t y method can gen e r a l l y provide two to three orders o f magnitude enhancement i n the s e n s i t i v i t y (28). The main problem i n t r a n s f e r r i n g t h i s technique t o SIRS i s t h a t the dye l a s e r can be s e r i o u s l y perturbed by the intense l a s e r at ω . A l s o , SIRS l i n e s from the s o l v e n t b and the dye may i n t e r f e r e . Werncke et a l . (l£) r e p o r t e d such an i n t r a c a v i t y study. They were able to i s o l a t e the ruby l a s e r from the dye c e l l t o a degree t o overcome the problem mentioned above. A l e n s - m i r r o r combination focuses both l a s e r s i n s i d e the sample, and i s found t o reduce g r e a t l y the i n t e r f e r e n c e e f f e c t s common i n broadband dye l a s e r s . Using t h i s experimental arrangement, they were able t o e s t a b l i s h a l i m i t o f d e t e c t i o n f o r benzene i n carbon t e t r a c h l o r i d e o f 1 mM, which i s two orders o f magnitude 0


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b e t t e r than the corresponding value o u t s i d e the dye l a s e r c a v i t y . A somewhat p u z z l i n g r e s u l t i s the f a c t t h a t the Raman i n a c t i v e 1

l i n e o f benzene at 1190 cm" i s not detected even though the d e t e c t a b i l i t y i s higher here than i n e a r l i e r experiments i n v o l v i n g picosecond pulses (29). For a l l o f the above i n t r a c a v i t y experiments, the s e n s i t i v i t y o b v i o u s l y depends on the gain o f the dye l a s e r (which depends on the degree o f pumping), and n o n l i n e a r behavior can g e n e r a l l y be expected. The i d e a l s i t u a t i o n i s one where the dye l a s e r i s b a r e l y l a s i n g , so t h a t any l o s s from the inverse Raman e f f e c t w i l l be g r e a t l y magnified i n the l a s e r out put. Yet another method f o r enhancing the d e t e c t i o n l i m i t i s t o make use o f the resonance Raman e f f e c t (30). When ω_ i s resonant L

or n e a r l y resonant w i t h an e l e c t r o n i c

t r a n s i t i o n o f the molecule

the Raman t r a n s i t i o n p r o b a b i l i t y few orders o f magnitude l a r g e r . T h i s then can be used t o i n crease the s i g n a l t o compensate f o r the low concentrations i n samples. I t has been shown (31) t h a t i n the resonant regime:

g

(8)

α (ω_ - ω ) L Ο

2

+ Γ

2

where ω and Γ are t h e frequency and the l i n e w i d t h o f the e l e c tronic transition, respectively. The important p o i n t i s t h a t the e l e c t r o n i c absorption must not be confused with the induced ab s o r p t i o n i n SIRS. Werncke e t a l . (31) note t h a t there i s a min imum d e t e c t a b l e c o n c e n t r a t i o n determined by g, and a maximum concentration l i m i t determined by the l i n e a r a b s o r p t i o n c o e f f i c i e n t o f t h e sample a t ω and a t ω_. T h i s i s c l e a r l y demonL S s t r a t e d i n the resonant SIRS i n the dye DTDC, where a spectrum ο

-6

can only be obtained i n the range o f 1 - 5

x

10" M.

With —6

p h o t o e l e c t r i c d e t e c t o r s , they p r e d i c t a d e t e c t a b i l i t y o f 10 M f o r SIRS o f t y p i c a l compounds. A p a r t i c u l a r l y i n t e r e s t i n g e f f e c t i n resonant SIRS i s the abnormal d i s t r i b u t i o n o f i n t e n s i t i e s i n the Raman spectrum. Since i s normally f i x e d , d i f f e r e n t modes ( d i f f e r e n t Raman s h i f t s , a) ) w i l l e i t h e r be r e s o n a n t l y enhanced or not enhanced depending on the value of ω , which i s equal t o ω + ω_. HowL o f t ever, i f one f i x e s ω_ and tunes ω i n s t e a d , the degree o f enR

τ

α

α

hancement w i l l be constant f o r modes w i t h the same symmetry. The o n l y r e p o r t e d study i n gases deals with molecular n i trogen i n a i r (32). There, a 30 cm f o c a l l e n g t h l e n s i s used t o


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focus a Q-switched ruby l a s e r t o 1 GW/cm^. The IV-F photographic p l a t e s r e g i s t e r an absorbance of 0 . 0 2 5 . Gases at concentrations much lower than t h i s w i l l be very d i f f i c u l t t o d e t e c t . Determination o f Raman C r o s s - s e c t i o n s . The measurement o f a b s o r p t i o n i n SIRS does not r e l y on absolute i n t e n s i t y c a l i b r a t i o n , which i s necessary i n the determination o f absolute Raman c r o s s - s e c t i o n s using conventional techniques. I t has been suggested (k) t h a t SIRS i s s u p e r i o r t o normal RS f o r t h i s purpose. In the p r e - l a s e r e r a o f Raman spectroscopy, i t was almost impossible t o measure absolute Raman c r o s s - s e c t i o n s . T h i s i s because the i n t e n s i t y o f the e x c i t a t i o n i s u s u a l l y d i f f i c u l t t o determine and because the i n t e r a c t i o n volume i s d i f f i c u l t t o define. The o n l y r e a l a l t e r n a t i v e i s t o measure r e l a t i v e s c a t t e r i n g i n t e n s i t i e s and r e l a t e thes t theoretical prediction Th r e l a t i v e strengths o f Rama c u l a t e d (33) i n p r i n c i p l e , presenc f o r c e s and short-range o r d e r i n g i n l i q u i d s makes such r e l a t i v e measurements u n r e l i a b l e . I t has a l s o been suggested t h a t the J=l t o J=3 t r a n s i t i o n i n molecular hydrogen be used as a standard f o r Raman i n t e n s i t i e s (3*0 » Again, t h i s cannot be used i n liquids. There are d i f f i c u l t i e s i n absolute Raman c r o s s - s e c t i o n measurements i n l a s e r Raman methods as w e l l , ( i ) The d e t e c t o r must be c a l i b r a t e d so t h a t the s i g n a l can be converted i n t o a measure o f the absolute photon f l u x reaching the d e t e c t o r . T y p i c a l l y a standard i n t e n s i t y source (black-body r a d i a t o r ) i s used. Such c a l i b r a t i o n procedures are subject t o v a r i o u s s y s tematic e r r o r s , ( i i ) The throughput o f the spectrometer must be determined a c c u r a t e l y . P a r t i c u l a r l y s e r i o u s a r e problems a s s o c i a t e d with p o l a r i z a t i o n e f f e c t s and v a r y i n g s p e c t r a l s l i t widths, ( i i i ) The s o l i d angle o f c o l l e c t i o n i s very d i f f i c u l t t o d e f i n e p r o p e r l y . A l s o , the m a g n i f i c a t i o n f a c t o r i n forming the image o f the i n t e r a c t i o n r e g i o n onto the s l i t and the r e l a t i o n o f the image s i z e t o the mechanical s l i t width must be known p r e cisely, ( i v ) The Raman s c a t t e r i n g i n t e n s i t y has an angular dependence t h a t i s normally neglected. In a t y p i c a l spectrometer, one c o l l e c t s s c a t t e r i n g from a cone around the 90° a x i s and a s sumes that t h e measured i n t e n s i t y corresponds t o 90° s c a t t e r i n g . In a c t u a l f a c t , an i n t e g r a t i o n over t h e d i s t r i b u t i o n o f angles a c t u a l l y c o l l e c t e d i s a p p r o p r i a t e , (v) The temperature dependence of the s c a t t e r i n g i s normally neglected. I t has been shown (35) t h a t Raman i n t e n s i t i e s can change by as much as a f a c t o r o f two over a 1 5 0 ° temperature range. I n using h i g h l a s e r powers t o obt a i n good s i g n a l - t o - n o i s e r a t i o s , the sample may be heated s i g n i f i c a n t l y t o produce such e r r o r s . The best attempt i n the determination o f absolute Raman c r o s s - s e c t i o n s t o date i s probably the one by Kato and Takuma (36). The s c a t t e r i n g from the sample i s c o l l e c t e d i n a symmetric arrangement w i t h a black-body r a d i a t o r and t h e two s i g n a l s are


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compared s i d e "by s i d e . This o p t i c a l arrangement e f f e c t i v e l y e l i m i n a t e s e r r o r s due t o ( i ) and ( i i ) , minimizes e r r o r s from ( i i i ) , but s t i l l s u f f e r s from the c o n t r i b u t i o n s o f ( i v ) and ( v ) . The l a r g e d i s c r e p a n c i e s i n the r e p o r t e d c r o s s - s e c t i o n s from the various l a b o r a t o r i e s c l e a r l y show the d i f f i c u l t i e s i n such mea surements. For example, f o r the benzene l i n e a t 992 cm" , Skinner e t a l . ( 3 χ ) obtained a value t h a t i s twice as l a r g e as the one r e p o r t e d by McClung e t a l . (3<S), even though the p r e c i s i o n o f the i n d i v i d u a l measurements i s much b e t t e r than t h i s . Table I I i n Ref. (3°_) again shows d i s c r e p a n c i e s much l a r g e r than the i n d i v i d u a l p r e c i s i o n s f o r f i v e independent p u b l i s h e d values of the same Raman l i n e . Yeung (k) measured the absolute Raman c r o s s - s e c t i o n o f the 1

3062 cm l i n e i n benzene u s i n g SIRS There a photographic p l a t e was c a l i b r a t e d u s i n of absorption can be measure the power, Ρ , o f the ruby l a s e r . This was accomplished 1

σ

by monitoring the p u l s e shape with an o s c i l l o s c o p e and the pulse energy with a thermopile simultaneously. Because the dye l a s e r p u l s e was roughly 7 ns and the ruby l a s e r was roughly 25 ns i n d u r a t i o n , P can be taken t o be the peak power o f the ruby l a s e r g

p u l s e . Temporal overlap o f the two l a s e r pulses was e m p i r i c a l l y confirmed. S p a t i a l o v e r l a p , however, was not guaranteed because of the multimode s t r u c t u r e o f these l a s e r s . The f a r - f i e l d p a t t e r n o f the dye l a s e r showed t h a t the dye l a s e r was l e s s inhomo geneous than the ruby l a s e r . T h i s gave a smoothing e f f e c t so t h a t the average power o f the ruby l a s e r over the cross s e c t i o n o f the beam can be used as P . c

To see t h a t there i s an averaging e f f e c t whenever one l a s e r beam i s s p a t i a l l y more homogeneous than the other, one can sub d i v i d e the cross s e c t i o n o f the l a s e r beams i n t o small segments each w i t h w e l l - d e f i n e d i n t e n s i t i e s , A. and Β r e s p e c t i v e l y . The

1

j

"average" powers experimentally determined are A = 1/iEA^ and Β = 1/jZB^., w i t h the i n v e r s e Raman absorption being the product o f these two. I f Β i s more homogeneous than A, one can average over a l l A^ w i t h i n each segment j o f B, so t h a t the i n t e r a c t i o n i s given by 1/jZA.B . As l o n g as j i s a much l a r g e r r e g i o n than J J i , and as long as A. and B. are not c o r r e l a t e d , A. w i l l be approximately equal t o the t r u e average power o f the e n t i r e l a s e r beam, A. Thus, the t o t a l i n t e r a c t i o n becomes A ( l / j l B . ) , o r simply AB. The r e s u l t s i n Ref. (h) are shown i n Table I . I t can be seen that there i s c l o s e agreement between the values obtained using SIRS and those obtained using normal RS. J


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Table I Raman Parameters f o r the 3065

cm" Line of Benzene 1

Ordinary Raman 1

Raman S h i f t

3062.o

(cm" )

Inverse Raman

3065.5

a

1

FWHM Bandwidth (cm" )

b

7.1*

T.U

Peak SIRS Absorption^

C

8.0*

1

(xiO"^ cm

MW" )

Absolute Raman (do/dQ) (xiO" a

3 0

2

1

cm sr" )

r e p o r t e d i n Ref.

