Topics in Applied Physics Volume 2

Transcription

1 Topics in Applied Physics Volume 2

2 Topics in Applied Physics Founded by Helmut K. v. Lotsch Volume 1 Dye Lasers Editor: F. P. Schăfer Volume 2 Volume 3 Laser Spectroscopy of Atoms and Molecules Editor: H. Walther Numerical and Asymptotic Techniques in Electromagnetics Editor: R. Mittra Volume 4 Interactions on Metal Surfaces Editor: R. Gomer Volume 5 M08sbauer Spectroscopy Editor: U. Gonser Volume 6 Picture Processing and Digital Filtering Editor: T. S. Huang Volume 7 Integrated Optics Editor: T. Tamir Volume 8 Light Scattering in Solids Editor: M. Cardona Volume 9 Volume 10 Laser Speckle and Related Phenomena Editor: J. C. Dainty Transient Electromagnetic Fields Editor: L. B. Felsen Volume 11 Digital Picture Analysis Editor: A. Rosenfeld Volume 12 Turbulence Editor: P. Bradshaw Volume 13 Volume 14 Volume 15 High-Resolution Laser Spectroscopy Editor: K. Shimoda Laser Monitoring of the Atmosphere Editor: E. O. Hinkley Radiationless Processes in Molecules and Crystals Editor: F. K. Fong

3 Laser Spectroscopy of Atoms and Molecules Edited by H. Walther With Contributions by F.A.Blum J.M.Cherlow B.Decomps M.Ducloy M.Dumont K.M.Evenson E. D. Hinkley K. W. Nill F. R. Petersen S. P. S. Porto K. Shimoda H. Walther With 137 Figures Springer-Verlag Berlin Heidelberg GmbH 1976

4 Professor Dr. HERBERT WALTHER Sektion Physik der Universităt Munchen D-8046 Garching, Am Coulombwall 1, Fed. Rep. of Germany ISBN ISBN (ebook) DOI / Library of Congress Cataloging in Publication Data. Main entry under ti tie: Laser spectroscopy of atoms and molecules. (Topics in applied physics; v. 2). lncludes bibliographical referenccs and index. 1. Laser spectroscopy. 2. Atomic spectra. 3. Molecular spectra.!. Walther, Herbert, Il. Blum, Fred A.,l939-. QC4S4.L3L37 S3S.S' This work is subiect to copyright. Ali rights are reserved, whether the whole or part of the material is concerned, specifically those of translation, reprinting. re-use of illustrations, broadcasting, reproduction by photocopying machine or similar means, and storage in data banks. Under S4 ofthe German Copyright Law where copies are made for other than private use, a fee is payable to the publisher, the amount of the fee to be determined by agreement with the publisher. by Springer-Verlag Berlin Heidelberg 1976 Originally published by Springer-Verlag Berlin Heidelberg New York in Softcover reprint ofthe hardcover lst edition 1976 The use ofregistered names. trademarks, etc. in this publication does not imply, even in the absence of a specific statcmcnt, that such namcs are exempt from the relevant protective laws and regulations and thercfore free for general usc. Monophoto typesetting, offset printing, and bookbinding: Brilhlsche Universitătsdruckerei, Giessen

5 Preface When the laser was invented one and a half decades ago it became obvious at once that this new device would be a unique light source for many spectroscopic applications. Spectroscopists were initially engaged in looking for new laser materials having transitions suita bie for laser action. Many new optica! transitions were found in this way and a large amount of new spectroscopic data especially in the infrared spectral region was gathered. Concurrently the interaction of intense laser radiation with matter was investigated, and many nonlinear effects explored. At the same time useful spectroscopic information was obtained by studying Raman spectra in solids, liquids and gases, using the laser as source of intense light. For the spectroscopy of atoms and molecules at this time the biggest step forward was the discovery of the "Lamb-dip". It opened up the possibility of eliminating the Doppler width and achieving a resolution corresponding to the natural line width. The applications of this phenomenon were more or less restricted to the investigation of the laser transitions themselves or to transitions in molecules having an accidental coincidence with laser lines. However, it was recognized soon that the phenomenon could play a significant role in the frequency stabilization of lasers. The corresponding techniques were developed rather rapidly; they have now reached such a state of development that the frequency stabilization of lasers has become extremely important for setting metrological standards. The steadily increasing number of available lasers and laser lines made it possible to use the accidental coincidences with electronic transitions of molecules in order to study molecular fluorescence. With pulsed lasers the relaxation of molecular vibrations has been studied. Meanwhile also techniques for the generation of pulses of picosecond duration were developed so as to open up relaxation studies in liquids and solids. Furthermore, the coherent optica! field provided by the laser was used to investigate a variety of optica! analogues of phenomena which previously have only been observed in magnetic resonances.

