Photodissociation of Van der Waals molecules. Adiabatic treatment

Abstract

In this paper an adiabatic distorted-wave treatment of vibrational predissociation in Van der Waals molecules is presented. The model considers the coupling between a vibration of a normal molecule and the Van der Waals stretch. In the zero-order approximation, the two motions are decoupled in a way analogous to the Born–Oppenheimer separation of electronic and nuclear motion. In contrast to the usual diabatic treatments (where the molecular vibration is considered to be independent of the Van der Waals bond length) the formulation presented on this paper explicitly takes into account the change of the molecular frequency and equilibrium internuclear distance as the Van der Waals bond stretches. The formalism is presented for harmonic as well as for an harmonic descriptions of the asymptotic intramolecular vibrations. In both cases analytic expressions for the non-adiabatic coupling terms are obtained. The vibrational predissociation rates are calculated using the first-order distored-wave approximation. A simple model leading to fully analytic results corresponds to a colinear X ⋯ BC molecule with linearization of the potential. It is shown that in this case the two treatments, adiabatic and diabatic, lead to exactly the same results. In all other cases an efficient procedure involving a fitting of the interaction terms is suggested which also leads to analytic expressions.