Δm_J Transitions in molecular scattering from isolated adsorbates on surfaces

Rotationally inelastic scattering of diatomic molecules from isolated adsorbates on a flat surface is studied, focusing on properties of Δm_J transitions (defined with respect to a quantization axis normal to the surface). A simple model is used, treating the molecule as a rigid pair of hard spheres and the adsorbate as a hard hemisphere. The sudden approximation is employed for the collision dynamics, resulting in simple expressions for the transition probabilities. Calculations are reported using model parameters that correspond to N₂ scattered from Ar adsorbates on a flat surface. The results show pronounced maxima of the transition probabilities for large Δm_J values, associated with a rainbow effect in the Δm_J variable. Since low-corrugation crystalline surfaces gives rise to very weak, low-order Δm_J transitions only, the results suggest that experimental studies of Δm_J transitions could provide a sensitive probe of isolated adsorbates on surfaces and of molecule-adsorbate interactions.