6 VI Preface A large widening of the application of lasers to spectroscopy was initiated in the late sixties when methods to frequency-tune laser radiation were found. Tunable lasers are now available from the near ultraviolet to the far infrared, as illustrated by the following diagram, so that the spectroscopic means for investigating atoms and molecules are almost unlimited oye Laser Diode Laser High Pressure Gas Laser 1.IIR 1111_1.11 Parametric Oscillator 1 Sum and Difference Frequencd Four-Wave Parametric Mixing 1 IIIIR ipin-fiip Raman Laser - H Dye LaserlPu~-~~n Laser 1 Polariton Laser Fower[W] cw pulsed n o' n tl n-1 1 Il' - 3-1)4 100 W::!velength [JJm 1 Available tunable laser sources [taken from the pa per of J. KuHL and W. ScHMIDT: Appl. Phys. 3, 251 (1974)]. Details on the infrared tunable lasers are given in Chapter 2 by HINKLEY, NILL, and BLUM. For details on dye lasers see the first volume of this series. In the wavelength regions where the black bars are interrupted tunable lasers are still under development or proposed In an other direction, a very important development presently under way is the direct measurement of the frequency of laser light. Apart from its metrological significance, this means that in the future spectroscopists may be able to replace the relatively inaccurate wavelength measurement by- a frequency measurement, providing a precision several orders of magnitude higher than that of conventional methods. The development of new experimental possibilities during recent years makes the field. of laser spectroscopy more exciting than it ever bas been. This is a good time to review the main achievements during the past few years so as to bring to light where more research is possible and needed.

7 Preface VII The first two chapters of this volume give a survey on laser spectroscopy in the visible and infrared spectral regions. The following two deal with the application of important methods to molecular spectroscopy. The fifth and sixth chapters finally discuss the linear and nonlinear effects in optica! pumping, and the direct measurement of the light frequency. No attempt was made by the editor to suppress the individuality of the different contributions. Therefore different notations may have been used for the same physical quantities; in addition, there is some overlap in the subjects discussed in different articles. This bas the great advantage that the reader is confronted with problems from a different point of view. 1 would like to thank the contributors to the book for writing the articles which were prepared much quicker than that ofthe editor. 1 would like to thank Dr. LoTSCH of Springer-Verlag for help and advice, and for eliminating some of the typical "German" sentences in my contribution. In ihis connection also the corrections of Dr. W. RASMUSSEN have been quite valuable. Occasionally my coworkers Dr. K. W. RoTHE and Dr. J. HĂGER helped to collect literature. 1 would like to thank them for their valuable assistance. Koln, September 1975 HERBERT WALTHER

8 Contents 1. Atomic and Molecular Spectroscopy with Lasers. By H. WALTHER (With 32 Figures) l.l. Doppler-Limited Spectroscopy Absorption and Emission Measurements 2 Absorption Spectroscopy Emission Spectroscopy Lifetime Measurements and Relaxation Studies 13 Lifetime Measurements Molecular Vibrational Relaxation Studies Further Collision Studies Photoionization and Photodetachment 25 Photoionization 25 Photodetachment Nonlinear Effects 33 Multiphoton Ionization 33 Two-Photon Spectroscopy 37 Higher-Harmonic Generation and Nonlinear Mixing Processes in Atomic Vapors Distortion of Atomic Structure in High lntensity EM Fields Application to Chemistry and Isotope Separation Photochemistry Laser Isotope Separation Survey of Isotope Separation Methods by Means of Lasers Separation by Radiation Pressure 58 Photochemical Separation 59 Separation by Pre-Dissociation 59 Separation by Photo-Isomerization 62 Separation by Two-Step Photoionization or Dissociation Economic Aspects of Laser Isotope Separation Product State Analysis by Tunable Lasers 64

