Non-adiabatic unimolecular dissociation of polyatomic molecules. I. General theory

Abstract

A golden-rule expression for the rate constant for unimolecular dissociation of a polyatomic molecule via a non-adiabatic process is expressed, in the harmonic oscillator approximation, in terms of a diatomic-like predissociation rate constant ( k diat), which contains all the electronic part of the dynamics of the process, and in terms of a product of (3 N - 7) Frank—Condon factors. The influence of a difference in the equilibrium geometries of the initial and final electronic states is pointed out. A suitably averaged rate constant is defined, and it is shown that the shortcomings of the separable hamonic oscillator model can be usefully corrected by appropriate rate constant averaging, which allows for the effect of anharmonicity as well as for the non-separability and possibly poor choice of normal coordinates. The theory presented is suitable for calculating rate constants in cases where information regarding enery partitioning among fragments is not required.