Vibrational predissociation quasiclassical tunnelling and classical diffusion

Abstract

A comparative study of vibrational predissociation dynamics is presented. Two collinear models of the van der Waals complex are used with a realistic medium-strength coupling parameter. The predissociation rates are calculated by four different approaches: an accurate quantum mechanical method by the complex-scaling technique, first-order approximations in the diabatic (FOD) and adiabatic (FOA) basis, and purely classically. It is shown that FOA within the improved semiclassical Landau method provides an excellent description of the dynamical tunnelling of the system from all the quasibound states into continuum; at the same time, FOD yields noticeably higher rates though the transition probabilities are very low. At low excitation energies of the van der Waals bond, the classical description yields zero rates in accord with the KAM theorem. At higher excitation energies, the classical rates are higher than the quasiclassical rates since the classical system dissociates via the diffusion through the holes in the phase space which are still too narrow to let the quantum system escape. A simple explanation of a parallelism between quantum and classical rates is suggested under a condition when the first-order quantum treatment is applicable.