Intramolecular Processes in Isolated Polyatomic Molecules

Abstract

This paper deals with certain rather well known unimolecular processes which occur in isolated polyatomic molecules. These processes are (i) chemical reactions, forming fragments with identifiable internal and translational energy distributions, and (ii) the coupling of vibrational and electronic degrees of freedom, a phenomenon which is responsible for so-called radiationless transitions. The work is mainly experimental, and capitalizes on the fact that infrared lasers can be used to excite the vibrations of polyatomics in the absence of collisions. Thus, parent translational and rotational energies are near ambient, while vibrations can be excited rather easily, allowing us to measure elementary unimolecular processes of these species without interference from collisions. Although molecular excitation cannot be adjusted to yield monoenergetic species of excitation, still the “average excitation level” can be controlled, enabling us to determine effects qualitatively. Above dissociation threshold, the balance between the optical pumping rate and the dissociation rate results in a rather narrow range of energies from which dissociation occurs, and by adjusting the laser intensity the mean unimolecular rate can be controlled.