^measured i n Ref.

(j+)

°calculated from Eq. f o r cug = 1UU03

(6)

1

cm""

A more r e f i n e d method f o r the determination of Raman c r o s s s e c t i o n s using SIRS i s given i n Ref. (22), which i s based on a g i a n t - p u l s e ruby l a s e r and a cw dye l a s e r . For a t y p i c a l s i g n a l t r a c e shown i n F i g u r e 5» one can determine both Ι (θ) and I ( & ) L L i n Eq. (5) f o r the i n v e r s e Raman absorption. I t i s most conve nient to use the absorption maximum, although i n p r i n c i p l e any part of the peak can be used. The corresponding power i n the ruby l a s e r , P^, can be d e r i v e d from the measured energy i n the pulse and the width and shape of the p u l s e . The l a t t e r information i s contained i n the same o s c i l l o s c o p e t r a c e because at every p o i n t i n time the absorption i s p r o p o r t i o n a l to the i n t e n s i t y . Alter n a t i v e l y , a second o s c i l l o s c o p e can be used to r e c o r d the pulse duration. There are c e r t a i n c l e a r advantages i n u s i n g SIRS f o r d e t e r mining Raman c r o s s - s e c t i o n s . Because of the low beam divergence and the r e l i a b l e alignment of the l a s e r beams r e l a t i v e to each other, the angular dependence of the s c a t t e r i n g process i s p r o p e r l y accounted f o r . The short pulse durations and the l a r g e r beam c r o s s - s e c t i o n a l areas reduce the extent of heating o f the sample, so t h a t temperature e f f e c t s on the s c a t t e r i n g i n t e n s i t y are minimized. The s p e c t r a l r e s o l u t i o n of the l a s e r s i n SIRS can be much higher than t h a t of the spectrometer i n normal RS. τ


T

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Compared t o e a r l i e r attempts f o r such a p p l i c a t i o n o f SIRS (k), the p u l s e d ruby + cw l a s e r arrangement i s s u p e r i o r . Spatial overlap o f the two beams i s more s a t i s f a c t o r y because o f the s i n g l e ( T E M ) mode nature o f the dye l a s e r . The multimode output o f the ruby l a s e r can be e f f e c t i v e l y averaged t o determine the power d e n s i t y . Any e f f e c t from s e l f - f o c u s i n g w i l l a l s o be minimized. Temporal o v e r l a p o f the two l a s e r s i s guaranteed because one l a s e r i s cw. There may i n f a c t be some high-frequency modulation i n the dye l a s e r i n t e n s i t y from the b e a t i n g o f the l o n g i t u d i n a l modes whenever more than one such mode i s present. Simple mathematical modeling shows that as long as the temporal c h a r a c t e r i s t i c s o f the two l a s e r s are u n c o r r e l a t e d , these f l u c t u a t i o n s w i l l be averaged out and w i l l not a f f e c t the measurements. A l t e r n a t i v e l y , one can use a s i n g l e l o n g i t u d i n a l mode l a s e r f o r these experiments We note a l s o that the p h o t o e l e c t r i c detector need not be extremel amount o f a b s o r p t i o n i term i n Eq. (5) w i t h the f i r s t two terms i n the expansion. The Raman c r o s s - s e c t i o n i s then determined by the r a t i o o f Q0

- Lj-Co)} and P . g

I f the time response o f the photodetector causes

i t t o underestimate the q u a n t i t y i n parentheses, our determination of Pg from the same o s c i l l o s c o p e t r a c e w i l l be low by the same r e l a t i v e amount. In t h i s way, the time response o f the d e t e c t o r i s not a c r i t i c a l requirement. Using t h i s procedure, the absolute Raman c r o s s - s e c t i o n o f the 1 -29 nitrobenzene l i n e a t 13^5 cm i s found t o be 1.38 ± 0.27 * 10

2

—1

cm s r . This i s t o be compared with p u b l i s h e d values of 1.56* (37) and 1.93 (38), i n the same u n i t s . I t i s expected that f u r t h e r refinement i n the experimental method, e.g. independent pulse width measurements and s c a l e expansion on the photodetector s i g n a l , can reduce the u n c e r t a i n t i e s even more. Study o f P o l a r i t o n s . Because o f the w e l l - d e f i n e d p o l a r i z a t i o n d i r e c t i o n s and the propagation d i r e c t i o n s o f the two photons i n SIRS, the study o f p o l a r i t o n s i n c r y s t a l s has been p a r t i c u l a r l y f r u i t f u l (l8). With the absence o f a t h r e s h o l d i n SIRS, the n e a r l y complete p o l a r i t o n and phonon s p e c t r a can be observed. For photons w i t h p a r a l l e l p o l a r i z a t i o n p l a n e s , the r e s u l t s a r e the same as f o r normal Raman s c a t t e r i n g . For photons w i t h d i f f e r e n t d i r e c t i o n s o f p o l a r i z a t i o n s , one can show (18) that i n u n i a x i a l c r y s t a l s SIRS and SRS r e s u l t i n complementary p o l a r i t o n frequencies. One such study was performed i n a hexagonal L i l O ^ c r y s t a l (l8). To enhance was p l a c e d i n s i d e d i r e c t i o n s o f the controllable. It

the s e n s i t i v i t y o f the experiment, the c r y s t a l the c a v i t y o f the dye l a s e r . The p o l a r i z a t i o n ruby and the dye l a s e r s were independently was p o s s i b l e t o observe the A(z) phonons by


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l 8 0 ° s c a t t e r i n g because of r e f l e c t i o n s at the c r y s t a l f a c e s . A l l of the observed phonon and p o l a r i t o n absorptions can be assigned and compared to the corresponding RS and SRS s p e c t r a . Only d i r e c t i o n s of propagation p a r a l l e l or p e r p e n d i c u l a r to the o p t i c a l axes -were used because of the l a r g e d i v i a t i o n s o f the beams at other d i r e c t i o n s . In p r i n c i p l e , one would l i k e to span a l l these d i r e c t i o n s to form a more complete p i c t u r e . Such experiments w i l l have to wait u n t i l higher s e n s i t i v i t y can be achieved t o compensate f o r the small i n t e r a c t i o n r e g i o n s . Study of Reaction T r a n s i e n t s . I t has been p o i n t e d out very e a r l y on (_l) that SIRS can be used f o r studying f r e e raducals and other s h o r t - l i v e d s p e c i e s . The time r e s o l u t i o n o f the technique i s determined only by the p u l s e d u r a t i o n of the l a s e r s . In p r i n c i p l e , t h e r e f o r e , one can o b t a i n time-resolved Raman s p e c t r a of r e a c t i o n t r a n s i e n t s a spectral resolution o advantag over other picosecond methods based on e l e c t r o n i c a b s o r p t i o n i s that more d e t a i l e d s t r u c t u r a l information can be obtained. To t h i s date, however, there has been no r e p o r t o f such s t u d i e s . This i s mainly due to t e c h n i c a l d i f f i c u l t i e s i n d e t e c t i n g i n v e r s e Raman absorption from species at low c o n c e n t r a t i o n s . In the next few y e a r s , i t may be reasonable t o expect some of these s t u d i e s to reach f r u i t i o n . High R e s o l u t i o n Spectroscopy. The s p e c t r a l r e s o l u t i o n o f SIRS i s determined by the two l a s e r s used, so t h a t t y p i c a l l y higher r e s o l u t i o n can be achieved compared to normal RS, where the monochromator s l i t s cannot be too narrow and the f-number cannot be too unfavorable f o r proper l i g h t c o l l e c t i o n . For s t u d i e s i n gas samples, the normal 90° c o l l e c t i o n geometry i n RS i n h e r e n t l y l i m i t s the r e s o l u t i o n to a Doppler width determined by the Stokes frequency of the Raman s c a t t e r i n g . For c o l i n e a r SIRS w i t h copropagating l a s e r beams (0° s c a t t e r i n g ) , i t can be r e a d i l y shown that the r e s i d u a l Doppler width i s t h a t corresponding to the frequency of the Raman t r a n s i t i o n i t s e l f , and i s one o r d e r - o f magnitude l e s s than t h a t i n t y p i c a l RS experiments. Similarly, f o r c o l i n e a r counter-propagating l a s e r beams, the r e s i d u a l Doppler width i s roughly two times t h a t i n normal RS. The high r e s o l u t i o n p o t e n t i a l of SIRS has not been explored to date, but i s c e r t a i n l y one o f the most u s e f u l f e a t u r e s inherent to the technique. Summary Spectroscopy by i n v e r s e Raman s c a t t e r i n g o f f e r s many unique advantages i n the study o f molecules, such as the r e j e c t i o n of f l u o r e s c e n c e i n Raman s p e c t r a , the ease f o r o b t a i n i n g q u a n t i t a t i v e i n f o r m a t i o n , the h i g h speed at which a Raman spectrum can be recorded, the h i g h s p e c t r a l r e s o l u t i o n a c h i e v a b l e , and the s i m p l i c i t y i n performing p o l a r i z a t i o n s t u d i e s . One can a n t i c i p a t e


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r a p i d growth i n t h i s f i e l d as l a s e r technology becomes b e t t e r and b e t t e r developed. Acknowledgement The author wishes t o thank the U.S. Department o f Energy, O f f i c e o f Basic Energy Sciences, Chemical Sciences D i v i s i o n , f o r research support that l e d t o p a r t o f t h i s work. References (1) (2) (3) (5) (6) (7)

