Springer-Verlag Berlin Heidelberg GmbH. Physics and Astronomy. Advanced Texts in Physics

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1 Laser Spectroscopy

2 Advanced Texts in Physics This program of advanced texts covers a broad spectrum of topics which are of current and emerging interest in physics. Each book provides a comprehensive and yet accessible introduction to a field at the forefront of modern research. As such, these texts are intended for senior undergraduate and graduate students at the MS and PhD level; however, research scientists seeking an introduction to particular areas of physics will also benefit from the titles in this collection. Springer-Verlag Berlin Heidelberg GmbH Physics and Astronomy ONLINE LlBRARY

3 Wolfgang Demtroder Laser Spectroscopy Basic Concepts and Instrumentation Third Edition With 7lO Figures, 16 Tables 93 Problems and Hints for Solution, Springer

4 Professor Dr. Wolfgang Demtröder Universität Kaiserslautern Fachbereich Physik Erwin-Schrödinger-Strasse Kaiserslautern, Germany LibraryofCongress Cataloging-in-Publication Data: Demtröder, W. Laser spectroscopy: basic concepts and instrumentation/ Wolfgang Demtröder. - 3rd ed. p. cm. ISBN (alk. paper) 1. Laser spectroscopy.l. Tide. QC 454.L3 D '6-dc ISSN This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer-Verlag. Violations are liable for prosecution under the German Copyright Law. ISBN ISBN (ebook) DOI / Springer-Verlag Berlin Heidelberg 1981, 1996, 2003 Originally published by Springer-Verlag Berlin Heidelberg New York in Softcover reprint ofthe hardcover 3rd edition 2003 The use of general descriptive names, 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. Typesetting: Data conversion by Fa. Le-TeX, Leipzig Cover design: design & production GmbH, Heidelberg Printed on acid-free paper SPIN /31u/ba

5 Preface to the Third Edition Laser Spectroscopy continues to develop and expand rapidly. Many new ideas and recent realizations of new techniques based on old ideas have contributed to the progress in this field since the last edition of this textbook appeared. In order to keep up with these developments it was therefore necessary to indude at least some of these new techniques in the third edition. There are, firstly, the improvement of frequency-doubling techniques in external cavities, the realization of more reliable cw-parametric oscillators with large output power, and the development of tunable narrow-band UV sources, which have expanded the possible applications of coherent light sources in molecular spectroscopy. Furthermore, new sensitive detection techniques for the analysis of small molecular concentrations or for the measurement of weak transitions, such as overtone transitions in molecules, could be realized. Examples are Cavity Ringdown Spectroscopy, which allows the measurement of absolute absorption coefficients with great sensitivity or specific modulation techniques that push the minimum detectable absorption coefficient down to 1O-14 cm-l! The most impressive progress has been achieved in the development of tunable femtosecond and subfemtosecond lasers, which can be amplified to achieve sufficiently high output powers for the generation of high harmonics with wavelengths down into the X-ray region and with pulsewidths in the attosecond range. Controlled pulse shaping by liquid crystal arrays allows coherent control of atomic and molecular excitations and in some favorable cases chemical reactions can already be influenced and controlled using these shaped pulses. In the field of metrology a big step forward was the use of frequency combs from cw mode-iocked femtosecond lasers. It is now possible to directly compare the microwave frequency of the cesium dock with optical frequencies, and it turns out that the stability and the absolute accuracy of frequency measurements in the optical range using frequency-stabilized lasers greatly surpasses that of the cesium dock. Such frequency combs also allow the synchronization of two independent femtosecond lasers. The increasing research on laser cooling of atoms and molecules and many experiments with Bose-Einstein condensates have brought about some remarkable results and have considerably increased our knowledge about the interaction of light with matter on a microscopic sc ale and the interatomic interactions at very low temperatures. Also the realization of coherent matter waves (atom lasers) and investigations of interference effects between matter waves have proved fundamental aspects of quantum mechanics.

