Rotationally inelastic collisions of LiH with He. II. Theoretical treatment of the dynamics

Abstract

Integral cross sections for the rotational excitation of LiH (j=0,1, and 2→j′) in collisions with He have been determined at collision energies of 0.2057, 0.3057, and 0.4057 eV. The rigid-rotor collision dynamics were solved within the coupled states (CS) approximation. The ab initio potential surface described in the preceding article was used. Sizeable inelastic flux occurs even for large changes in the rotational quantum number, which is probably a reflection of the extreme anisotropy in the potential surface. The dependence of the cross sections on j′, the final rotational quantum number, is not monotonic but displays a pronounced oscillatory structure which is a sensitive function of collision energy. Use of the infinite order sudden approximation results in a disappearance of this oscillatory structure as well as a pronounced lowering in the actual magnitudes of the inelastic cross sections. By contrast excellent agreement with the CS j=1,2→j′ cross sections is provided by sudden limit scaling relations with the CS j=0→j′ cross sections as input. Neither the exponential-gap nor power-law parametrizations provide an adequate fit to the CS integral cross sections. The rotationally inelastic cross sections obtained with an electron-gas potential surface are significantly smaller, because the surface is less repulsive. Thermally averaged cross sections are determined for comparison with the results of the experiment described in the following article.