Jones, W. J . , and S t o i c h e f f , B. P., Phys. Rev. L e t t e r s , (1964), 13, 657. Grun, J. B., McQuillan, Α. Κ., and S t o i c h e f f , B. P., Phys. Rev., (1969), 180, 61. Placzek, G., Handbuc Yeung, E. S., J. Fenner, W. R., Hyatt, Η. Α., Kellam, J. Μ., and Porto, S. P. S., J. Opt. Soc. Am., (1973), 6 3 , 7 3 . Owyoung, Α., IEEE J . Quantum E l e c t . , (1978), QE-14, 192. Maker, P. D., and Terhune, R. W., Phys. Rev., (1965), 137 A801. P e t i c o l a s , W. L., Ann. Rev. Phys. Chem., (1967), 18, 233. Honig, B., J o r t n e r , J . and Szoke, Α., J. Chem. Phys., (1967)

(8) (9) 46, 2714. (10) Yeung, E. S., and Moore, C. B., in "Fundamental and A p p l i e d Laser Physics," Ed. F e l d , K u r n i t and Javan, p. 223, WileyI n t e r s c i e n c e , New York, 1972. (11) Hochstrasser, R. Μ., Sung, Η. Ν., and Wessel, J. E., J. Chem. Phys., (1973), 58, 4694. (12) S t o i c h e f f , B. P., Phys. L e t t e r s , (1963), 7 , 186. (13) Duardo, J. Α., Johnson, F. Μ., and El-Sayed, Μ. Α., Phys. L e t t e r s , (1966), 2 1 , 168. (14) Gadow, P., Lau, Α., Thuy, Ch. T., Weigmann, H. J., Werncke, W., Lenz, K. and Pfeiffer, M., Optics Comm., (1971), 4, 226. (15) McLaren, R. Α., and S t o i c h e f f , B. P., Appl. Phys. L e t t e r s , (1970), 1 6 , 140. (16) A l f a n o , R. R., and Shapiro, S. L., Phys. Rev. L e t t e r s , (1970), 24, 592. (17) K l e i n , J . , Werncke, W., Lau, Α., Hunsalz, G., and Lenz, Κ., Exp. Technik der Phys., (1974), 2 2 , 565. (18) Kneipp, Κ., Werncke, W., Ponath, Η. E., K l e i n , J . , Lau, Α., and Thuy, C. D., Phys. S t a t . S o l . (1974), 64, 589. (19) Werncke, W., K l e i n , J . , Lau, Α., Lenz, Κ., and Hunsalz, G., O p t i c s Comm., (1974), 1 1 , 159. (20) Tsunoda, Y., Jap. J. Appl. Phys., (1972), 1 1 , 1293. (21) L y t l e , F. E., "Inverse Raman Studies o f Chemical L a s e r s , " UCRL-13557 T e c h n i c a l Report, (1972), 8. (22) Hughes, L. J., Steenhoek, L. Ε., and Yeung, E. S., Chem. Phys. L e t t e r s , (1978), t o be p u b l i s h e d .


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Lau, Α., Werncke, W., Pfeiffer, Μ., Lenz, Κ., and Weigmann, H. J., Sov. J. Quant. E l e c t r o n . , (1976), 6, 402. (24) K e l l e y , P. L., Phys. Rev. L e t t e r s , (1965), 15, 1005. (25) Dowley, M. W., E i s e n t h a l , Κ. B., and P e t i c o l a s , W. L., Phys. Rev. L e t t e r s , (1967), 18, 531. (26) McQuillan, R. Α., and Stoicheff, B. P., Appl. Phys. Letters, (1970), 16, 140.

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Weigmann, H. J., Lenz, Κ., Werncke, W., Lau, Α., Pfeiffer, Μ., and Gadow, P., Z. Chem., (1971), 11, 36. Peterson, N. C., K u r y l o , M. J., Braun, W., Bass, Α. Μ., and K e l l e r , R. Α., J. Opt. Soc. Am., (1971), 61, 746. Werncke, W., Lau, Α., Pfeiffer, Μ., Lenz, Κ., Weigmann, H. J., and Thuy, C. D., O p t i c s Comm., (1972), 4, 413. Behringer, J., and Brandmuller, J . , Z. Elektrochem, (1956),

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Werncke, W., Lau Hunsalz, G., an VonHolle, W., U.S. Ballistic Research L a b o r a t o r i e s Memorandum Report No. 2 6 0 7 , (1976). Brandmuller, J . , and S c h r o t t e r , Η., Z. Physik, (1957), 149, 131. Udagava, Y., Mikami, N., Kaya, Κ., and I t o , M., J. Raman Spectrosc., (1973), 1, 341. Kondilenko, I . I . , and Babich, I . L., U k r a i n s ' k i i Fizichnii Zhurnal, (1960), 5, 532. Kato, Υ., and Takuma, H., J. Opt. Soc. Am., (1971), 61, 347. Skinner, J. G., and N i l s e n , W. G., J. Opt. Soc. Am., (1968),

58, 113.

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McClung, F. J., and Weiner, D., J. Opt. Soc. Am.,

(1964),

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Owyoung, Α., and Peercy, P. S., J. A p p l . Phys., 674.

RECEIVED August 25, 1978.


(1977),

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12 3

Time-Resolved Resonance Raman Spectroscopy (TR ) and Related Vidicon Raman Spectrography: Vibrational Spectra in Nanoseconds 1

WILLIAM H. WOODRUFF and STUART FARQUHARSON Department of Chemistry, Universit V i b r a t i o n a l Raman spectroscopy i s a rich source of s t r u c t u r a l information, which is particularly v a l u a b l e as a probe f o r s t r u c t u r e s of species i n s o l u t i o n . Raman s c a t t e r i n g can e a s i l y be observed under experimental c o n d i t i o n s , such as aqueous s o l u t i o n s or low v i b r a t i o n a l frequency, wherein the observation of d i r e c t i n f r a r e d absorption is difficult or impossible. Although normal Raman s c a t t e r i n g is a weak e f f e c t , r e q u i r i n g r e l a t i v e l y high concentrations of s c a t t e r i n g s p e c i e s , the resonance Raman e f f e c t can enormously increase the sensitivity of Raman s p e c t r o s copy. In the resonance Raman case ( l a s e r e x c i t a t i o n w i t h i n an e l e c t r o n i c t r a n s i t i o n of a s c a t t e r i n g chromophore) d e t e c t i o n l i m i t s lower than 10 M can be a t t a i n e d , and spectra can commonly be obtained i n the 10 - 10 M c o n c e n t r a t i o n range. The resonance Raman e f f e c t has been the subject of numerous recent reviews (e.g., Reference 1 ) . Raman s c a t t e r i n g occurs on an extremely short timescale (10 or less). By v i r t u e of t h i s property combined with the sensitivity and selectivity f o r a given chromophore of the resonance Raman e f f e c t , resonance Raman spectroscopy o f f e r s an a t t r a c t i v e probe f o r s t r u c t u r a l information on t r a n s i e n t chemical species such as r e a c t i o n intermediates or e x c i t e d states. In p r a c t i c e , however, the resonance Raman e f f e c t has seldom been a p p l i e d to chemical t r a n s i e n t s . This i s due to two f a c t o r s : p r i m a r i l y , the long (minutes to hours) i n s t r u m e n t a l l y -imposed time r e q u i r e d to o b t a i n a Raman spectrum using convent i o n a l techniques; and, s e c o n d a r i l y , the s p e c i a l o p t i c a l r e q u i r e ments of the resonance Raman experiment wherein the sample t y p i c a l l y absorbs 90% of the i n c i d e n t l a s e r l i g h t w i t h i n the Raman s c a t t e r i n g volume. The few s t u d i e s which have used e s s e n t i a l l y conventional instrumentation to record resonance Raman spectra of t r a n s i e n t s have employed e i t h e r continuous-flow techniques (_2,3) or r e p e t i t i v e (electrochemical) r e a c t i o n i n i t i a t i o n plus l o c k i n or boxcar d e t e c t i o n ( 4 ) . -7

-6

-5

-14

S

'Author to whom correspondence should be addressed.

0-8412-0459-4/78/47-085-215$05.25/0 © 1978 American Chemical Society In New Applications of Lasers to Chemistry; Hieftje, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.

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R e c e n t l y , a f e w g r o u p s have a p p l i e d i m a g e - i n t e n s i f i e d v i d i c o n d e t e c t o r s o r r e l a t e d d e v i c e s t o Raman s p e c t r o s c o p y , w i t h t h e o b j e c t i v e o f d r a s t i c a l l y r e d u c i n g the time r e q u i r e d t o o b t a i n Raman s p e c t r a ( 5 - 1 1 ) . These d e t e c t o r s a r e employed i n t h e same manner a s a p h o t o g r a p h i c p l a t e , i n c o n j u n c t i o n w i t h s p e c t r o g r a p h i c d i s p e r s i o n o f t h e Raman s p e c t r u m . T h i s r e s u l t s i n a m u l t i p l e x advantage i n s p e c t r a l a c q u i s i t i o n time analogous to t h e F e l l g e t t advantage o f a F o u r i e r transform spectrometer. In a d d i t i o n t o the s p e c t r a l m u l t i p l e x advantage, v i d i c o n Raman s p e c t r o g r a p h y o f f e r s t h e p o t e n t i a l f o r a c q u i s i t i o n o f Raman s p e c t r a i n e x t r e m e l y s h o r t t i m e s . This p o s s i b i l i t y e x i s t s when t h e v i d i c o n s p e c t r o g r a p h i s used i n c o n j u n c t i o n w i t h p u l s e d l a s e r e x c i t a t i o n o f Raman s c a t t e r i n g . U n d e r t h e s e c o n d i t i o n s , t h e t i m e r e s o l u t i o n o f a Raman e x p e r i m e n t becomes i n p r i n c i p l e t h e same a s t h e d u r a t i o n o f t h e l a s e r p u l s e s Due t o t h e m u l t i p l e x nature of th r e c o r d a s i z a b l e segmen of a s i n g l e l a s e r p u l s e . This combination o f p u l s e d - l a s e r e x c i t a t i o n ( w h e t h e r s i n g l e - p u l s e o r r e p e t i t i v e - p u l s e ) and v i d i c o n s p e c t r o g r a p h i c d e t e c t i o n i s t h e approach which has g e n e r a l l y b e e n t a k e n i n t h e a c q u i s i t i o n o f t i m e - r e s o l v e d Raman s p e c t r a (5,7-11). Formidable experimental d i f f i c u l t i e s , i n a d d i t i o n t o those e n c o u n t e r e d i n n o r m a l Raman s p e c t r o s c o p y , a r e e n c o u n t e r e d i n t h e t i m e - r e s o l v e d r e s o n a n c e Raman (TR^) c a s e . I n t h e n o r m a l Raman c a s e t h e s a m p l e g e n e r a l l y does n o t a b s o r b l i g h t a t t h e l a s e r w a v e l e n g t h , w h i l e i n t h e r e s o n a n c e Raman c a s e i t i s t y p i c a l f o r v i r t u a l l y a l l o f t h e l i g h t a t t h e l a s e r w a v e l e n g t h t o be a b s o r b e d w i t h i n t h e s c a t t e r i n g v o l u m e o f t h e s a m p l e . When p u l s e d - l a s e r e x c i t a t i o n i s e m p l o y e d w i t h an a b s o r b i n g s a m p l e , w h i c h i s t h e u s u a l T R r e q u i r e m e n t , c o m p l i c a t i o n s c a n be e x p e c t e d f r o m sample h e a t i n g and p h o t o c h e m i s t r y . Furthermore, o p t i c a l requirements c o u p l e d w i t h h i g h peak l a s e r power c a n l e a d t o damage t o sample and c e l l m a t e r i a l . The two p r e v i o u s r e p o r t s o f t i m e - r e s o l v e d r e s o n a n c e Raman s p e c t r a m i n i m i z e d t h e s e p r o b l e m s e i t h e r by e m p l o y i n g many r e p e t i t i v e e x c i t a t i o n p u l s e s a t r a t h e r l o w p e r p u l s e energy t o g a i n 5 ns time r e s o l u t i o n ( 4 ) , o r by u s i n g a r e l a t i v e l y l o n g - d u r a t i o n (0.6 y s ) s i n g l e - p u l s e e x c i t a t i o n t o m i n i m i z e peak p u l s e power a t r e l a t i v e l y h i g h p e r - p u l s e e n e r g y . One o f o u r o b j e c t i v e s h a s b e e n t o o b t a i n TR3 s p e c t r a w i t h t i m e r e s o l u t i o n o f a few nanoseconds w h i l e r e t a i n i n g t h e a b i l i t y t o a c q u i r e a s p e c t r u m e x c i t e d by a s i n g l e l a s e r p u l s e . P r e l i m i n a r y communication o f t h i s work has appeared elsewhere (12). I n t h e p r o c e s s o f d e v e l o p i n g t h e b a s i c TR^ i n s t r u m e n t a t i o n and t e c h n i q u e s , we h a v e d e m o n s t r a t e d t h a t t h e v i d i c o n Raman spectrograph i s e n t i r e l y superior t o a conventional scanning s p e c t r o m e t e r a s a d e t e c t i o n s y s t e m f o r most r o u t i n e Raman spectroscopy. I n a d d i t i o n , we h a v e i d e n t i f i e d c e r t a i n c o n d i t i o n s of p u l s e d - l a s e r e x c i t a t i o n under w h i c h i n t e r f e r e n c e s from non l i n e a r Raman e f f e c t s , p a r t i c u l a r l y s t i m u l a t e d Raman e m i s s i o n , J


