Irreversible Electronic Relaxation in Polyatomic Molecules

Abstract

The general phenomena which are classified as involving radiationless transitions, e.g., intersystem crossing, internal conversion, are discussed in terms of a unified Green's function formalism for the cases of the large, intermediate, and small molecule limits for both isolated molecules and molecules with external perturbers. Because of the existence of a natural time limit on any experiment, e.g., an isolated molecule description is only valid for a limited time, irreversibility can “effectively” occur on these relevant time scales in molecules which only have discrete energy levels. The introduction of this natural time limit allows the derivation of completely general criteria for the occurrence of irreversible electronic relaxation in isolated and perturbed polyatomic molecules. The dependence of these criteria on the type of radiationless process is elucidated in a general manner. For completeness, the excitation process is treated as well as the decay. Because we wish to examine the general phenomena encompressed in “radiationless transitions,” only a simplified model for the excitation is used. However, this simple model is sufficient to examine those general aspects of “radiationless transitions” which are independent of the “fine details” of the exciting light.