Monte Carlo trajectory study of Ar + H₂ collisions. I. Potential energy surface and cross sections for dissociation, recombination, and inelastic scattering

Modified statistical electron-gas calculations using the methods of Gordon, Kim, Rae, Cohen, and Pack are carried out to obtain the interaction energy of Ar with H₂ as a function of geometry. The results are combined with the accurate pairwise interactions, the long-range nonpairwise interaction, and the potential LeRoy and van Kranendonk fit to spectral data on the van der Waals' complex to obtain a potential energy surface which is as accurate as possible at all geometries. This surface and the pairwise additive surface are then used in a Monte Carlo quasiclassical trajectory study of the cross sections (under shocktube high-energy collision conditions) for complete dissociation, for production of quasibound states of H₂, and for V-T, R-T, and V-R-T energy transfer. Except for R-T energy transfer, the accurate surface yields smaller cross sections than the pairwise additive surface does. The cross sections for dissociation are much smaller than predicted by the available-energy hard-sphere model but are larger than the inelastic cross sections for excitation to the highest bound vibrational energy levels. Initial vibrational excitation energy is more effective than rotational energy or relative translational energy in causing dissociation. Using the full potential surface the recombination cross section of the V = 13, j = 8 quasibound state of H₂ is calculated at E_rel = 0.026 eV and is in good agreement with the result previously calculated by Whitlock, Muckerman, and Roberts using a less accurate, pairwise additive potential surface.