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may be expected. A range of commonly encountered types of resonant and nonresonant Raman samples are i n v e s t i g a t e d , and a systematic comparison of the c a p a b i l i t i e s of the v i d i c o n Raman spectrograph and the conventional scanning spectrometer are presented. Our s i n g l e - p u l s e TR^ spectra represent a ten order of magnitude improvement over scanning spectrometry i n the time required to o b t a i n comparable s p e c t r a , and a two order of magni tude improvement i n time r e s o l u t i o n over p r e v i o u s l y reported s i n g l e - p u l s e TR^ s p e c t r a (9). Experimental Section F i g u r e 1 shows a schematic diagram of the apparatus used i n the present study. The v i d i c o n spectrograph c o n s i s t s of a standard SPEX 1870 0.5 m spectrograph with the camera attachment removed. An adapter f o the p h o t o s e n s i t i v e surfac spectrographic image plane. The v i d i c o n detector was a P r i n c e t o n Applied Research 12051 two-stage image i n t e n s i f i e d (ISIT) detector head, with a vacuum-deposited u l t r a v i o l e t s c i n t i l l a t o r for adequate response below 400 nm. The d i f f r a c t i o n g r a t i n g i n the spectrograph i s a 1800 groove/mm h o l o g r a p h i c a l l y recorded d i f f r a c t i o n g r a t i n g (HRDG) i n s t e a d of the standard 1200 groove/ mm c o n v e n t i o n a l l y r u l e d g r a t i n g . This s u b s t i t u t i o n i s necessary to e l i m i n a t e g r a t i n g ghosts (6). In some cases, i t was necessary to employ a c u t o f f f i l t e r (Schott 0G-550) i n f r o n t of the entrance s l i t s to reduce s t r a y l i g h t at the l a s e r wavelength. The sample stage and c o l l e c t i o n o p t i c s were a modified v e r s i o n of the Nestor design (13). T r a n s l a t i o n of the sample stage i n Χ,Υ,Ζ coordinates was accomplished with A r d e l T-50 micrometer t r a n s l a tor modules. The o p t i c a l bench and lens t r a n s l a t o r s were E a l i n g components. O p t i c a l components were s u p r a s i l or quartz through out. The lenses (ESCO Products, S1UV) were 2 1/2" diameter, the c o l l e c t i o n lens being f/1 and the matching lens f/7. The o b j e c t i v e lens was t r a n s l a t a b l e i n the Χ,Υ,Ζ d i r e c t i o n s v i a E a l i n g v e r t i c a l and transverse motions. The p o l a r i z i n g f i l t e r was an E a l i n g u l t r a v i o l e t p o l a r i z e r , and the scrambler was a quartz wedge from Lambda/Airtron. The l a s e r s employed to e x c i t e Raman s c a t t e r i n g i n t h i s study included Spectra-Physics 164 Argon i o n and Krypton i o n l a s e r s f o r CW. e x c i t a t i o n and, f o r TR s t u d i e s , a Quanta-Ray DCR-1 Qswitched Nd:YAG o s c i l l a t o r . The gas l a s e r beams were r i d of plasma emission l i n e s by a Burke f i l t e r ( d i s p e r s i v e d i r e c t viewing prism) assembly, and were d i r e c t e d and focussed u s i n g conventional Raman i l l u m i n a t o r o p t i c s . The 9 nsec pulses a t the Nd:YAG fundamental wavelength (1.064 ym) r e q u i r e d s p e c i a l handling. To provide a s u i t a b l e Raman e x c i t a t i o n wavelength i n the v i s i b l e r e g i o n , the beam was passed through a second harmonic generator (type 2 potassium dideuterium phosphate, KD*P, from Quanta-Ray). This arrangement produced a maximum of 75 mJ 3


In New Applications of Lasers to Chemistry; Hieftje, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978. Timing Circuitry

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of l a s e r energy i n seven-nanosecond pulses at 5318 8. However, a f t e r frequency doubling the Nd:YAG beam s t i l l contains sub s t a n t i a l (>50%) r e s i d u a l 1.064 ym r a d i a t i o n . This i s i n s u f f e r a b l e from a spectroscopic point of view, and a l s o i n c r e a s e s the t e c h n i c a l d i f f i c u l t y of handling the Nd:YAG p u l s e s , inasmuch as the i n f r a r e d r a d i a t i o n i s much more damaging to o p t i c a l components than the v i s i b l e . We solve t h i s problem by u s i n g , l i t e r a l l y , a chemist's s o l u t i o n to remove the 1.064 ym r a d i a t i o n from the d e s i r e d 5318 8. This we denote the F e r r i s c e l l (to acknowledge the c o n t r i b u t i o n s of N.S. F e r r i s ) , which i s a c y l i n d r i c a l c e l l of 20 cm pathlength f i l l e d with a 10% w/w s o l u t i o n of Fe(NH^) (S0^) '6H 0 i n 1 M degassed, aqueous Η Ν 0 · This s o l u t i o n has an absorbance of approximately u n i t y per cm a t 1.064 ym and zero at 5318 X. Thus the F e r r i s c e l l transmits v i r t u a l l y a l l of the 5318 8 Nd:YAG beam and none o s o l u t i o n s can be devise YAG harmonics. Samples were contained i n 1 mm i n s i d e diameter m e l t i n g p o i n t c a p i l l a r i e s f o r C.W. experiments. However, these c a p i l l a r i e s were destroyed by the t i g h t l y focussed Nd:YAG beam, thus f o r the pulsed experiments 3 mm i n s i d e diameter, 1 mm w a l l t h i c k n e s s quartz c a p i l l a r i e s were g e n e r a l l y used. For pure l i q u i d s or c o l o r l e s s s o l u t i o n s , a long pathlength c y l i n d r i c a l c e l l was sometimes used, with the l a s e r beam i n c i d e n t upon the f l a t window, i t s f o c a l point s e v e r a l centimeters i n s i d e the l i q u i d . The Raman-scattered l i g h t was c o l l e c t e d p e r p e n d i c u l a r to the a x i s of the c y l i n d e r . The v i d i c o n - d e t e c t e d s p e c t r a were processed by the standard PAR OMA console, and d i s p l a y e d on an o s c i l l o s c o p e f o r immediate viewing and on a X-Y p l o t t e r f o r a permanent copy. F o r s i n g l e pulse experiments, the scanning f u n c t i o n of the OMA d e t e c t o r was synchronized with the l a s e r pulse v i a the OMA console such that the l a s e r pulse a r r i v e d a t the sample w i t h i n the 600 ys r e t r a c e time of the v i d i c o n s e l e c t r o n beam. This was done by using the DELINHDI s i g n a l from the OMA to t r i g g e r a p p r o p r i a t e timing and l e v e l conversion c i r c u i t r y , and u l t i m a t e l y to f i r e the l a s e r v i a the l a s e r ' s remote t r i g g e r i n g i n p u t s . For r e p e t i t i v e - p u l s e or C.W.-excited s p e c t r a using the v i d i c o n spectrograph no attempt was made to synchronize the v i d i c o n scans and the l a s e r operation. In s e v e r a l cases, c o n v e n t i o n a l l y scanned s p e c t r a were obtained f o r comparison to the performance of the v i d i c o n spectrograph. These conventional s p e c t r a were obtained using e i t h e r a SPEX 1401 spectrometer as described elsewhere (14) or a Cary 82 with a cooled ITT FW-130 p h o t o m u l t i p l i e r and photon counting d e t e c t i o n . Laser e x c i t a t i o n f o r the conventional spectra was provided by the C.W. Ar or K r l a s e r s d e s c r i b e d above. V i s i b l e and u l t r a v i o l e t absorption s p e c t r a o f sample s o l u t i o n s were obtained using Cary 14, 15, or 118 2