6 VI Preface to the Third Edition The largest expansion of laser spectroscopy can be seen in its possible and already realized applications to chemical and biological problems and its use in medicine as a diagnostic tool and for therapy. Also, for the solution of technical problems, such as surface inspections, purity checks of sampies or the analysis of the chemical composition of sampies, laser spectroscopy has offered new techniques. In spite of these many new developments the representation of established fundamental aspects of laser spectroscopy and the explanation of the basic techniques are not changed in this new edition. The new developments mentioned above and also new references have been added. This, unfortunately, increases the number of pages. Since this textbook addresses beginners in this field as weil as researchers who are familiar with special aspects of laser spectroscopy but want to have an overview on the whole field, the author did not want to change the concept of the textbook. Many readers have contributed to the elimination of errors in the former edition or have made suggestions for improvements. I want to thank all of them. The author would be grateful if he receives such suggestions also for this new edition. Many thanks go to all colleagues who gave their permission to use figures and results from their research. I thank Dr. H. Becker and T. Wilbourn for critical reading of the manuscript, Dr. H.J. Kölsch and C.-D. Bachern of Springer-Verlag for their valuable assistance during the editing process, and LE-TeX Jelonek, Schmidt and Vöckler for the setting and layout. I appreciate, that Dr. H. Latsch, who has taken care for the foregoing editions, has supplied his computer files for this new edition. Last, but not least, I would like to thank my wife Harriet who made many efforts in order to give me the necessary time for writing this new edition. Kaiserslautern, April 2002 Wolfgang Demträder

7 Preface to the Second Edition During the past 14 years since the first edition of this book was published, the field of laser spectroscopy has shown a remarkable expansion. Many new spectroscopic techniques have been developed. The time resolution has reached the femtosecond scale and the frequency stability of lasers is now in the millihertz range. In particular, the various applications of laser spectroscopy in physics, chemistry, biology, and medicine, and its contributions to the solutions of technical and environmental problems are remarkable. Therefore, a new edition of the book seemed necessary to account for at least part of these novel developments. Although it adheres to the concept of the first edition, several new spectroscopic techniques such as optothermal spectroscopy or velocitymodulation spectroscopy are added. A whole chapter is devoted to time-resolved spectroscopy including the generation and detection of ultrashort light pulses. The principles of coherent spectroscopy, which have found widespread applications, are covered in a separate chapter. The combination of laser spectroscopy and collision physics, which has given new impetus to the study and control of chemical reactions, has deserved an extra chapter. In addition, more space has been given to optical cooling and trapping of atoms and ions. I hope that the new edition will find a similar friendly acceptance as the first one. Of course, a texbook never is perfect but can always be improved. I, therefore, appreciate any hint to possible errors or comments concerning corrections and improvements. I will be happy if this book helps to support teaching courses on laser spectroscopy and to transfer some of the delight I have experienced during my research in this fascinating field over the last 30 years. Many people have helped to complete this new edition. I am grateful to colleagues and friends, who have supplied figures and reprints of their work. I thank the graduate students in my group, who provided many of the examples used to illustrate the different techniques. Mrs. Woll scheid who has drawn many figures, and Mrs. Heider who typed part of the corrections. Particular thanks go to Helmut Lotsch of Springer-Verlag, who worked very hard for this book and who showed much patience with me when I often did not keep the deadlines. Last but not least, I thank my wife Harriet who had much understanding for the many weekends lost for the family and who helped me to have sufficient time to write this extensive book. Kaiserslautern, June 1995 Wolfgang Demtröder

