A quantitative comparison of multiple-photon absorption (MPA) and dissociation (MPD) has been performed for experiments with a number of different polyatomic molecules. Appropriate normalization techniques for the absorption and dissociation parameters are formulated to account for the different conditions of the experiment and the molecular parameters. This procedure in a first approximation, accounts for the effects of independent variables such as gas pressure, optical bandwidth, optical pulse duration, excitation frequency, spectral width of the optical absorption band, absorption strength of the transition, bond strength, and the density of states in the molecule. The theoretical description of the dynamics of the absorbing ground state is considered and used to provide the rationale for the normalization procedure. The results of this analysis indicate that the functional dependence of the number of photons absorbed per molecule with fluence is qualitatively the same for the most molecules. Similarly, the probability for dissociation of most molecules can be related to the density of vibrational states, the bond strength, the number of photons absorbed per molecule, and the width of the absorbing transition. The functional relationships derived for MPA and MPD can be related to several theoretical aspects of the optical interaction process. In particular, the implications for the basic absorption mechanisms and the distribution of vibrational energy in the molecule will be addressed.

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 1 aser sep aration of isotopes.

The dynamics of multiphoton absorption and dissociation of polyatomic ions will be studied to characterize the details of the process. Ion fragments will be identified and the energy released into translation will be measured using a coaxial laser ion beam spectrometer. Single photon absorption studies of positive and negative polyatomic ions failed to reveal any vibrational absorptions in the infrared region investigated. A single photon electronic transition absorption in O2(+) was found which resulted in dissociation. (Author).

The procedure of maximal entropy is applied to characterize the three distributions over energy states which are of direct interest for studies of multiple photon excitation of polyatomic molecules. These distributions are the original population of the different energy states given the mean number of photons absorbed, the distribution over the final energy states after a single collision, given a well defined initial energy state and the mean energy transfer per collision, and the time evolution of the population of the different energy states due to collisional deactivation, given the mean energy transfer per collision. Good agreement with experimentally determined values of (.delta.E), the average amount of energy removed in a collision, are obtained for deactivation of sec-butyl, cyclohexane, .beta.-hexyl, and .beta.-naphthylamine by structureless collision partners such as He or H/sub 2/. The vibrational relaxation surprisal parameter is found to be lambda/sub 1/ approximately equal to 0.1 for all these systems. This is much closer to a statistical, or strong-collision limit, than vibrational deactivation of diatomics by atoms, for which lambda/sub 1/ is approximately equal to 1.0. Deactivation most likely proceeds through a sequence of maximal-entropy distributions.

Photodissociation induced by the absorption of single photons permits the detailed study of molecular dynamics such as the breaking of bonds, internal energy transfer and radiationless transitions. The availability of powerful lasers operating over a wide frequency range has stimulated rapid development of new experimental techniques which make it possible to analyse photodissociation processes in unprecedented detail. This text elucidates the achievements in calculating photodissociation cross-sections and fragment state distributions from first principles, starting from multi-dimensional potential energy surfaces and the Schrödinger equation of nuclear motion. Following an extended introduction in which the various types of observables are outlined, the book summarises the basic theoretical tools, namely the time-independent and the time-dependent quantum mechanical approaches as well as the classical picture of photodissociation. The discussions of absorption spectra, diffuse vibrational structures, the vibrational and rotational state distributions of the photofragments form the core of the book. More specific topics such as the dissociation of vibrationally excited molecules, emission during dissociation, or nonadiabatic effects are also discussed. It will be of interest to graduate students and senior scientists working in molecular physics, spectroscopy, molecular collisions and molecular kinetics.