Influence of the Distorted Wave Approximation in Calculations of Chemical Reaction Cross Sections; Ar++HD

Abstract

Two different approximations to the formal quantum theory of rearrangement collisions are applied to the same reactive system. The comparison enables some conclusions to be drawn concerning the effects of approximations to perturbed translational motion on predicted product energy distributions. In both schemes, the exact wavefunction is approximated by a product of a translational and a molecular wavefunction, where the molecular wavefunction is treated in the perturbed-stationary-state approximation. In one scheme (DWBA PSS) the translational wavefunction is expressed as a WKB solution to the appropriate elastic scattering problem, while in the other (BA PSS), free translational motion is assumed. Calculations are performed for the system Ar++HD using an assumed potential surface. Significant differences are noted in predicted distributions of translational exoergicity and product vibrational and rotational energy. In particular, the behavior of the most probable translational exoergicity with respect to incident energy predicted by the DWBA PSS is in substantial agreement with experiment, whereas the BA PSS consistently overestimates product excitation. Isotope effects predicted by both schemes are in serious disagreement with experiment; it is concluded that the source of this discrepancy lies largely in the neglect of the noncentral nature of the forces in this system.