8 Preface to the First Edition The impact of lasers on spectroscopy can hardly be overestimated. Lasers represent intense light sources with spectral energy densities which may exceed those of incoherent sources by several orders of magnitude. Furthermore, because of their extremely small bandwidth, single-mode lasers allow a spectral resolution which far exceeds that of conventional spectrometers. Many experiments which could not be done before the application of lasers, because of lack of intensity or insufficient resolution, are readily performed with lasers. Now several thousands of laser lines are known which span the whole spectral range from the vacuum-ultraviolet to the far-infrared region. Of particular interst are the continuously tunable lasers which may in many cases replace wavelength-selecting elements, such as spectrometers or interferometers. In combination with optical frequency-mixing techniques such continuously tunable monochromatic coherent light sources are available at nearly any desired wavelength above 100 nm. The high intensity and spectral monochromasy of lasers have opened a new dass of spectroscopic techniques which allow investigation of the structure of atoms and molecules in much more detail. Stimulated by the variety of new experimental possibilities that lasers give to spectroscopists, very lively research activities have developed in this field, as manifested by an avalanche of publications. A good survey about recent progress in laser spectroscopy is given by the proceedings of various conferences on laser spectroscopy (see "Springer Series in Optical Sciences"), on picosecond phenomena (see "Springer Series in Chemical Physics"), and by several quasi-mongraphs on laser spectroscopy published in "Topics in Applied Physics". For nonspecialists, however, or for people who are just starting in this field, it is often difficult to find from the many articles scattered over many journals a coherent representation of the basic principles of laser spectroscopy. This textbook intends to dose this gap between the advanced research papers and the representation of fundamental principles and experimental techniques. It is addressed to physicists and chemists who want to study laser spectroscopy in more detail. Students who have some knowledge of atomic and molecular physics, electrodynamics, and optics should be able to follow the presentation. The fundamental principles of lasers are covered only very briefly because many excellent textbooks on lasers already exist. On the other hand, those characteristics of the laser that are important for its applications in spectroscopy are treated in more detail. Examples are the frequency spectrum of different types of lasers, their linewidths, amplitude and frequency stability, tunability, and tuning ranges. The optical compo-

9 x Preface to the First Edition nents such as mirrors, prisms, and gratings, and the experimental equipment of spectroscopy, for example, monochromators, interferometers, photon detectors, etc., are discussed extensively because detailed knowledge of modem spectroscopic equipment may be crucial for the successful performance of an experiment. Each chapter gives several examples to illustrate the subject discussed. Problems at the end of each chapter may serve as a test of the reader's understanding. The literature cited for each chapter is, of course, not complete but should inspire further studies. Many subjects that could be covered only briefly in this book can be found in the references in a more detailed and often more advanced treatment. The literature selection does not represent any priority list but has didactical purposes and is intended to illustrate the subject of each chapter more thoroughly. The spectroscopic applications of lasers covered in this book are restricted to the spectroscopy of free atoms, molecules, or ions. There exists, of course, a wide range of applications in plasma physics, solid-state physics, or fluid dynamics which are not discussed because they are beyond the scope of this book. It is hoped that this book may be of help to students and researchers. Although it is meant as an introduction to laser spectroscopy, it mayaiso facilitate the understanding of advanced papers on special subjects in laser spectroscopy. Since laser spectroscopy is a very fascinating field of research, I would be happy if this book can transfer to the reader some of my excitement and pleasure experienced in the laboratory while looking for new lines or unexpected results. I want to thank many people who have helped to complete this book. In particular the students in my research group who by their experimental work have contributed to many of the examples given for illustration and who have spent their time reading the galley proofs. I am grateful to colleages from many laboratories who have supplied me with figures from their publications. Special thanks go to Mrs. Keck and Mrs. Ofiiara who typed the manuscript and to Mrs. Woll scheid and Mrs. Ullmer who made the drawings. Last but not least, I would like to thank Dr. U. Hebgen, Dr. H. Lotsch, Mr. K.-H. Winter, and other coworkers of Springer-Verlag who showed much patience with a dilatory author and who tried hard to complete the book in a short time. Kaiserslautern, March 1981 Wolfgang Demträder

10 Contents 1. Introduction Absorption and Emission of Light Cavity Modes Thermal Radiation and Planck's Law Absorption, Induced, and Spontaneous Emission Basic Photometrie Quantities Definitions Illumination of Extended Areas Po1arization of Light Absorption and Emission Spectra Transition Probabilities Lifetimes, Spontaneous and Radiationless Transitions Semiclassical Description: Basic Equations Weak-Field Approximation Transition Probabilities with Broad-Band Excitation Phenomenological Inclusion of Decay Phenomena Interaction with Strong Fields Relations Between Transition Probabilities, Absorption Coefficient, and Line Strength Coherence Properties of Radiation Fields Temporal Coherence Spatial Coherence Coherence Volume The Coherence Function and the Degree of Coherence Coherence of Atomic Systems Density Matrix Coherent Excitation Relaxation of Coherently Excited Systems Problems Widths and Profiles of Spectral Lines Natural Linewidth Lorentzian Line Profile of the Emitted Radiation Relation Between Linewidth and Lifetime Natural Linewidth of Absorbing Transitions Doppler Width... 68

