Vibrational predissociation of the Ar⋯Cl2 van der Waals complex: The small molecule limit for intramolecular vibrational redistribution

Abstract

A converged three-dimensional quantum treatment of vibrational predissociation in the Ar⋯Cl2(B 3Π0u+;,v′) van der Waals complex is presented. The potential energy surface used is a sum of pairwise Morse atom-atom interactions adjusted asymptotically to a C 6/R6 + C8/R8 anisotropic van der Waals form. Calculations have been performed in the energy region of Ar⋯Cl2(B1v′ = 6, 10, and 11) excited levels. In agreement with the experimental findings, the final rotational distribution of Cl2 is found to be strongly dependent on the initial v′ state being excited, as well as on the number of vibrational quanta lost in the vibrational predissociation process. The role of intramolecular vibrational redistribution for v′ = 10 and 11 for which the Δv= -1 channel is closed is also studied. It is found that the vibrational predissociation (VP) dynamics are dominated by the coupling of the zero-order "bright" state with a single "dark" state from the v′ -1 manifold of van der Waals vibrationally excited states which then decays to the continuum, and that the product state distribution is determined by the dissociation of the dark state. This is characteristic of the sparse limit for intramolecular vibrational redistribution. It also implies that the dissociation rate is not governed by a simple function of the initial quantum numbers such as the one given by the energy gap law. The golden rule approximation gives surprisingly accurate results for Ar⋯Cl2 dynamics. This will be very useful for fitting a potential energy surface to experimental results. © 1992 American Institute of Physics.