Stereodynamics of cold HD and D2 collisions with He.

Abstract

We present a comprehensive quantum mechanical study of stereodynamic control of HD + He and D2 + He collisions that have been probed experimentally by Perreault et al. [J. Phys. Chem. Lett. 13, 10912 (2022)] using Stark-induced adiabatic Raman passage (SARP) techniques. Our calculations utilize a highly accurate full-dimensional H2 + He interaction potential with diagonal Born-Oppenheimer correction appropriate for HD and D2 isotopomers. The results show that rotational quenching of HD from j = 2 → j' = 0 in v = 2, j = 2 → j' = 1 in v = 2 and v = 4, and j = 4 → j' = 3 in v = 4 is dominated by an l = 1 shape resonance located between 0.1 and 1.0 cm-1. For collision energies less than 0.1 cm-1, isotropic scattering prevails. An l = 1 resonance centered around 0.02 cm-1 is also found to dominate the j = 2 → j' = 0 and j = 4 → j' = 2 transitions in v = 4 for He-D2 collisions consistent with our prior studies of Δj = -2 transition in He + D2(v = 2, j = 2) collisions. Our analysis does not support the hypothesis of Perreault et al. [J. Phys. Chem. Lett. 13, 10912 (2022)] that a strong l = 2 resonance controls the angular distribution for Δj = -2 transition for both systems. Despite improvements in the development of the potential energy surface, a good agreement with SARP experiments for v = 2 is achieved only when contributions from collision energies less than 1.0 cm-1 were excluded in the computation of velocity averaged differential rate coefficients for both systems. This could be due to some uncertainties in the velocity spread in the experiment that employs co-propagation of the collision partners and possibly, the neglect of transverse velocities in the simulation of the experiment.