Role of rotational alignment in molecule–surface interaction for CH3F and OCS

Abstract

The influence of rotational alignment on molecule–surface interaction is studied for CH3F and OCS colliding with a glass surface. Experiments were performed at 285 K with the technique of surface light-induced drift using a flat channel, the alignment being produced through excitation by linearly polarized light. For molecules having rotational energy well below thermal, it is found that the accommodation coefficient for parallel momentum α, which can be related to the trapping/desorption probability, is larger if the angular momentum J is parallel to the surface (‘‘cartwheeling motion’’) than if perpendicular (‘‘helicopters’’). For CH3F the experiments indicate that this difference decreases strongly with increasing K, denoting the component of J along the principal molecular axis. Experiments on OCS confirm this behavior. For molecules having rotational energy well above thermal, however, the reverse behavior is found, viz., α is larger for helicopters than for cartwheels. This is consistent with molecular beam data on the system NO/Pt(111) studied by Jacobs et al. [J. Chem. Phys. 91, 3182 (1989)]. A possible explanation of the observations is given in terms of the role of rotational (de-)excitation in the accommodation process.