2

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spectrophotometers. Results and D i s c u s s i o n Pure L i q u i d s . Figure 2 shows Raman spectra of carbon t e t r a c h l o r i d e obtained using the v i d i c o n spectrograph with both CW. (Figure 2A) and pulsed (Figure 2, B-E) e x c i t a t i o n . The two CW. e x c i t e d spectra show the s i g n a l - t o - n o i s e r a t i o (S/N) a t t a i n a b l e using both maximum s i g n a l accumulation with the OMA and a s i n g l e v i d i c o n exposure of 33 m i l l i s e c o n d s . The s p e c t r a l range shown i s that which i s detectable simultaneously with the v i d i c o n spectrograph, 381 cm~l using 5145 8. e x c i t a t i o n . The spectra c l e a r l y show the s u p e r i o r speed due to the s p e c t r a l m u l t i p l e x advantage of the v i d i c o n spectrograph: the 33 msec v i d i c o n exposure i n Figure 2(A) corresponds to a conventional scan speed of greater tha The carbon t e t r a c h l o r i d s p e c t r per formance of the v i d i c o n spectrograph near the l a s e r e x c i t a t i o n frequency. These s p e c t r a were obtained without any attempt to suppress l a s e r l i g h t e n t e r i n g the spectrograph, because any such suppression (step f i l t e r s of fore-monochromators) w i l l l i m i t the s p e c t r a l range of the spectrograph. I t can be seen t h a t , even though the sample geometry was a transverse c a p i l l a r y , which i s a notorious source of excessive s t r a y l i g h t , the 218 cm ! peak of C C l ^ i s observed with e s s e n t i a l l y the same S/N as would be seen i n a conventional double spectrometer. Furthermore, peaks at j u s t s l i g h t l y above 100 cm~^, i f present, could e a s i l y be observed. Figure 2(B-D) shows the e f f e c t of i n c r e a s i n g l a s e r pulse energy upon the p u l s e d - l a s e r e x c i t e d spectrum of carbon t e t r a c h l o r i d e . These s p e c t r a were observed using the c y l i n d r i c a l c e l l described above. The normal i n t e n s i t y p a t t e r n of the C C l ^ spectrum may be seen by comparing the V2(218 cm~^-), v^(314 cm~l) , and v^(459 cm~l) peaks i n the CW. spectrum, 1(A). Essentially the same i n t e n s i t y p a t t e r n i s observed when pulsed e x c i t a t i o n i s employed at an energy of 5 mJ/pul'e, F i g u r e 1(B). In F i g u r e 1(C), however, obtained using 16 mJ p u l s e s , the v-^ CCI4 peak i s s i g n i f i c a n t l y i n t e n s i f i e d r e l a t i v e to V£ and v^, compared to the CW. and 5 mJ/pulse s p e c t r a . When the per-pulse energy i s increased to 27 mJ, V]_ i s s t i l l more s t r o n g l y enhanced and the peak has begun to narrow r e l a t i v e to the spontaneous Raman l i n e w i d t h . These c h a r a c t e r i s t i c s ; d e f i n i t e energy threshold f o r i n t e n s i t y enhancement, t h r e s h o l d dependent upon c r o s s s e c t i o n f o r spontaneous Raman ( i . e . , the strongest spontaneous Raman mode i n a given spectrum w i l l become enhanced at the lowest energy t h r e s h o l d ) , nonlinear dependence of Raman i n t e n s i t y upon e x c i t a t i o n energy, and narrowing of the enhanced peak, are a l l c h a r a c t e r i s t i c of the stimulated Raman e f f e c t (15,16). However, our observations were made at 90° to the l a s e r beam, whereas stimulated Raman emission i s propagated i n a cone whose -


12.



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axis i s c o l i n e a r with the l a s e r beam. Thus we are observing 90° Rayleigh s c a t t e r i n g of stimulated Raman emission. This i s a r e f l e c t i o n of the p o t e n t i a l s e n s i t i v i t y (when observed d i r e c t l y ) of stimulated Raman emission and r e l a t e d e f f e c t s such as CARS, which may be e x p l o i t e d i n T R ^ - l i k e experiments. Figure 2(E) shows a spectrum of CCI4 obtained with a s i n g l e , 7_ nanosecond l a s e r pulse, showing stimulated Raman enhancement of 1· In view of the stimulated Raman emission observed i n the spectra i n Figure 1, and s i m i l a r e f f e c t s observed i n pulsede x c i t e d spectra of other pure l i q u i d s , i t i s f a i r to ask whether s i n g l e - p u l s e e x c i t a t i o n w i l l ever r e s u l t i n a spectrum e x h i b i t i n g normal, spontaneous Raman i n t e n s i t y p a t t e r n s . Figure 3 shows the spectrum of nitrobenzene obtained using r e p e t i t i v e pulse, 2(A), and s i n g l e - p u l s e the 1344 cm"^ and 100 thresholds f o r stimulated Raman emission (16), the r e l a t i v e i n t e n s i t i e s of a l l of the modes are the same as one observes when C.W. e x c i t a t i o n i s employed. This i s achieved by a v o i d i n g t i g h t f o c u s s i n g of the pulsed l a s e r beam, and thus remaining below the energy density threshold f o r n o n l i n e a r Raman e f f e c t s . This s c a t t e r i n g arrangement has the a d d i t i o n a l advantage of minimizing l a s e r damage to sample c e l l m a t e r i a l s . F i g u r e 3(B) demonstrates that time-resolved normal Raman s p e c t r a can be obtained with good S/N and without n o n l i n e a r Raman e f f e c t s . ν

Nonresonant Solutes: Lysozyme. As a more c h a l l e n g i n g t e s t of the performance of the v i d i c o n spectrograph with nonresonant Raman samples, C.W. e x c i t a t i o n was used to o b t a i n the spectrum of a biomolecule i n aqueous s o l u t i o n and i n the absence o f resonance enhancement. We wished to e s t a b l i s h the a b i l i t y o f the v i d i c o n spectrograph to handle high l i g h t l e v e l s , both at the l a s e r frequency (stray l i g h t ) and at frequencies s h i f t e d from that of the l a s e r (background due to f e a t u r e l e s s s c a t t e r i n g or luminescence), and s t i l l detect weak Raman peaks. These sample c h a r a c t e r i s t i c s are amply met by s o l u t i o n s o f macromolecules (e.g., p r o t e i n s , DNA). We chose a 30% s o l u t i o n of the nonchromophoric p r o t e i n lysozyme as a t e s t sample. In a d d i t i o n to intense Rayleigh s c a t t e r i n g expected of a l a r g e molecule, lysozyme e x h i b i t s background s c a t t e r i n g l e v e l s almost ten times as high as the Raman peaks, as shown on the v e r t i c a l a x i s o f the c o n v e n t i o n a l l y scanned spectrum, F i g u r e 4(A). Both F i g u r e s 4(A) and 4(B) were obtained using C.W. l a s e r e x c i t a t i o n at 4880 X, but Figure 3(B) was obtained using the v i d i c o n s p e c t r o graph. I t can be seen that a l l of the major f e a t u r e s i n the scanned spectrum are v i s i b l e i n the v i d i c o n spectrum, even though the v i d i c o n spectrum was obtained 100 times f a s t e r per v i d i c o n frame than the scanned spectrum. Longer accumulation times would have improved the S/N i n the v i d i c o n spectrum, but t h i s i s not p o s s i b l e without an e x t e r n a l s i g n a l averager due to


222

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Figure 2. Raman spectra of the 100-500 cm' region of carbon tetrachloride taken using the vidicon spectrograph. (A) (above) Top two traces; cw excitation, Ar laser, 5145 A. (B, above; C, D, E, right; +


12. WOODRUFF AND FARQUHARSON

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Figure 3. Pulse-excited Raman spectra of nitrobenzene, ND:YAG laser, 5318 A. (A) Full-scale OMA signal accumulation; 10 nsec pulses, 10 pulses/sec; 90 mW ave. power; 140 sec accumulation time; 1.4 X 10~ sec exposure time. (B) Single laser pulse, 7 X 10~ sec; 0.9 X 10' sec acquisition time. The double-ended arrows de note the point where adjacent vidicon exposures were joined. This notation is used throughout. 6

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Resonance Raman Spectroscopy 225

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PHOTON COUNTS/SEC H 2500

Ή 2250 1300

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Figure 4. Comparison of scanned and vidicon-detected normal Raman spectra of lyso zyme, 80% lysozyme in H 0. (A) Scanning double monochromator (SPEX 1401), photomultipler detection; conventional scan, 4880 A excitation, 85 mW cw, 4800 sec scan time. (B) Vidicon spectrograph, ISIT detection, full-scale OMA signal accumulation; 4880 A excitation; 500 mW cw, 44 sec accumulation time. 2


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the high background l i g h t l e v e l of the lysozyme sample and the l i m i t e d count range (100,000 counts f u l l s c a l e ) of the memories of the OMA. CW. E x c i t a t i o n o f Resonant Solutes: Cytochrome c. Figure 5 shows the c o n v e n t i o n a l l y scanned (5A) and v i d i c o n detected (5B) resonance Raman s p e c t r a of the heme p r o t e i n cytochrome c_, concentration 5.5 χ 10"^ M, e x c i t e d with the 5145 8 l i n e of the A r l a s e r . I t i s evident from comparison of F i g u r e s 5 (A) and 5 (B) that the v i d i c o n spectrograph i s an e n t i r e l y superior Raman d e t e c t i o n system, y i e l d i n g both f a s t e r s p e c t r a l a c q u i s i t i o n and b e t t e r S/N than the c o n v e n t i o n a l double s p e c t r o meter under s i m i l a r c o n d i t i o n s . The p o t e n t i a l s p e c t r a l m u l t i p l e x advantage of the v i d i c o n spectrograph over the double mono chromator, roughly a f a c t o f 100 dependin natural l i n e widths of the sample an l a t e s i n t o approximately improvemen a c q u i s i t i o n time f o r comparable S/N. The f a c t o r of 10 d i f f e r e n c e between the p o t e n t i a l and a c t u a l advantage o f the v i d i c o n spectrograph i s due i n part to lower per-detection-element s e n s i t i v i t y of the ISIT v i d i c o n compared to a p h o t o m u l t i p l i e r , which amounts to a f a c t o r of between 1.5 and 2. The remainder of the d i f f e r e n c e may be a s c r i b e d to the o b v i o u s l y poorer s t r a y l i g h t r e j e c t i o n of the spectrograph. Again, no attempt was made to suppress s t r a y l i g h t f o r these measurements. We wished to e s t a b l i s h using CW. e x c i t a t i o n the minimum l a s e r energy required to record a resonance Raman spectrum with the v i d i c o n spectrograph, and a l s o to e s t a b l i s h the best time r e s o l u t i o n a t t a i n a b l e using the i n t e r n a l t i m i n g of the OMA d e t e c t i o n system. The r e s u l t s are shown i n F i g u r e 5(C), wherein the resonance Raman spectrum of cytochrome c_ was recorded i n a s i n g l e , 33 m i l l i s e c o n d scan of the v i d i c o n ' s e l e c t r o n beam across i t s photodiode array. This i s the e l e c t r o n i c a l l y - i m p o s e d l i m i t on time r e s o l u t i o n which can be obtained u s i n g any PAR OMA system's response to continuous i l l u m i n a t i o n , without g a t i n g the e l e c t r o n o p t i c s of the image i n t e n s i f i e r stages. Most of the features of the cytochrome c^ spectrum a r e c l e a r l y d i s c e r n i b l e i n Figure 5(C), although only 1.8 m i l l i j o u l e s o f l a s e r energy e x c i t e d the s c a t t e r i n g . Comparison of F i g u r e s 5(A) and 5(C) shows an improvement of almost f i v e orders o f magnitude i n the temporal a c q u i s i t i o n of resonance Raman s p e c t r a i n the v i d i c o n spectrograph compared to conventional scanning under s i m i l a r e x c i t a t i o n c o n d i t i o n s , a l b e i t with a c o n s i d e r a b l e s a c r i f i c e i n S/N i n the v i d i c o n case. +

