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.

The possibility of initiating chemical reactions by high-intensity laser exci tation has captured the imagination of chemists and physicists as well as of industrial scientists and the scientifically informed public in general ever since the laser first became available. Initially, great hopes were held that laser-induced chemistry would revolutionize synthetic chemistry, making possible "bond-specific" or "mode-specific" reactions that were impos sible to achieve under thermal equilibrium conditions. Indeed, some of the early work in this area, typically employing high-power continuous-wave sources, was interpreted in just this way. With further investigation, however, a more conservative picture has emerged, with the laser taking its place as one of a number of available methods for initiation of high-energy chemical transformations. Unlike a number of these methods, such as flash photolysis, shock tubes, and electron-beam radiolysis, the laser is capable of a high degree of spatial and molecular localization of deposited energy, which in turn is reflected in such applications as isotope enrichment or localized surface treatments. The use of lasers to initiate chemical processes has led to the discovery of several distinctly new molecular phenomena, foremost among which is that of multiple-photon excitation and dissociation of polyatomic molecules. This research area has received the greatest attention thus far and forms the focus of the present volume.

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.