Rovibrational excitation in collisions of He on D2 at energies less than 2 keV.

Abstract

This work presents a theoretical study of rovibrational excitation in electronically elastic collisions of He on either H{sub 2} or D{sub 2} in the collision energy range 100 eV{le}{ital E}{sub inc}{le}2 keV. Earlier classical trajectory collision calculations, which predicted the rovibrational excitation energy to follow a universal scaling law, are here generalized to (1) encompass vibrational excitation caused by a three-body force representing the He interacting with the H{sub 2} covalent bond, and (2) incorporate quantization of vibrational excitation by a semiquantal ansatz imposed on the otherwise classical calculations. Quantization of vibrational excitation is shown to cause a breakdown of the scaling law predicted by the fully classical theory. The collisional excitation calculations utilize straight-line trajectories in the framework of the impulsive approximation, and are based on an improved parametric fit to the {ital ab} {ital initio} interaction energy surface for the HeH{sub 2} triatomic molecular system. In addition to the usual two-body Bohr and Born-Mayer terms describing the He-H interactions, the new interaction potential incorporates a three-body term to describe the contribution arising from the He interacting with the H-H covalent bond. The three-body force gives rise to some vibrational excitation phenomena, which are here explored. Good agreement ismore » obtained with experimental measurements on rovibrational excitation energies produced in quasielastic collisions of He on D{sub 2}« less