TR^ Detection of Nonreacting S o l u t e s : Cytochrome c. Figure 6 shows spectra of cytochrome c_ recorded using conven t i o n a l scanning with 5309 8 CW. e x c i t a t i o n , and using the TR apparatus with 5318 8 pulsed e x c i t a t i o n i n both r e p e t i t i v e and s i n g l e - p u l s e modes. Comparing the CW. spectrum to the

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227

r e p e t i t i v e - p u l s e d one, s e v e r a l p o i n t s can be made. F i r s t , the spectra are e s s e n t i a l l y the same feature f o r f e a t u r e , i n d i c a t i n g that the per-pulse energy which we employ, 11 mJ i n the present case, does not degrade or s i g n i f i c a n t l y perturb the p h o t o l y t i c a l l y - s t a b l e heme chromophore of t h i s p r o t e i n . The shoulder on the low-frequency side of peak C i s an a r t i f a c t i n the v i d i c o n spectrograph, and the d i f f e r e n c e i n r e l a t i v e areas of peaks D and Ε i s due to a p o l a r i z a t i o n anomaly i n the Cary 82 spectrometer. Second, c o n s i d e r i n g the improved S/N i n the p u l s e - e x c i t e d spectrum and the l a s e r powers employed i n each case, the v i d i c o n Raman spectrograph r e a l i z e s approximately a f a c t o r of 14 improvement over conventional scanning i n s p e c t r a l a c q u i s i t i o n time due to the m u l t i p l e x advantage, without regard to whether the Raman experiment i s C.W. or time-resolved. Third, the r e p e t i t i v e - p u l s e TR spectrum was obtained by e x c i t a t i o n from 1300 pulses of 7 Thus the sample was expose 10~5 s out of a t o t a l s p e c t r a l a c q u i s i t i o n time of 130 S, and time r e s o l u t i o n of 7 ns i s p o s s i b l e f o r any process which can be r e p e t i t i v e l y i n i t i a t e d the r e q u i r e d number of times. 3

To o b t a i n T R s p e c t r a i n processes which cannot be r e p e t i t i v e l y i n i t i a t e d , s i n g l e - s h o t a c q u i s i t i o n of TR spectra i s r e q u i r e d . The bottom spectrum i n Figure 6 was obtained using a s i n g l e e x c i t a t i o n pulse from the Nd:YAG o s c i l l a t o r , d e l i v e r e d during the 0.6 ys r e t r a c e time of the OMA detector. Otherwise the c o n d i t i o n s were i d e n t i c a l to those employed f o r the r e p e t i t i v e - p u l s e spectrum. The S/N of the s i n g l e shot spectrum, 3.5 f o r peaks C and H, i s the expected f a c t o r of 36 poorer than f o r the r e p e t i t i v e - p u l s e spectrum, and approximately a f a c t o r o f 8 poorer than the C.W., scanned spectrum. Neverthe l e s s , the main f e a t u r e s , peaks C, Ε, H, and perhaps F, are v i s i b l e i n the s i n g l e - s h o t spectrum. The a c q u i s i t i o n time f o r the s i n g l e - s h o t spectrum i s a f l a t f a c t o r of 7 χ i o n shorter than that f o r the C.W., scanned spectrum i n Figure 1. The S/N d e f i c i t s u f f e r e d by the s i n g l e - s h o t spectrum would allow the C.W. spectrum to be acquired 64 times f a s t e r than that i n Figure 1 to give S/N equal to the s i n g l e - s h o t spectrum. Considering t h i s S/N d i f f e r e n c e , the s i n g l e - s h o t spectrum represents a r e a l improvement of ten orders of magnitude over the conventional Cary 82 spectrometer i n the temporal a c q u i s i t i o n of comparable s p e c t r a . Compared to the previous TR studies (8,9), these r e s u l t s represent a conspicuous improvement i n S/N f o r r e p e t i t i v e - p u l s e TR-* f o r comparable time r e s o l u t i o n , and an improvement of two orders of magnitude i n time r e s o l u t i o n over the only p r e v i o u s l y reported s i n g l e - s h o t TR spectra ( 9 ) . J

TR^ D e t e c t i o n o f T r a n s i e n t s : The P h o t o d i s s o c i a t i o n of ΟΡ Η emo g lob i n . The cooperative binding of four ligands such as dioxygen or carbon monoxide by the hemoglobin tetramer i s a w e l l known and p h y s i o l o g i c a l l y v i t a l phenomenon (17). Conformational


NEW


CHEMISTRY

COUNTS/SEC

_l 1200

I 1300

I 1400

I 1500

1 1600

Figure 5. Comparison of scanned and vidicon-detected, cw-excited resonance Raman spectra of cytochrome c; 5.5 X 10' M. (A) (above) Scanning double monochromator (SPEX 1401), photomultiplier detection; conventional scan, 5145 À excitation, 40 mW cw, 2550 sec scan. (B) (top right) Vidicon spectrograph, ISIT detection, full-scale OMA signal accumulation; 5145 A excitation, 55 mW cw, 147 sec accumulation time/frame. (C) (bottom right) Single OMA frame scan, 33 msec spectral acquisition time; 5145 À excitation, total energy = 1.8 mJ. 4


12.



ι

I

I

1200

1300

1400

I

1500

I

1600

Δ ^ , cm-1


229

I

1700

NEW

I

1200

1

1

1300

1400


I

I

1500

1600

CHEMISTRY

I

1700

Figure 6. Conventional (cw scanned) and time-resolved resonance Raman spectra of cytochrome c.(Top) cw excitation by 5309 A Kr laser line, scanning spectral detection by Cary 82 spectrometer, 80 mW cw, 5000 sec scan time. (Top right) 5318 Â repetitive-pulse TR spectrum, vidicon spectrographic detection; 10 nsec pulses, 10 pulses/sec, 110 mW ave. power, 130 sec accumulation time, 1.3 X 10' sec exposure time. (Bottom right) single-pulse, 7 nsec TR spectrum, 5320 A excitation, 0.9 X 10' sec acquisition time. +

3

5

3

8


12.


1200

1300


1400

1500

1600

A*,cm-'


231

1700

NEW

232


CHEMISTRY

i n t e r p l a y between the heme and p r o t e i n s t r u c t u r e s i s thought to be responsible f o r t h i s c o o p e r a t i v i t y . The most g e n e r a l l y accepted model proposes that the "stereochemical t r i g g e r " f o r the l i g a t e d (oxy t e r t i a r y , "Relaxed" quaternary) to u n l i g a t e d (deoxy t e r t i a r y , "Tense" quaternary) p r o t e i n conformational change i s the s p i n - s t a t e change of the i r o n atom upon d e l i g a t i o n at the a x i a l s i t e trans to the proximal h i s t i d i n e (18,19). This s p i n - s t a t e change i s accompanied (or followed) by displacement of the i r o n atom out of the heme plane toward the proximal h i s t i d i n e , and probably by appreciable lengthening of the i r o n imidazole bond. The heme s t r u c t u r e change i s transmitted to the p r o t e i n t e r t i a r y and quaternary s t r u c t u r e s , e f f e c t i n g the observed c o o p e r a t i v i t y of l i g a n d binding i n the tetrameric p r o t e i n . The temporal r e l a t i o n s h i p s among the events of heme d e l i g a t i o n , s p i n - s t a t e change heme s t r u c t u r e change and p r o t e i n s t r u c t u r e chang standing the r e l a t i o n s h i p function. Inasmuch as C.W. resonance Raman spectroscopy i s a w e l l - e s t a b l i s h e d probe f o r e q u i l i b r i u m heme e l e c t r o n i c s t a t e s and s t r u c t u r e s (1), time-resolved resonance Raman spectroscopy may be expected to provide s i m i l a r information on heme t r a n s i e n t s and t h e i r temporal behavior. Present evidence on the dynamics of the p h o t o l y s i s of carbonmonoxy hemoglobin (COHb) suggests that the f o l l o w i n g events occur : Reaction Reference 1.

COHb

hv, 0 - 0.5 3°0xy ! > CO + Hb t , < 0.5 ps °R y

(20)

4

l y

3

2.

Hb I°

J.

Hb 4

4.

5.

xy

2

(Fe in-plane)

R

3

Hb ° *°R

40 ns < t D e o x

1

/

> t i / 2 t

1

/

2

Hb + C0(1 atm)

c

o

«

m

p

l

e

t

e

i

n

4°R

^ Hb ο * K

3

Hb ° °

D e 0 X y

0

4

< 1 ms

H b

>

X y

(

F

e

( U

o u t

o f

"" plane) (21) 2

2

)

;

(23,24)

T

COHb

(17)

100 ms

where 3° and 4° r e f e r to the t e r t i a r y and quaternary s t r u c t u r e s of the p r o t e i n . In a t y p i c a l experiment, we i l l u m i n a t e the Raman s c a t t e r i n g volume of our sample with 7 ns l a s e r pulses of 5318 8 wavelength, 10 mJ energy, and 10 Hz r e p e t i t i o n r a t e . Each pulse i s s u f f i c i e n t l y intense to photodissociate a l l of the COHb w i t h i n the s c a t t e r i n g volume i n approximately 300 ps. The remaining 6.7 ns of the l a s e r pulse "sees" only the product of


12.


Resonance Raman Spectroscopy θ α

ι 1250

1 1300

1

1 1400

1

1 1500

1

1 1600

—i

Γ 1700

Δ*, cm-'

Figure 7. Time-resolved resonance Raman spectra of (A) oxy hemoglobin, (B) deoxyhemoglobin, and (C) photodissociated carbonmonoxyhemoglobin (see text), in the frequency region of the structure-sensitive "indicator bands." Abbreviations: ρ = polarized, dp — depolarized, ap = anomolously (in versely) polarized. Conditions: 5318 Â excitation, 7 sec pulses, pulse repetition frequency 10 Hz, pulse energy 10 m], accumuhtion time 165 sec.


