Creation and Disposal of Vibration Energy in Polyatomic Molecules

Abstract

The importance of vibrational excitation in chemical reactions has been known for many years, having been recognized first in the study of the unimolecular reactions of polyatomic molecules. Because a nonlinear polyatomic molecule of N atoms has 3N 6 vibrational degrees of freedom (compared to three rotations and three translations), it is clear that much of the phenomenology of excited molecules must reside in vibration. As in most fields of chemical physics the study of vibrational excitation has grown together from two widely separated areas. Bulk kinetic data (thermal unimolecular reaction rates and sound dispersion) and static spectroscopic energy level experiments are now combined into a large area of interest dominated by spectroscopic measurements of dynamic processes. A knowledge of the chemical, energetic, and spectral properties of poly atomic molecules is important for studies of chemical reactivity, isotope separation, combustion processes, chemical lasers and other technologies, as well as being a splendid stimulus to the ever expanding predictive abilities of chemical theorists. The importance of a knowledge of vibrational processes has increased rapidly with the possibility of using lasers to induce state-specific chemical reactions. Such excitation can result in interesting selectivity only if the resulting molecules retain their state-specific excitation long enough to react. In contrast to the extremely large number of highly selective electronic photochemical reactions, the only comparably selec­ tive effect for vibrational excitation is the isotope selectivity of infrared multiphoton dissociation. Although vibrational excitation is known to enhance strongly the rate of a few bimolecular reactions, there are to date no data showing chemical selectivity based on state-specific excita-