Table Of Content35
Topics in
Current Physics
Topics in Current Physics Founded by Helmut K. V. Lotsch
Volume 39 Nonequilibrium Vibrational Kinetics
Editor: M. Capitelli
Volume 40 Microscopic Methods in Metals
Editor: U. Gonser
Volume 41 Structure and Dynamics of Surfaces I
Editors: W. Schommers and P. v. Blanckenhagen
Volume 42 Metallic Magnetism I
Editor: H. Capellmann
Volumes 1-38 are listed on the back inside cover
Multiple-Photon Excitation
and Dissociation
of Polyatomic Molecules
Edited by C. D. Cantrell
With 139 Figures
Springer-Verlag Berlin Heidelberg New York
London Paris Tokyo
Professor Dr. Cyrus D. Cantrell
Center for Applied Optics, University of Texas at Dallas, P.O. Box 830688,
Richardson, TX 75083-0688, USA
ISBN-13: 978-3-642-82294-0 e-ISBN-13: 978-3-642-82292-6
001: 10.1007/978-3-642-82292-6
Library of Congress Cataloging-in-Publication Data. Main entry under title: Multiple-photon excitation and
dissociation of polyatomic molecules. (Topics in current physics; 35.) Includes index. 1. Multiphoton pro
cesses. 2. Molecules. 3. Laser beams. I. Cantrell, C. D. (Cyrus D.), 1940-. QC793.5.P428M85
1986 539.7'217 85-17275
This work is subject to copyright. All rights are reserved, whether the whole or part of the material is con
cerned, specifically those of translation, reprinting, reuse of illustrations, broadcasting, reproduction by
photocopying machine or similar means, and storage in data banks. Under § 54 of the German Copyright
Law where copies are made for other than private use, a fee is payable to "Verwertungsgesellschaft
Wort", Munich.
© Springer-Verlag Berlin Heidelberg 1986
Softcover reprint of the hardcover 1s t edition 1986
The use of registered names, trademarks, etc. in this publication does not imply, even in the absence of a
specific statement, that such names are exempt from the relevant protective laws and regulations and
therefore free for general use.
2153/3150-543210
Preface
In the early 1970s, researchers in Canada, the Soviet Union and the United States
discovered that powerful infrared laser pulses are capable of dissociating mole
cules such as SiF4 and SF6' This result, which was so unexpected that for some
time the phenomenon of multiple-photon dissociation was not recognized in many cir
cumstances in which we now know that it occurs, was first publicized at a time when
the possibility of using lasers for the separation of isotopes had attracted much
attention in the scientific community. From the mid-1970s to the early 1980s, hun
dreds of experimental papers were published describing the multiple-photon absorp
tion of C02 laser pulses in nearly every simple molecule with an absorption band
in the 9 - 11 jJm region. Despite this impressive volume of experimental results,
and despite the efforts of numerous theorists, there is no agreement among re
searchers in the field on many fundamental aspects of the absorption of infrared
laser light by polyatomic molecules.
This book is devoted to reviells of the experimental and theoretical research
that provides the foundations for our current understanding of molecular multiple
photon exc itat i on, and to rev i ews of research that is pert i nent to the 1a ser sep
aration of isotopes. Although no book could provide complete coverage of the field
of molecular multiple-photon excitation and its applications, the reviews pub-
1 ished here provide much of the background required for researchers and graduate
students in the field to appreciate the current literature. This volume includes a
review of experimental measurements of multiple-photon absorption performed with
pulsed infrared lasers, with an emphasis on the universality of the general fea
tures of the dependence of energy absorpt i on and react i on probab i 1 i ty on 1 aser
fluence for different molecular species. Other chapters review molecular-beam ex
periments on multiple-photon excitation, the two-frequency method for obtaining
high isotopic selectivity, the observation of sharp multi photon resonances in the
SF6 molecule, rate-equation theories of molecular multiple-photon excitation, the
enhancement of isotopic selectivity through control of the laser-pulse shape, and
techniques for the calculation of laser-pulse propagation effects.
I thank all of the authors for their contributions and for their patience dur
ing the preparation of this volume.
v
The support of the United States National Science Foundation under Grants CHE-
8017324 and CHE-8215245, the United States Office of Naval Research under Grant
N00014-82-K-0628, and the Robert A. Welch Foundation under Grant AT-873, are grate
fully acknowledged.