11 XII Contents 3.3 Collisional Broadening of Spectral Lines Phenomenological Description Relations Between Interaction Potential, Line Broadening, and Shifts Collisional Narrowing of Lines Transit-Time Broadening Homogeneous and Inhomogeneous Line Broadening Saturation and Power Broadening Saturation of Level Population by Optical Pumping Saturation Broadening of Homogeneous Line Profiles Power Broadening Spectral Line Profiles in Liquids and Solids Problems Spectroscopic Instrumentation Spectrographs and Monochromators Basic Properties Prism Spectrometer Grating Spectrometer Interferometers Basic Concepts Michelson Interferometer Mach-Zehnder Interferometer Multiple-Beam Interference Plane Fabry-Perot Interferometer Confocal Fabry-Perot Interferometer Multilayer Dielectric Coatings Interference Filters Birefringent Interferometer Tunable Interferometers Comparison Between Spectrometers and Interferometers Spectral Resolving Power Light-Gathering Power Accurate Wavelength Measurements Precision and Accuracy of Wavelength Measurements Today's Wavemeters Detection of Light Thermal Detectors Photodiodes Photodiode Arrays Photoemissive Detectors Detection Techniques and Electronic Equipment Conclusions Problems

12 Contents XIII 5. Lasers as Spectroscopic Light Sources Fundamentals of Lasers Basic Elements of a Laser Threshold Condition Rate Equations Laser Resonators Open Optical Resonators Spatial Field Distributions in Open Resonators Confocal Resonators General Spherical Resonators Diffraction Losses of Open Resonators Stable and Unstable Resonators Ring Resonators Frequency Spectrum of Passive Resonators Spectral Characteristics of Laser Emission Active Resonators and Laser Modes Gain Saturation Spatial Hole Buming Multimode Lasers and Gain Competition Mode Pulling Experimental Realization of Single-Mode Lasers Line Selection Suppression of Transverse Modes Selection of Single Longitudinal Modes Intensity Stabilization Wavelength Stabilization Controlled Wavelength Tuning of Single-Mode Lasers Continuous Tuning Techniques Wavelength Calibration Linewidths of Single-Mode Lasers Tunable Lasers Basic Concepts Semiconductor-Diode Lasers Tunable Solid-State Lasers Color-Center Lasers Dye Lasers Excimer Lasers Free-Electron Lasers Nonlinear Optical Mixing Techniques Physical Background Phase Matching Second-Harmonic Generation Quasi Phase Matching Sum-Frequency and Higher-Harmonic Generation X-Ray Lasers Difference-Frequency Spectrometer

13 XIV Contents Optical Parametric Oscillator Tunable Raman Lasers Gaussian Beams.... Problems Doppler-Limited Absorption and Fluorescence Spectroscopy with Lasers Advantages of Lasers in Spectroscopy High-Sensitivity Methods of Absorption Spectroscopy Frequency Modulation Intracavity Absorption Cavity Ring-Down Spectroscopy (CRDS) Direct Determination of Absorbed Photons Fluorescence Excitation Spectroscopy Photoacoustic Spectroscopy Optothermal Spectroscopy Ionization Spectroscopy Basic Techniques Sensitivity of Ionization Spectroscopy Pulsed Versus CW Lasers for Photoionization Resonant Two-Photon Ionization Combined with Mass Spectrometry Thermionic Diode Optogalvanic Spectroscopy Velocity-Modulation Spectroscopy Laser Magnetic Resonance and Stark Spectroscopy Laser Magnetic Resonance Stark Spectroscopy Laser-Induced Fluorescence Molecular Spectroscopy by Laser-Induced Fluorescence Experimental Aspects of LIF LIF of Polyatomic Molecules Determination of Population Distributions by LIF Comparison Between the Different Methods Problems Nonlinear Spectroscopy Linear and Nonlinear Absorption Saturation of Inhomogeneous Line Profiles Hole Burning Lamb Dip Saturation Spectroscopy Experimental Schemes Cross-Over Signals Intracavity Saturation Spectroscopy Lamb-Dip Frequency Stabilization of Lasers

