Stereo-selective partitioning of translation-to-internal energy conversion in gas ensembles

A recent computational study of translation-to-internal energy transfer to H2 (v = 0,j = 0), hereinafter denoted H2 (0;0), in a bath of H atoms [A. J. McCaffery and R. J. Marsh, J. Chem. Phys. 139, 234310 (2013)] revealed an unexpected energy partitioning in which the H2 vibrational temperature greatly exceeds that of rotation. This occurs despite rotation and vibration distributions being close to Boltzmann from early in ensemble evolution. In this work, the study is extended to include H2 (0;0), O2 (0;0), and HF (0;0) in a wide range of atomic bath gases comprising some 22 ensembles in all. Translation-to-internal energy conversion in the systems studied was found to be relatively inefficient, falling approximately with (√μ')(-1) as bath gas mass increases, where μ' is the reduced mass of the diatomic-bath gas pair. In all 22 systems studied, T(v) exceeds T(r)--by a factor > 4 for some pairs. Analysis of the constraints that influence (0;0) → (1;j) excitation for each diatomic-atom pair in momentum-angular momentum space demonstrates that a vibrational preference results from energy constraints that limit permitted collision trajectories to those of low effective impact parameter, i.e., to those that are axial or near axial on impact with the Newton surface. This implies that a steric constraint is an inherent feature of vibration-rotation excitation and arises because momentum and energy barriers must be overcome before rotational states may be populated in the higher vibrational level.