Dallas, April 1986 C.D. Cantrell
VI
Contents
1. Molecular Multiphoton Excitation. By C. D. Cantrell (Wi th 1 Fi gure) ......... .
1.1 Introductory Comments ...........•.................................... 1
References .............•.......................... , ........•.. , . . . . . .. •. . . 7
2. Single-l nfrared-Frequency Studies of Multiple-Photon Excitation
and Dissociation of Polyatomic Molecules
By J.L. Lyman, G.P. Quigley, and O.P. Judd (With 34 Figures) ...•.......... 9
2.1 Overview .........•.........•......•..............•................... 9
2.2 Data-Reduction Techniques ............................................ 11
2.2.1 Absorption of Nonuniform Optical Beams by Nonlinear Media 11
2.2.2 Data-Analysis Techniques for Reaction Studies ............•..•. 16
2.3 A Quantitative Comparison of Absorption Data ......................... 19
2.3.1 Interaction Dynamics and Normalization Concepts ............... 20
a) General Considerations for MPE Processes ................... 20
b) Molecular Absorption in a Coupled Two-Level Approximation 22
c) Generalization to the MPE Processes ........................ 24
2.3.2 MPA in Different Polyatomic Molecules ......................... 25
a) Absorption Characteristics of Polyatomic Molecules ....•..•• 26
b) Calculation of <f> for SF6 ................................. 29
c) Discussion ............................... .................. 30
2.4 A Review of MPA Data for SF6 ......................................... 32
2.4.1 Low-Fluence Spectral Absorption .......................... ..... 33
2.4.2 Fluence-Dependent Absorption at 300 K . •.•••...•••••••••••.•••• 35
2.4.3 Fluence-Dependent Absorption at 140 K ......................... 40
2.4.4 Effect of Optical Pulse DUration and Shape in MPA Processes 41
2.4.5 Discussion and Summary....................... ................. 43
2.5 A Comparison of Multiple-Photon Dissociation Data ................•..• 44
2.5.1 Experimental Techniques ....................................... 45
2.5.2 Molecular Properties ............................... ........... 48
2.5.3 Comparison of MPD Results ..................................... 51
2.5.4 Concluding Remarks ............................................ 61
2.6 Collisional Effects in Multiple-Photon Absorption and
Dissociation Processes 61
VII
2.6.1 Collisional Effects on Multiple-Photon Absorption 63
2.6.2 The Influence of Collisions on Multiple-Photon
Dissociation 68
2.6.3 Collisional Effects in Laser-Induced Isomerization
Reactions and Molecular Elimination 69
2.6.4 Secondary Processes 70
2.7 Conclusion 71
2.8 Appendix 72
References 87
3. Molecular-Beam Studies of Laser-Induced Multiphoton Dissociation
By A.S. Sudb~, P.A. Schulz, Y.R. Shen, and Y.T. Lee
(With 18 Figures) ......................•................................. 95
3.1 0jverview ...........................................................• 95
3.2 Theory ............•..•...........•.................................. 97
3.3 Experimental Arrangement ............................................ 106
3.4 Experimental Resul ts ................................................ 108
3.5 Discussion 113
3.6 Concluding Remarks 120
References 121
4. Two-Frequency Technique for Multiple-Photon Dissociation and
Laser Isotope Separation. By R.V. Ambartzumian (With 20 Figures) ........ 123
4.1 Background ..............•........................................... 123
4.2 Basic Concepts of Two-IR-Frequency Dissociation 125
4.3 Selectivity of Dissociation 126
4.3.1 Spectral Measurements 126
4.3.2 Isotope Enrichment Experiments ...............••.............. 129
4.4 Investigation of Multiple-Photon Excitation ......................... 133
4.4.1 Evaluation of q and 1 in Os04 .......•........................ 134
4.5 Interaction of the Nonresonant Pulse with Excited Molecules 138
4.5.1 Absorption Measurements ..•..•................................ 138
4.5.2 Effects of Variation of ")2 ...............................••.. 141
4.6 Concluding Remarks 146
References 146
5. Excitation Spectrum of SF6 Irradiated by an Intense IR Laser Field
By S.S. Al impiev, N. V. Karlov, E.M. Khokhlov, S.M. Nikiforov,
A.M. Prokhorov, B.G. Sartakov, and A.L. Shtarkov (With 7 Figures) 149
5.1 Background 149
5.2 The Experiment 150
VIII
5.3 Experimental Results and Discussion 153
5.4 Conclusion •........•.................•........•..................•.• 157