14 Contents XV 7.4 Polarization Spectroscopy Basic Principle Line Profiles of Po1arization Signals Magnitude of Polarization Signals Sensitivity of Polarization Spectroscopy Advantages of Polarization Spectroscopy Multiphoton Spectroscopy Two-Photon Absorption Doppler-Free Multiphoton Spectroscopy Influence of Focusing on the Magnitude of Two-Photon Signals Examples of Doppler-Free Two-Photon Spectroscopy Multiphoton Spectroscopy Special Techniques of Nonlinear Spectroscopy Saturated Interference Spectroscopy Doppler-Free Laser-Induced Dichroism and Birefringence Heterodyne Polarization Spectroscopy Combination of Different Nonlinear Techniques Conclusion Problems Laser Raman Spectroscopy Basic Considerations Experimental Techniques of Linear Laser Raman Spectroscopy Nonlinear Raman Spectroscopy Stimulated Raman Scattering Coherent Anti-Stokes Raman Spectroscopy Resonant CARS and BOX CARS Hyper-Raman Effect Summary of Nonlinear Raman Spectroscopy Special Techniques Resonance Raman Effect Surface-Enhanced Raman Scattering Raman Microscopy Time-Resolved Raman Spectroscopy Applications of Laser Raman Spectroscopy Problems Laser Spectroscopy in Molecular Beams Reduction of Doppler Width Adiabatic Cooling in Supersonic Beams Formation and Spectroscopy of Clusters and Van der Waals Molecules in Cold Molecular Beams Nonlinear Spectroscopy in Molecular Beams

15 XVI Contents 9.5 Laser Spectroscopy in Fast Ion Beams Applications of FIBLAS Spectroscopy of Radioactive Elements Photofragmentation Spectroscopy of Molecular Ions Laser Photodetachment Spectroscopy Saturation Spectroscopy in Fast Beams Spectroscopy in Cold Ion Beams Combination of Molecular Beam Laser Spectroscopy and Mass Spectrometry Problems Optical Pumping and Double-Resonance Techniques Optical Pumping Optical-RF Double-Resonance Technique Basic Considerations Laser-RF Double-Resonance Spectroscopy in Molecular Beams Optical-Microwave Double Resonance Optical-Optical Double Resonance Simplification of Complex Absorption Spectra Stepwise Excitation and Spectroscopy of Rydberg States Stimulated Emission Pumping Special Detection Schemes of Double-Resonance Spectroscopy OODR-Polarization Spectroscopy Polarization Labeling Microwave-Optical Double-Resonance Polarization Spectroscopy Hole-Burning and Ion-Dip Double-Resonance Spectroscopy Triple-Resonance Spectroscopy Problems Time-Resolved Laser Spectroscopy Generation of Short Laser Pulses Time Profiles of Pulsed Lasers Q-Switched Lasers Cavity Dumping l.1.4 Mode Locking of Lasers l.5 Generation of Femtosecond Pulses l.l.6 Optical Pulse Compression Sub 10-fs Pulses with Chirped Laser Mirrors l.1.8 Fiber Lasers and Optical Solitons Shaping of Ultrashort Light Pulses l.1.10 Generation of High-Power Ultrashort Pulses

16 Contents XVII 11.2 Measurement of Ultrashort Pulses Streak Camera Optical Correlator for Measuring Ultrashort Pulses Lifetime Measurement with Lasers Phase-Shift Method Single-Pulse Excitation Delayed-Coincidence Technique Lifetime Measurements in Fast Beams Pump-and-Probe Technique Pump-and-Probe Spectroscopy of Collisional Relaxation in Liquids Electronic Relaxation in Semiconductors Femtosecond Transition State Dynamics Real-Time Observations of Molecular Vibrations Transient Grating Techniques Problems Coherent Spectroscopy Level-Crossing Spectroscopy Classical Model of the Hanle Effect Quantum-Mechanical Models Experimental Arrangements Examples Stimulated Leve1-Crossing Spectroscopy Quantum-Beat Spectroscopy Basic Principles Experimental Techniques Molecular Quantum-Beat Spectroscopy Excitation and Detection of Wave Packets in Atoms and Molecules Optical Pulse-Train Interference Spectroscopy Photon Echoes Optical Nutation and Free-Induction Decay Heterodyne Spectroscopy Correlation Spectroscopy Basic Considerations Correlation Spectroscopy of Light Scattered by Microparticles Homodyne Spectroscopy Heterodyne Correlation Spectroscopy Fluorescence Correlation Spectroscopy and Single Moleeule Detection Problems