9 X Contents 1.3. Coherent Transient Effects Feynman-Vernon-Hellwarth Representation Photon-Echoes Other Coherent Transient Phenomena 70 Optical Nutation and Free Induction Decay 70 Coherent Raman Beats Two-Pulse Optica} Nutation and Multiple-Pulse Spin Echoes Superradiance Spontaneous Decay of Coherently Excited Atoms Natural Linewidth-Limited Spectroscopy High Resolution Spectroscopy with Broad-Band Excitation Optical-Radio-Frequency Double Resonance, OpticalPumping,Anti-Crossing Level-Crossing, Quantum Beats, Periodic Excitation High-Resolution Spectroscopy with Narrow-Band Excitation High-Resolution Spectroscopy by Means of Atomic and Molecular Beams 93 Fluorescence Line Narrowing Saturated Absorption Two-Photon Spectroscopy, Stepwise Excitation 105 References lnfrared Spectroscopy with Tunable Lasers. By E. D. HINKLEY, K. W. NILL, and F. A. BLUM (With 26 Figures) 2.1. Tunable Infrared Lasers Semiconductor Diode Lasers Spin-Flip Raman Lasers Nonlinear Optica} Devices. 134 Optica} Parametric Oscillator 134 Difference-Frequency Generator 135 Two-Photon Mixer 136 Four-Photon Mixer Gas Lasers Zeeman-Tuned Gas Laser 137 High-Pressure Gas Lasers Spectroscopic Techniques Resolution Capabilities of Tunable Lasers Laser Tuning Characteristics Experimental Techniques of Laser Spectroscopy 150

10 Contents XI Experimental Arrangement. 150 Calibration of Laser Tuning Rate. 151 Laser Modulation and Detection. 153 Heterodyne Detection of Infrared Emission Lines. 155 Nonlinear Spectroscopy Infrared Laser Spectroscopy Lineshapes, Linewidths, and Intensities. 161 Gaussian Profile-Doppler Broadening.161 Lorentzian Profile- Collision Broadening. 162 Collisional Narrowing Natural Linewidth and Saturation Spectroscopy. 166 Laser Gain Lineshapes Line Intensities Line Identification and Band Analysis Rotational Fine Structure. 176 A-Type Doubling in NO Nuclear Hyperfine Structure. 178 Zeeman Splitting Stark Splitting Spectroscopic Detection of Trace Gases- Application to Air Pollution Monitoring Point Sampling at Reduced Pressure. 183 In situ Monitoring by Resonance Absorption. 184 Single-Ended Remote Sensing Conclusion. 188 References Double-Resonance Spectroscopy of Molecules by Means of Lasers. By K. SHIMODA (With 35 Figures) 3.1. Theory of Double Resonance Semiclassical Theory Quantum-Mechanical Analysis Rate-Equation Approximation 3.2. Infrared-Infrared Double Resonance Doppler Effect in Infrared-lnfrared Double Resonance Unidirectional Amplification in Optically Pumped HF Gas Double Resonance within the Doppler Width Infrared-Microwave Double Resonance Infrared-Microwave Double Resonance in H 2 CO Experimental Results

11 XII Contents Theoretical Analysis Evaluation of Infrared Field Intensity Rotational Transitions of Vibrationally Excited Molecules Double-Resonance Experiments in HDCO Microwave Spectroscopy of HCOOH in the Excited State Observation of Very Weak Microwave Transitions in CH Modulation Doubling of the Inverted Lamb Dip Raman-Type Double Quantum Transition Relaxation and Transient Effects Lifetime Measurement of Laser-Excited Levels. 238 Relaxation Study of Molecular Laser Levels. 238 Time-Resolved Infrared Double Resonance Collision-Induced Transitions Double Resonance in Four Levels. 241 Time-Resolved Study ofrotational Energy Transfer. 244 Double-Resonance Experiments Using a Two-Photon Technique Coherent Transients in Double Resonance. 248 References Laser Raman Spectroscopy of Gases. By J. M. CHERWW and S.P.S.PORTO (With 12 Figures) 4.1. High Resolution Spectroscopy Techniques New Results Cross Sections and Depolarization Ratios References Linear and Nonlinear Phenomena in Laser Optica! Pumping. By B. DECOMPS, M. DUMONT, and M. DUCWY (With 27 Figures) 5.1. Experimental Features of Laser Optical Pumping of a Resonant Atomic Medium General Survey of the Observed Phenomena Experimental Apparatus. Internat Cell Configuration Externa! Cell Configuration Observation of Linear Effects Hanle Effect

