RELAXATION PHENOMENA IN EXCITED MOLECULES

Abstract

This paper reviews the interpretation of radiationless transitions, thermal reactions and photochemical rearrangements and decompositions, developed in the last three years. In each of these cases, the relaxation process involved can be described within a common theoretical framework by simply observing that the system of interest is prepared in a compound (nonstationary) state. We begin (part II) by briefly discussing the role of zero-order properties (e.g. interaction energies and densities of states) in determining the time scales appropriate to the observation of irreversible decay in isolated molecules. We then (part III) identify the various ways in which the usual classification of excited molecular states breaks down. A simple, but quite general, energy level model is shown to represent the essential features of the spectrum of zero-order vibronic manifolds in polyatomic molecules: absorption line shapes, interference effects, and emission lifetimes and quantum yields, are briefly discussed. The experimentally observed dependencies of the nonradiative transition rate on electronic energy gap, molecular geometry and frequency changes, and isotope effects, are shown to follow directly from a many-phonon description of the electronic relaxation process. We discuss further the way in which this theoretical approach provides quantitative predictions of the dependence of the radiationless lifetimes (within a given electronic state) on vibrational excitation.