Potential surface dependence of vibrationally inelastic collisions between He and H2

Abstract

We have carried out a careful study of He–H2 vibrationally inelastic collisions based on five different analytic fits to the Gordon-Secrest [J. Chem. Phys. 52, 120 (1970)] ab initio interaction potential. Within a model which freezes the atom-diatom orientation, we solve the collision dynamics fully quantum mechanically. Both for the 0→1 and 3→4 vibrational transitions, we find that the use of different analytic surfaces leads to substantial qualitative and quantitative differences in the calculated transition probabilities. In particular, we observe an anomalous angular dependence in the case of the original Gordon-Secrest analytic surface. We show that this arises from difficulties in the extrapolation of the ab initio points in the classically forbidden region at small center-of-mass separation. These results have important implications for future theoretical study of molecular collisions.

Ne–H–H potential energy surface including electron correlation

“…The least squares parameters obtained for a fit to this form of the interaction potential are A = 2.2828, ~ = 1.500 (10) This form of the interaction potential is desirable since only two .. order of magnitude larger than that of those fits previously described, however.…”

Vib-rotational relaxation in Li + N2collisions Calculations with the optical-potential method

“…The use of a model (with realistic features) as a replacement for the (unavailable) truly quantitative potential is acceptable here, since our main purpose is to study trends and properties of vib-rotational cross sections, which are presumably not specific to the true potential. We assume that the potential surface for Li+ N z is of the analytical .4B-4 form previously employed for He + H 2 [10] :…”

Inelastic molecular collisions: applications of theoretical methods to problems in relaxation phenomena, laser operation, and combustion kinetics. Progress report, February 1--November 1, 1978. [Summaries of research activities at the University of Wisconsin]

“…A fundamentally important consideration is the functional form of the fit to the interaction; it determi nes the quality of the fit and, importantly, the nature of the extrapolation beyond the ab initio points. Recently, this latter point was underscored in a study conducted by Alexander and Berard (18) of.the sensitivity of inelastic collisions in He-H2 to several analytic fits of an ab initio surface.• They traced serious discrepancies in the theoretically observed collisional behavior to difficulties arising in the extrapolation of the ab initio points. to small intermolecular separations corresponding to the classically forbidden region.…”