12 Contents XIII Magnetic Resonance and Level-Crossing Experiments Modulation Phenomena at the Beat Frequency between Modes Nonlinear Effects Pure Nonlinear Effects Method for Obtaining Reliable Experimental Results with Linear Effects Semiclassical Equations: Hypotheses and Notations Density Matrix Excitation Matrix Hamiltonian Relaxation Spontaneous Emission Developed Equations Perturbation Method Iteration Linear Response of Atomic System 304 de Terms: Hanle Effect Modulated Terms Fourth-Order Effects: Saturation Resonances 309 Monomode Laser (Simplified Model: la=o, Jb= 1) 311 Multimode Laser (Simplified Model) 314 Behavior of the Saturation Resonances with a Multimode Laser Case la and JbtO: Lande g Factor Measurements Nonperturbative Theory Broad-Line Approximation Exact Calculation for a J = 1-J =0 Transition Saturation Experiments on J = 1-J =0 Transitions s 2-2p 1 Neon Transition (A.= 1.52 Jlm) 328 The 2s 2 Hanle Effect and Saturation Resonance Line Shapes Hanle-Effect Width and Saturation-Resonance Amplitude: Variations with Laser Intensity and Pressure s 2-2p 1 Neon Transition (A.= 7305 Â) Relaxation Rate of the 3s 2 Level 332 Pumping Rate of the 7305 Â Laser Line J a= 1 - J b = 1 Laser Transition Higher-Order Non! inear Effects in J = 1-J = 2 Transitions Laser-Induced Hexadecapole Moment in the Neon 2p 4 Level

13 XIV Contents Anomalous Behavior of the Saturation Resonance of the 2p 4 -+ ls 5 Fluorescent Line (A.= 5944 Â) Appendix 1. General Formula for the Fluorescence Light. The Standard Components of the Polarization Vector. 344 Appendix 2. General Formula for the Linear Response. 346 References Laser Frequency Measurements, the Speed of Light, and the Meter. By K. M. EVENSON and F. R. PETERSEN (With 5 Figures) 6.1. Stabilization of Lasers by Saturated Absorption Frequency Measurement Techniques High-Frequency Diodes The Speed of Light Methane Frequency Measurement Possible New Standard of Length. 364 References Additional References with Titles. 369 Subject Index

14 Contributors BLUM,F.A. Science Center, Rockwell International,Thousand Oaks, CA 91360, USA CHERLOW,JOEL M. Departments of Physics and Electrical Engineering, University of Southem California, University Park, Los Angeles, CA 90007, USA DECOMPS, BERNARD Universite Paris-Nord, Laboratoire de Physique des Lasers, Place du 8 Mai 1945, F-93206, St. Denis-Cedex 01, France DucLOY, MARTIAL Laboratoire de Spectroscopie Hertzienne de l'e.n.s., Associe au C.N.R.S., 24 rue Lhomond, F Paris-Cedex 05, France DUMONT, MI CHEL Universite Paris-Nord, Laboratoire de Physique des Lasers, Place du 8 Mai 1945, F-93206, St. Denis-Cedex 01, France EVENSON,KENNETH M. Precision Laser Metrology Section, National Bureau of Standards, Boulder, CO 80302, USA HINKLEY,E.DAVID MIT, Lincoln Laboratory, Lexington, MA 02173, USA NILL,K.W. Laser Analytics, Inc., Lexington, MA 02173, USA PETERSEN, F. RUSSEL Precision Laser Metrology Section, National Bureau of Standards, Boulder, CO 80302, USA

15 XVI Contents PoRTO, Sergio P. S. Departments of Physics and Electrica! Engineering, University of Southem California, University Park, Los Angeles, CA 90007, USA SHIMODA, KOICHI Dept. of Physics, University of Tokyo, Hongo Bunkyo-ku, Tokyo 113, Japan VVALTHER,HERBERT Sektion Physik der Universitat Munchen, D-8046 Garching, Am Coulombwall 1, Fed. Rep. of Germany (The book was prepared during H. W. was still at the Universităt Koln, Fed. Rep. of Germany)