Nonlinear Laser Chemistry

Prefaces are usually written when a manuscript is finished. Having finished this book I can clearly see many shortcomings in it. But if I began to eliminate them I would probably write quite a different book in another two years; indeed, this has already happened once. In 1979, when I finished the first version of this book, it was much broader in scope and was to be titled "Laser Photochemistry." Corrections and additions to that unpublished manuscript gave rise to the present book with its revised title and more specific subject matter. I resolved to have it published in exactly this form, despite the fact that it concerns a dynamically developing field of research and will soon make way for other works. This book contains the basic ideas and results I have been developing with my colleagues, friends and students at the Institute of Spectroscopy, USSR Academy of Sciences, in the town of Troitsk since 1970. It deals with the interaction of light with atoms and molecules via multiple-phonon inter­ action. Nonlinear processes in the resonant interaction are used to illustrate the physical mechanisms involved and to indicate how these processes have led to modern applications such as isotope separation, detection of single atoms and molecules, and chemical and biochemical synthesis.

Inhalt

1. Introduction.- 1.1 General Concepts of Selective Photophysics and Photochemistry.- 1.1.1 Inter- and Intramolecular Selectivity.- 1.1.2 Photochemical and Photophysical Processes.- 1.1.3 History of Selective Photochemistry and Photophysics.- 1.2 Classification of Selective Molecular Photoprocesses Induced by Laser Radiation.- 1.2.1 Laser Radiation Properties.- 1.2.2 Nonlinear (Multi-Step and Multi-Photon) Photoexcitation.- 1.2.3 Type of Selective Molecular Photoprocess.- 1.3 Selectivity and Yields of Photochemical Processes.- 1.3.1 Processes Leading to Losses in Selectivity.- 1.3.2 Photochemical Collisional Reaction.- 1.3.3 Photochemical Monomolecular Reactions.- 1.3.4 The Yield of the Photoprocess with Multi-Photon Excitation.- 1.4 Applications of Selective Laser Photochemistry.- 1.4.1 Various Aggregate States of Substance.- 1.4.2 Various Trends of Applications.- 2. Selective Photoexcitation of Atoms and Molecules.- 2.1 Isotopic Selectivity in Linear Photoexcitation of Atoms and Molecules.- 2.1.1 Isotopic Shifts for Atoms.- 2.1.2 Hyperfine Structure and Nuclear Spin for Atoms.- 2.1.3 Isotope Shifts in Molecular Spectra.- 2.1.4 Nuclear Spin Effects in Molecules.- 2.2 Limitations of Linear Selectivity of Photoexcitation and Methods for Its Enhancement.- 2.2.1 Overlapping of Spectral Lines. Atomic Beams.- 2.2.2 Overlapping of Vibrational-Rotational Absorption Bands. Cooling of Molecules.- 2.3 Methods for the Enhancement of Selectivity with Nonlinear Photoexcitation.- 2.3.1 Selectivity Multiplication in a Multi-Step Process.- 2.3.2 Selectivity of Two-Photon Excitation of Overlapping Doppler-Broadened Lines.- 2.3.3 Selectivity of Multi-Step Excitation of Overlapping Nonhomogeneously Broadened Lines.- 2.3.4 Selectivity of Multi-Step Photoexcitation with Temporal Pulse Selection.- 2.4 Overall Selectivity of Photochemical Processes.- 3. Multi-Step Selective Photoionization of Atoms.- 3.1 Introduction.- 3.1.1 Qualitative Considerations.- 3.1.2 First Experiments.- 3.2 Characteristics of Multi-Step Photoionization.- 3.2.1 Kinetics of Two-Step Ionization.- 3.2.2 Coherent Interaction Effects.- 3.2.3 Various Methods of Ionizing Excited Atoms.- 3.3 Photoionization from Excited States to the Continuum and Autoionization States.- 3.3.1 Measurement of the Photoionization Cross-Section by the Method of Quantum State Depletion.- 3.3.2 Frequency Dependence of the Photoionization Cross-Section.- 3.3.3 Autoionization Resonances of Excited States.- 3.4 Ionization of Highly Excited (Rydberg) Atomic States.- 3.4.1 Properties of Highly Excited Atoms.- 3.4.2 Electric Field Ionization.- a) Theoretical Treatment.- b) Experimental Results.