233

NEW

234


CHEMISTRY

e o x y

r e a c t i o n (1) or (2) plus any H b ^ ° R which has been produced by r e a c t i o n (3). Our time-resolved resonance Raman spectra of ycOHb", t h e r e f o r e , are at l e a s t 86% HbÔxy, i than 10% 4°R ° ' approximately 4% COHb. A d d i t i o n a l l y , since COHb i s completely reformed i n l e s s than 0.1 s, 10 Hz l a s e r pulses can be repeated i n d e f i n i t e l y i n order to improve s p e c t r a l s i g n a l - t o - n o i s e r a t i o by s i g n a l accumulation. Figure 7 shows the time-resolved resonance Raman s p e c t r a of the s t r u c t u r e - s e n s i t i v e 1300-1700 c n T s p e c t r a l regions of Û2Hb, Deoxy Hb, and "COHb" (photodissociated as noted above). In these experiments, Û2Hb remains l i g a t e d due to i t s r e l a t i v e l y low quantum y i e l d f o r p h o t o d i s s o c i a t i o n . The spectra of Û2Hb and Deoxy Hb appear i n s i g n i f i c a n t l y d i f f e r e n t from the r e s p e c t i v e CW. spectra e x c i t e d at s i m i l a r l a s e r wavelengths. The three i n d i c a t o r bands which r e f l e c t heme e l e c t r o n i c and/or geometrical s t r u c t u r e s h i f t from 1377 1552, and 1607 cm" i highest-frequency v i b r a t i o n s are thought to be s e n s i t i v e p r i m a r i l y to displacement of the i r o n atom i n or out of the heme plane, p a r t i c u l a r l y when the i r o n atom and the heme are f a r from coplanar (as i n Deoxy Hb) (25-27). The spectrum of photodissociated COHb i n Figure 7 i s e s s e n t i a l l y the same as that of Deoxy Hb. This suggests that the s t r u c t u r a l r e l a x a t i o n s of the heme group following photod i s s o c i a t i o n of COHb are complete i n much l e s s than 7 nsec, despite the evidence that the p r o t e i n s t r u c t u r a l r e l a x a t i o n s take place i n longer times (vide supra, equations 2,3). Two points are c l e a r from t h i s r e s u l t : f i r s t , the proposed s t e r e o chemical t r i g g e r f o r coopérâtivity i n hemoglobin (the heme s t r u c t u r e change) i s temporally decoupled from the p r o t e i n r e o r g a n i z a t i o n s which i t purportedly t r i g g e r s ; and, second, the nonequilibrium g l o b i n s t r u c t u r e i n photodissociated Hb^ ^xy apparently exerts no s i g n i f i c a n t t r a n s i e n t c o n s t r a i n t upon the heme s t r u c t u r e (at l e a s t , on the timescale which we are able to observe). Concerning the second p o i n t , i t has been c l e a r l y demonstrated (26) that s t a t i c g l o b i n c o n s t r a i n t s r e s u l t i n no resonance Raman-detectable d i s t o r t i o n of the heme group of carp hemoglobin (which can e x i s t i n the Τ or R p r o t e i n conformations, independent of the l i g a t i o n s t a t e of i t s hemes). I t was conceivable, however, that such an e f f e c t of g l o b i n c o n s t r a i n t might be observed i n an experiment s e n s i t i v e to the dynamics o f the s t r u c t u r a l r e o r g a n i z a t i o n s . Our r e s u l t s i n d i c a t e that no such dynamic e f f e c t occurs. The i m p l i c a t i o n s of these r e s u l t s i n questions of hemoglobin c o o p e r a t i v i t y are discussed elsewhere (21). For the present purposes, our r e s u l t s c l e a r l y demonstrate the power of TR as a s t r u c t u r e probe f o r t r a n s i e n t s . e

H b

e

X y

a

n

s

s

d

1

1

Q

Acknowledgement. This work was supported by NSF Grants CHE77-15220 and CHE78-09338.



12.


235

L i t e r a t u r e Cited 1.

S p i r o , T.G. and Loehr, T.M. in "Advances in I n f r a r e d and Raman Spectroscopy", V o l . 1, C l a r k , R.J.H. and Hester, R.E., Eds., Heyden, London, 1975, Chapter 3.

2.

Woodruff, W.H. and Spiro, T.G., Appl. Spectrosc., (1974) 28 576.

3.

Hester, R.E., Grossman, W.E.L., and Ernstbrummer, E. i n "Proceedings of the 5th I n t e r n a t i o n a l Conference on Raman Spectroscopy", p. 13, Schmid, E.D., et al., Eds., Schulz V e r l a g , F r e i b u r g , 1976.

4.

Jeanmarie, D.L., Suchanski M.R. and Van Duyne R.P. J . Am. Chem. Soc. Van Duyne, R.P., See a l s o more recent r e p o r t s in t h i s s e r i e s by Van Duyne and co-workers.

Delhaye, M. in "Proceedings of the Fifth I n t e r n a t i o n a l Conference on Raman Spectroscopy", p. 747, Schmid, E.D., al., Eds., Schulz V e r l a g , F r e i b u r g (1976).

5. et

6.

Woodruff, W.H. and Atkinson, G.H., A n a l . Chem., (1976) 48 186.

7.

Bridoux, M., D e f f o n t a i n e , Α., and R e i s s , C., Compt. Rend. C., (1976) 282 771.

8.

Campion, Α., Terner, J . , and El-Sayed, M.A., Nature, 265 659.

9.

(a) Wilbrandt, R., Pagsberg, P., Hansen, K.B., and Weisberg, C.V., Chem. Phys. L e t t . , (1975) 36 76; (b) Pagsberg, P., Wilbrandt, R., Hansen, K.B., and Weisberg, K.V., ibid., (1976) 39 538.

10. et

(1977)

P e t i c o l a s , W.L. in "Proceedings of the Fifth International Conference on Raman Spectroscopy", p. 163, Schmid, E.D., al., Eds., Schulz V e r l a g , F r e i b u r g , 1976; P e t i c o l a s , W.L., personal communication, 1978.

11.

Lyons, K.B., C a r t e r , H. L., and F l e u r y , P.A. in "Light S c a t t e r i n g i n S o l i d s " , B a l k a n s k i , M., L e i t e , R.C.C., and Porto, S.P., Eds., p. 244, Flammarion, P a r i s , 1976.

12.

Woodruff, W.H. and Farquharson, 1389.

S., A n a l . Chem., (1978) 50



236

13.

Nestor, J.R., P r i n c e t o n U n i v e r s i t y , personal communication, 1974.

14.

Woodruff, W.H., Pastor, R.W., Chem. Soc., (1976) 98 7999.

15.

Bloembergen, Ν., Amer. J . Phys., (1967) 35 989.

16.

Maier, Μ., Appl. Phys., (1976) 11 209.

17.

A n t o n i n i , E. and B r u n o r i , Μ., "Hemoglobin and Myoglobin i n t h e i r Reactions with Ligands", North-Holland, Amsterdam, 1971.

18.

Hoard, J.L. in "Heme d Hemoproteins" Chance B. Estabrook, R.W., an New York, 1966, pp Hamor, T.Z., and Caughey, W.S., J . Am. Chem. Soc., (1965) 87 2312.

19.

Perutz, M.F., Nature, (1970) 228 726.

20.

Shank, C.V. and Ippen, E.P., and Bersohn, R., Science, (1976) 193 50.

21.

Woodruff, W.H. and Farquharson, S., Science, (1978) 201 831.

22.

A l p e r t , B., Banerjee, R., and L i n d q v i s t , L., Proc. Nat. Acad. S c i . USA, 71, 558 (1974).

23.

Sawicki, C.A. and Gibson, Q.H., J. Biol. Chem., (1976) 251 1533.

24.

Ferrone, F.A. and H o p f i e l d , J . J . , Proc. Nat. Acad. S c i . USA, (1976) 73 4497.

25.

S p i r o , T.G. and Strekas, T.C., J. Am. Chem. Soc., (1974) 96 338.

26.

S c h o l l e r , D.M. and Hoffman, B.M. in "Porphyrin Chemistry", Longo, F.R., Ed., 1978, Ann Arbor Sciences.

27.

Spiro, T.G. and Burke, J.M., J. Am. Chem. Soc., (1976) 98 5482.

and Dabrowiak, J.C., J. Am.

RECEIVED September 11, 1978.


INDEX A

Absorption interferences at emission wavelengths 42 interferences at excitation wavelengths 40 saturated 22 studies of SF , double resonance .... 20 two-photon 27 process 35 Acetone, spectral data for PPO in 46,47 Acid, sialic 10 Aflatoxin-free sample of yellow Aflatoxin Bi 84, 87 in contaminated corn, detection of .80, 83 Aflatoxins B , d , and G 84 Aggregation Ill Albumin 106 Analysis (es) in absorbing solvents,fluorimetric.. 45 in complex samples, fluorimetric ... 38 in complex samples via two-photon spectroscopy, quantitative 38 Analyte 1 Analytes associated with lanthanides 5 Analytical calibration curves 66-69 Analytical problems 186 Anemometry, laser Doppler 121 Angles, scattering 104 Antibodies 106 Argon ion laser 130 Aspergillus fungi 80 Atomic fluorescence flame spectrometry 60 laser-excited 61—63 furnace spectrometry 73 spectrometry, pseudo-continuum laser excited 51 Autocorrelator 104 6

2

2

Β

Benzene, Raman parameters for 210 Biological surfaces 102 Bipyridine, absorption spectrum of .... 41 bisMSB, emission spectrum of 43 bisMSB, excitation spectrum of 35, 43 Bloodcell(s) 104 red 106 white 109,112 Blood plasma, human 106,107

2

Ca 115 Carbon tetrachloride, Raman spectra of 222, 223 Carbonmonoxyhemoglobin, timeresolved resonance Raman spectra of photodissociated 233 Carboxyhemoglobin tetramers 107 Carcinogens 80 CARS ( see Coherent anti-Stokes Raman scattering) adhesion I l l , 114 Β 106,108 electrochemical 132 photoresistor 104 photovoltaic 104 Τ 106,108 Chamber, electrophoretic light-scattering 105 Chromaffin granules 115,116 Clinical test 106 CO-hemoglobin, photodissociation of 227 Coherent anti-Stokes Raman scattering (CARS) experiment 180 description of 172 resonance 187 signal, coherence of 173 signal, frequency dependence of ... 179 spectroscopy 171 Concentration, C 0 15 Concentration for a tunable diode laser system, minimum detectable 16 Condon-Franck factors 187 "Continuum," anti-Stokes 198 Convolution 119 Corn, aflatoxin-free sample of yellow 88 Corn, aflatoxin Bi detection in contaminated 80, 83 Cottrell plot of current—time data 161 Current(s) AC-coupled photoemission 145 photo-related 143,147-149,156,157 for chopped CW laser 154,155 DC-coupled 153 transient 150 -time data, Cottrell plot of 161 transient photoemission-related 151 1 4