17 XVIII Contents 13. Laser Spectroscopy of Collision Processes High-Resolution Laser Spectroscopy of Collisional Line Broadening and Line Shifts Sub-Doppler Spectroscopy of Collision Processes Combination of Different Techniques Measurements of Inelastic Collision Cross Seetions of Excited Atoms and Moleeules Measurements of Absolute Quenching Cross Sections Collision-Induced Rovibronic Transitions in Excited States Collisional Transfer of Electronic Energy Energy Pooling in Collisions Between Excited Atoms Spectroscopy of Spin-Flip Transitions Spectroscopic Techniques for Measuring Collision-Induced Transitions in the Electronic Ground State of Moleeules Time-Resolved Infrared Fluorescence Detection Time-Resolved Absorption and Double-Resonance Methods Collision Spectroscopy with Continuous-Wave Lasers Collisions Involving Moleeules in High Vibrational States Spectroscopy of Reactive Collisions Spectroscopic Determination of Differential Collision Cross Seetions in Crossed Mo1ecular Beams Photon-Assisted Collisiona1 Energy Transfer Photoassociation Spectroscopy of Colliding Atoms Problems New Developments in Laser Spectroscopy Optical Cooling and Trapping of Atoms Photon Recoil Measurement of Recoil Shift Optical Cooling by Photon Recoil Experimental Arrangements Threedimensional Cooling of Atoms; Optical Mollasses Cooling of Molecules Optical Trapping of Atoms Optical Cooling Limits Bose-Einstein Condensation Evaporative Cooling Applications of Cooled Atoms and Molecules

18 Contents XIX 14.2 Spectroscopy of Single Ions Trapping of Ions Optical Sideband Cooling Direct Observations of Quantum Jumps Formation of Wigner Crystals in Ion Traps Laser Spectroscopy in Storage Rings Optical Ramsey Fringes Basic Considerations Two-Photon Ramsey Resonance Nonlinear Ramsey Fringes U sing Three Separated Fields Observation of Recoil Doublets and Suppression of One Recoil Component Atom Interferometry Mach-Zehnder Atom Interferometer Atom Laser The One-Atom Maser Spectral Resolution Within the Natural Linewidth Time-Gated Coherent Spectroscopy Coherence and Transit Narrowing Raman Spectroscopy with Subnatural Linewidth Absolute Optical Frequency Measurement and Optical Frequency Standards Microwave-Optical Frequency Chains Frequency Comb from Femtosecond Laser Pulses Squeezing Amplitude and Phase Fluctuations of a Light Wave Experimental Realization of Squeezing Application of Squeezing to Gravitational Wave Detectors Applications of Laser Spectroscopy Applications in Chemistry Laser Spectroscopy in Analytieal Chemistry Single-Molecule Deteetion Laser-Induced Chemie al Reactions Coherent Control of Chemical Reactions Laser Femtosecond Chemistry Isotope Separation with Lasers Summary of Laser Chemistry Environmental Research with Lasers Absorption Measurements Atmospheric Measurements with LIDAR Spectroscopic Detection of Water Pollution Applications to Technical Problems Spectroscopy of Combustion Processes

19 xx Contents Applications of Laser Spectroscopy to Materials Science Measurements of Flow Velocities in Gases and Liquids Applications in Biology Energy Transfer in DNA Complexes Time-Resolved Measurements of Biological Processes Correlation Spectroscopy of Microbe Movements Laser Microscope Time-Resolved Spectroscopy of Biological Processes Medical Applications of Laser Spectroscopy Applications of Raman Spectroscopy in Medicine Heterodyne Measurements of Ear Drums Cancer Diagnostics and Therapy with the HPD Technique Laser Lithotripsy Laser-Induced Thermotherapy of Brain Cancer Fetal Oxygen Monitoring Concluding Remarks References Subject Index