Vibrational energy relaxation of diatomic molecules isolated in rare-gas matrices: Role of orientational motions

Abstract

A quantum-mechanical model which accounts for the dynamical role of orientational motions in vibrational energy relaxation (VER) of diatomic impurities isolated in rare-gas solids is developed. A simple closed expression for the VER rate constant is derived upon the following principal assumptions: (i) the lattice is planar, the diatomic a plane vibrotor; (ii) dynamical participation of local translational and bulk lattice modes is neglected. Results for the systems NH and ND in Ar, OH and OD in Ne, HCl and DCl in Ar and CO in Ar are in reasonable agreement with experiment. In particular, observed isotopic effects seem to be well described by the model. However, there are a couple of discrepancies between calculated and observed ratios of VER rate constants for the systems NH, ND/Kr and HCl, DCl/Ar. It is surmised that these are due to features not included in the model, e.g., lattice imperfections, van der Waals molecule formation and/or rotational–translational coupling.