2

239


240

NEW


CHEMISTRY

CW excitation of resonant solutes: Fe ( II ) concentration ( s ) cytochrome c 226 dependence to theory, comparison CW laser, chopped 'of ~. 164 intensity 143 from flash photoreduction, irradiation 141 determination of 160 photo-related current for 154, 155 vs. pathlengths 162 source 142 on pathlength, dependence of 163 results with 138,146 Fe(III) oxalate system 129 Cytochrome c Fibroblasts 114 CW excitation of resonant solutes .. 226 Flame(s) resonance Raman spectra of 228, 229 laser-enhanced ionization for trace conventional and time-resolved 230, 231 metal analysis in 91 TR detection of nonreacting solutes 226 spectrometry 64, 65 atomic fluorescence 60 laser-excited 61-63 D techniques, L E I and other 95 Debye-Hiickel constant I l l Flashlamps, pulsed xenon 127 Deoxyhemoglobin, time-resolved resonance Raman spectra o Detection limits, comparative 9 Detection, low level 12 Fluctuations 106 Determinations, trace 205 Fluorescence Diffusion 104 correlation spectroscopy 121 Dirac theory 33 emission profiles of sodium D lines 72 Dissociation, photo 22 excitation profiles of sodium D lines 72 Doppler excitation spectrum 70 effect 102,103 two-photon excited molecular 24 -free spectroscopy 22 Fluorimeter velocimetry, laser 102 laser 81 Dropping mercury electrode (DME) 127 response 84 Dye(s) 30 signal 87 laser 91,199 two-photon 29 nitrogen pumped 130 Fluorimetry data 44, 47 source, results with pulsed 144 Fluorimetry, laser 80 synchronously pumped 31 excited 74,75 Franck-Condon factors 187 Ε Frequency dependence of the CARS signal 179 Electrode 105 80 dropping mercury ( D M E ) 127 Fungi, Aspergillus 64, 65 Electrophoresis 102 Furnace spectrometry atomic fluorescence 73 Electrophoretic light scattering 102 apparatus 103 chamber 105 G spectrum 107,109 Electrophoretic mobility 105,115 Gallium 71 distributions 109-113 Globulin 106 Electrostatic forces I l l Granules, chromaffin 116 Energy Granulocytes Ill conversion, solar-to-chemical 126 Grating ghosts 217 conversion, solar-to-electrical 126 level diagram 183 H for naphthalene, electronic 37 Erythrocytes, rabbit and human 109 He-Ne laser 104 Exocytosis 116 Heme group 234 Hemoglobin 105 F —CO, photodissociation of 227 FAST (see Fluctuation analysis Herzberg-Teller coupling 187 spectroscopic techniques ) Hiickel-Debye constant Ill 3


241

INDEX

Laser(s) (continued) I photolysis studies Impulse-response function 119 instrumentation for 134 Inductively coupled plasma 64, 65 procedures for 135 Instrumentation, analytical 13 quantum yield 128 Ion(s) source, results with pulsed 152 lanthanide 1 dye 144 laser, argon 130 synchronously pumped CW dye ... 28 luminescence, selective excitation synchronously pumped dye 31 of probe 1 tunable diode 12 Ionization for trace metal analysis in pollution monitoring system, flames, laser-enhanced 91 diagram of 14 spectroscopy 12 system, minimum detectable J concentration for 16 Joule heating 104 transmission spectrum 17,18 LEI figure of merit, factors L

Lamb dip spectroscopy 22 Lanthanide(s) analysis 3 analytes associated with 5 ions 1 Laser(s) applications in photoelectro chemistry 126 argon ion 130 chopped CW intensity 143 irradiation 141 photo-related current for 154, 155 source 142 results with 138,146 Doppler anemometry 121 Doppler velocimetry 102,121 dye 91,199 -enhanced ionization for trace metal analysis in flames 91 excitation molecular luminescence spectrometry, narrow line 54 excitation, selective 2 -excited atomicfluorescenceflame spectrometry 61-63 atomicfluorescencespectrome try, pseudo-continuum 51 fluorimetry 74,75 luminescence spectrometry 50 multiple photon 59 narrow-line 58 feedback stabilized 82 fluorimetry 80, 81 inducedfluorescence( LIF ) spectrometer 100 induced reaction 22 intensity, pulsed 150,156,157 Ne-He 104 nitrogen pumped dye 130

Leukemia 108, 111 LIF (Laser-induced fluorescence) 100,114 Light scattering, electrophoretic 102 apparatus 103 chamber 105 spectrum 107,109 Linear response theory 119 Liquids, pure 220 Luminescence lifetime measure ment, FAST 121 Lymphocytes 106,108 Lymphokines 114 Lysozyme, nonresonant solutes 221 Lysozyme, normal Raman spectra of 225 M +2

Mg 115 Mg-ATP 115 Manganese 71 Measurement of transient chemical events 118 Merit, figure of 97 Metal analysis inflames,laserenhanced ionization for trace 91 Metal cations, transition 146 Mobility, electrophoretic 105 Molecular beams 22 state selective excitation of 22 Molecular luminescence spectrometry 76 narrow line laser excitation 54 Multiplex advantage, spectral 216 Ν

N 0 solutions N 0 solutions Ne-He laser Naphthalene, electronic energy level diagram for 3

2


138 138 104 37

NEW

242

Naphthalene spectrum one-photon excitation two-photon excitation Nestor design Neuraminidase Neurotransmitters Nitrobenzene, Raman spectra of Nitrogen-pumped dye laser

36 39 41 217 108, 111 114 224 130

Ο

Optics, linear 25 Optics, nonlinear 24, 25,172 Optogalvanic effect 91 Oscillator beam, local 102,104 Oxyhemoglobin, time-resolved reso nance Raman spectra of 233 Ρ

Pathlength, dependence of Fe(II) concentration on 163 Pathlength vs. Fe(II) concentration .. 162 Perturbation diagrams, time ordered 180,181 Photocurrents 136 Photochemistry 12 IR 19 Photodissociated carbonmonoxyhemoglobin, time-resolved resonance Raman spectra of 233 Photodissociation of CO-hemoglobin 227 Photoelectrochemistry, laser applications in 126 Photoemission current 139 AC-coupled 145 Photoemission studies 126 discussion of 158 instrumentation for 130 procedures for 133 results of 136 Photolysis studies, laser instrumentation for 134 procedures for 135 quantum yield 128 Photomultiplier tube 104 Photon counting 104 oneexcitation spectrum of naphthalene 39 resonance enhancement 171 twoabsorption 27 process 35 excitation spectrum of naphthalene 41 excited molecularfluorescence.... 24 fluorimeter 29


CHEMISTRY

Photon, two- ( continued ) spectroscopy 28 qualitative analysis via 33 quantitative analysis in complex samples via 38 transition moment 33 Photoreduction, determination of Fe(II) concentration from flash .. 160 Photoresistor cell 104 Photovoltaic cell 104 Plasma, inductively coupled 64, 65 Plasma membrane vesicles 115 Plateau's unduloid 81 Polargram, DC 140 Polargraph interface 132 Polaritons, study of 211 Polarization 158 Pollution monitors, air 12 PPO in acetone, spectral data for 46, 47 PPO, excitation and emission spectra for 43 Pressure determination, partial 15 Pressures, partial vapor 19 Properties, temporal 30 Proteins 104,234 Q

Quantum yields, experimentally determined

166

R Raman cross-sections, determination of 208 effect, stimulated 220 parameters for benzene 210 scattering coherent anti-Stokes ( see _ Coherent anti-Stokes Raman scattering ) inverse (SIRS) 200 normal (RS) 200 spectroscopy, coherent anti-Stokes 171 spectroscopy by inverse (SIRS).. 193 stimulated (SRS) 200 spectra acquisition of 216 of carbon tetrachloride 222, 223 of cytochrome c, resonance 228-231 of lysozyme, normal 225 of nitrobenzene 224 time-resolved resonance of deoxyhemoglobin 233 of oxyhemoglobin 233 of photodissociated carbonmonoxyhemoglobin 233


INDEX

243

Raman (continued) spectrography, vidicon spectroscopy, time-resolved resonance (TR ) Rayleigh scattering Reaction transients, study of Repulsions, coulombic Resolution Response function 3

S

Spectroscopy ( continued ) site selective 2 time-resolved resonance Raman (TR ) 215,232 215,232 tunable diode laser 12 174, 221 two-photon 28 212 qualitative analysis via 33 114 quantitative analysis in complex 104 samples via 38 119 Spectrum(a) analyzer 102,104 of bipyridine, absorption 41 of bisMSB, emission 43 21 of bisMSB, excitation 35, 43 114 electrophoretic light-scattering ...107,109 108 excitation 7,9 87 fluorescence excitation 70 72 215

SF Secretion, vesicular Sialic acid Signal, fluorimeter Sodium D lines, profiles of Solutes cytochrome c, CW excitatio resonant 226 cytochrome c, TR detection of nonreacting 226 nonresonant: lysozyme 221 Solvents,fluorimetricanalyses in absorbing 45 Spectral coverage 30 Spectrography, vidicon Raman 215 Spectrometer, laser induced fluorescence (LIF) 100 Spectrometry atomic fluorescence flame 60 atomicfluorescencefurnace 73 flame 64,65 furnace 64, 65 laser-excited atomic fluorescence flame 61-63 atomicfluorescence,pseudocontinuum 51 luminescence 50 multiple photon 59 narrow line 58 molecular luminescence 76 narrow-line laser excitation 54 Spectroscopic techniques, fluctuation analysis (FAST) (see also Fluctuation analysis spectro scopic techniques) 119 Spectroscopy of aqueous solutions, IR 19 coherent anti-Stokes Raman scattering (CARS) 171 Doppler-free 22 fluorescence correlation 121 high resolution 212 by inverse Raman scattering (SIRS) 193 Lamb dip 22 nonlinear 12,19 G

3

3

Raman acquisition of 216 of carbon tetrachloride 222, 223 of cytochrome c 228-231 of deoxyhemoglobin, timeresolved resonance 233 of lysozyme, normal 225 of nitrobenzene 224 of oxyhemoglobin, timeresolved resonance 233 of photodissociated carbonmonoxyhemoglobin, timeresolved resonance 233 of p-terphenyl 41 tunable diode laser transmission .17,18 vibrational 215 Strength, ionic 104, 111, 114 Symmetry considerations 34 Τ

Teller-Herzberg coupling 187 p-Terphenyl 43 excitation and emission spectra of .. 41 Tetramers, carboxyhemoglobin 107 Theory comparison of Fe(II) concentration dependence to 164 Dirac 33 linear response 119 Thermal perturbation 159 Time -current data, Cottrell plot of 161 -ordered perturbation diagrams 180,181 -resolved resonance Raman ( TR ) apparatus 218 detection of nonreacting solutes: cytochrome c 226 detection of transients 227 spectroscopy 215,232


3

NEW

244

Time (continued) response 123 function 119 TR (see Time-resolved resonance Raman) Transfer function 119 Transformation, viral 114 Transients(s) chemical events, measurement of .. 118 photoemission-related current 151 photo-related current 150 TR detection of 227 Transition moment, two-photon 33 Tunability 158 3

3

CHEMISTRY

V

van der Waals forces Velocimetry, laser Doppler Vesicles Vidicon Raman spectrography

114 121 114 215

W

Wave mixing, fourWave mixing, threeWavelengths, absorption interferences at emission Wavelengths, absorption interferences at excitation X

U

Urea treatment


11


26 171 42 40

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