References .•... .. . . .. . . .. . . .•. .. . .. . . . .. . . .. . .. . . .. .• .. . . ... . . . .. ... ...•. 157
6. Laser-Induced Decomposition of Polyatomic Molecules:
A Comparison of Theory with Experiment
By M.F. Goodman, J. Stone, and E. Thiele (With 23 Figures) ..........•... 159
6.1 Brief Historical Review .......................•..................... 159
6.2 Models of a Complex Polyatomic Molecule •...•..•..•.•.......••...•••• 160
6.2.1 Laser Excitation Mechanisms .................................. 160
6.2.2 The Heat Bath Feedback Model ...........•.................•••. 163
6.2.3 Some Microscopic Aspects of the Theory •...•..•...........•.•• 167
6.3 Comparisons of Theory with Experiment ...............•.........••••.. 169
6.3.1 Laser-Induced Decomposition of CF2HCl ....................•... 169
a) Coherent Discrete Level Pumping and Interface with
Rate Equation ..•..•......•....................•.•.....•••. 170
b) Rotational Hole Filling by Collisions 172
c) A Comparison of Theory with Experiment for CF2HCl ....•••.. 173
d) Additional Theoretical Predictions ....•............•.••..• 174
6.3.2 Laser Intensity Versus Fluence Effects ....................... 175
6.3.3 High-Pressure Fallof Reaction Rate .......................•... 177
6.3.4 Laser-Induced Decomposition of SF6 179
a) Discrete Level Spectrum and Tuning Curve 179
b) Dissociation Yield and Cross Section 182
6.3.5 Laser Frequency Effects 183
6.4 Effects Specific to Laser Excitation .........................•....•. 185
6.4.1 Energy Distribution: Laser Versus Thermal ...•............•.•• 186
6.4.2 Microstate Formalism ................................•...•.•.. 190
a) Intramolecular Relaxation .•................•.............• 190
b) Relation to RRK and RRKM Theory .......................•.•. 191
c) A Discriminating Reaction Mechanism ....................... 193
d) Multiple Reaction Channels and Product Selectivity ..•.•... 196
6.5 Phase Coherence and the Transition to Incoherent Pumping 198
6.5.1 The Generalized Master Equation 200
6.5.2 Fermi, Golden Rule Considerations 205
6.5.3 Coherent Quasi-Continuum Pumping - an Open Question .•....•••. 208
6.6 Addendum . . . . . • . . . . . . . . . . . . . . . . . . . . . • . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 209
References 211
IX
7. A Method of Laser Isotope Separation Using Adiabatic Inversion
By G.L. Peterson and C.D. Cantrell (With 6 Figures) .•..•.•••••••••••••••. 215
7.1 Background •.•• ••.• . •• • ••. . . .• • • •. . . . • • •• •• • • •. • . ... • •• • •• . .• • ••• •• •• 215
7.2 Physical Principles .•...•...•.••...••••••••••.••..•••••••....••.•••. 216
7.3 Multilevel Systems Under the Adiabatic Approximation •.•••••••••••••. 218
7.4 Conditions for Effective Adiabatic Laser Isotope Separation 219
References •.• , •••••.•••.• , ••.......••••.•••••••••••• , ..• , • • ••• •••. • •• •. .. 221
8. Three-Level Superfluorescence. By F.P. Mattar, P.R. Berman, A.W. Matos,
Y. Claude, C. Goutier, and C.M. Bowden (With 30 Figures) •.••••.••.••••••• 223
8.1 Background .••••••• " •• " •••••••..•.•••..•.••••• , ..• , ••••••••.•••••.• '224
8.2 Pump Dynamics Effects in Three-Level Superfluorescence ..••.•...•••.. 231
8.3 Semiclassical Equations of Motion and Computational Method •••••••••• 233
8.4 Deterministic Effects of Pump Dynamics in the Nonlinear Regime
of Superfl uorescence • . • • • • • • . . • • • • • • . . . • • • . . . • . • • • . • . • • • • • • . • • . • • • •• 240
8.5 Conclusions Concerning Deterministic Three-Level Superfluorescence •• 262
8.6 Quantum Initiation: Calculational Results and Delay-Time Statistics 262
8.7 Conclusions Concerning Effects of Resonance Diffraction and
Copropagation on Superfluorescence Evolution 266
8.8 The Role of DisperSion ••••••.•.••••....•.....••••....••••••.•..•..•• 268
8.9 Conclusions Concerning the Mutual Influence of Dispersion and
Diffraction on the Superfl uorescence Buil dup •••••..•.•.••••••..••••. 276
8.10 Final Conclusion .•••••.••••.•••••••..••.••....••••..••.••••.•.••..•• 276
References • • • . . • . . • • . • • • . • . • • . • • • • • . . • • • • . . . • • • • • • • • • • • • • • . . • • • . . • • . • . . •• 277
Additional References •••..••.••.•••.•••••••••••..••••••.•.•.••••••.•••.•• 281
Subject Index ................................................................ 285
x
