Effective potential study of rotationally-vibrationally inelastic collisions between He and H2

Abstract

Within the Rabitz effective potential formalism [J. Chem. Phys. 57, 1718 (1972)] we have carried out a three‐dimensional, quantum mechanical study of He–H₂ collisions, concentrating on rotationally‐vibrationally inelastic processes. We treat the H₂ molecule as a rotating Morse oscillator and use an interaction potential which is an analytic fit to the ab initio potential surface of Gordon and Secrest. Employing a modified version of the Gordon algorithm to solve the close‐coupled equations, we have performed a series of calculations involving the collision of He with both ortho‐ and para‐hydrogen at collision energies ranging 1.2–2.0 eV. We have carefully investigated the convergence properties of the close‐coupled expansion. We present cross sections for all relevant rotational processes associated with the ν = 1 → ν =0 collision‐induced vibrational relaxation of the H₂ molecule. Our major conclusions are: (1) ∼20 rotational‐vibrational states must be included; (2) large changes in the molecular angular momentum may accompany vibrational relaxation; (3) the degree of resonance of a particular rotation‐vibration process may play an important role in determining the probability of the transition; and (4) the total cross section for vibrational relaxation increases significantly with increasing molecular rotational angular momentum.