- c) Total Ionization Yield.- 3.4.3 Photoionization Through IR Radiation.- 3.5 Collision Processes in the Multi-Step Ionization of Atoms.- 3.5.1 Resonance Energy Transfer Between Atoms.- 3.5.2 Collisional Ionization of Excited Atoms.- a) Electron Capture.- b) Associative Ionization.- c) Electron Escape.- 3.5.3 Resonant Charge Transfer.- 4. Selective Monomolecular Photoprocesses with Nonlinear Excitation of Electronic States.- 4.1 Methods Used in the Multi-Step Excitation of Molecular Electronic States.- 4.2 Photodissociation of Molecules by Two-Step Excitation Through Vibrational States.- 4.2.1 Electronic Absorption Spectrum from Excited Vibrational States.- 4.2.2 Effect of the Rotational "Bottleneck" on Photoexcitation of Vibrations.- 4.2.3 Selectivity of Two-Step IR-UV Photoexcitation.- a) Ultimate Spectral Selectivity.- b) Influence of Thermal Excitation of Vibrations.- 4.2.4 Isotopically-Selective IR-UV Photodissociation.- 4.2.5 Loss of Selectivity Due to Collisions.- a) V-V Exchange Between Lower Vibrational Levels.- b) Secondary Photochemical Processes.- 4.3 Monomolecular Photoprocesses with Multi-Photon Vibrational and Subsequent Electronic Excitation.- 4.3.1 Distortion of the Electronic Absorption Spectrum Due to Multi-Photon Excitation of Vibrations.- 4.3.2 Selective IR-UV Photodissociation of Isotopic Molecules.- 4.3.3 UV-IR Photoisomerization of Molecules.- a) Schemes of Photoisomerization.- b) Stepwise IR-UV Photoisomerization.- c) Competition Between Isomerization and Dissociation.- 4.3.4 IR-VUV Photoionization of Molecules.- 4.4 Photoionization of Molecules Through Nonlinear Excitation of Electronic States.- 4.4.1 Two-Step UV-VUV Photoionization.- 4.4.2 Photoionization and Photofragmentation Using Intense Resonant UV Laser Light.- 4.4.3 Multi-Photon Photoionization Using Very Intense Laser Light.- 5. Multi-Photon Monomolecular Photoprocesses in the Ground Electronic State.- 5.1 Introduction to IR Multi-Photon Laser Chemistry.- 5.1.1 Early Papers and the First Experiments.- 5.1.2 Basic Processes.- 5.2 Multi-Photon (MP) Absorption of IR Radiation and Excitation of Molecules Through Lower Vibrational Levels.- 5.2.1 MP Absorption Characteristics.- 5.2.2 Fraction of Excited Molecules.- 5.2.3 Theory of the Excitation of Lower Vibrational Levels.- a) Model for MP Transitions.- b) Model for Anharmonicity Compensation.- c) Model for Weak Transitions.- 5.2.4 Comparison of Theory with Experiment.- 5.3 Multi-Photon Excitation of Molecules in the Vibrational Quasi-Continuum and Distribution of Vibrational Energy.- 5.3.1 Properties of the Vibrational Quasi-Continuum.- 5.3.2 Excitation and Vibrational Distribution of Molecules in the Quasi-Continuum.- 5.3.3 Stochastization of Vibrational Energy.- 5.3.4 Comparison of Theory with Experiment.- 5.4 Dissociation of Highly Excited Molecules.- 5.4.1 Characteristics of the MP Dissociation.- 5.4.2 MP Dissociation in Two-Frequency IR Fields.- 5.4.3 Statistical Theory of Monomolecular Decay.- 5.4.4 Kinetic Model of the MP Dissociation.- 5.4.5 MP Dissociation Products.- 5.4.6 Collision Effects.- a) Collisions with Buffer Gas.- b) Collisions Between Molecules.- c) Secondary Processes.- 5.5 Molecular Isomerization under MP Excitation.- 5.5.1 Features of the MP Isomerization.- 5.5.2 Experimental Data.- 5.6 Isotopic Selectivity of the MP Dissociation.- 5.6.1 Methods of Measuring the Selectivity.- 5.6.2 Dependence of Selectivity on Frequency and Intensity of Laser Pulses.- a) Spectral Dependence.- b) Intensity Dependence.- c) Selectivity of the MP Dissociation Using IR Pulses of Two Different Frequencies.- 5.6.3 Role of V-V Exchange.- 6. Laser Photoseparation on an Atomic and a Molecular Level.- 6.1 Introduction to Methods of Laser Photoseparation.- 6.1.1 Characteristics of the Separation Cell.- a) Selectivity of Isotope Separation.- b) Degree of Isotope Extraction.- c) Energy Consumption of the Photoseparation.- 6.1.2 Potential Advantages of Laser Separation.- 6.2 Separation of Atoms Through Photoionization.- 6.…