collisions*

The Stark-induced adiabatic Raman passage (SARP) technique has become a powerful tool to probe stereodynamics of cold molecular collisions. Recent experiments using SARP techniques have focused on cold collisions of  aligned HD with H₂, D₂, and He as well as collisions of two aligned D₂ molecules. Here, we report full-dimensional quantum calculations of stereodynamic control of rotational quenching of HD(v=1,j=2) by collisions with D₂ that is probed experimentally [1,2]. Both △j=-1 and △j=-2 transitions in HD were explored experimentally. The latter can occur with concurrent rotational excitation of D₂ from j=0 to 2. Our calculations reveal that this inelastic process has a cross section that is 5 times larger than elastically scattered D₂ at collision energies involved in the experiment but it is not considered in the analysis of the experimental data. Differential rates for both horizontal (H-SARP) and vertical (V-SARP) alignments of the HD molecule are computed and compared against experimental findings. While reasonable agreement with experiment is obtained for the △j=-1  transition in HD, strong discrepancy is observed for the △j=-2 transition. The discrepancy persists whether or not the D₂ excitation channel is considered in the theoretical simulations. 

[1] W. E Perreault, N. Mukherjee, and R. N. Zare, Quantum control of molecular collisions at 1 kelvin, Science, 358, 356–359, (2017).

[2] W. E Perreault, N. Mukherjee, and R. N. Zare, Cold quantum-controlled rotationally inelastic scattering of HD with H₂ and D₂ reveals collisional partner re-orientation. Nature Chemistry, 10, 561–567, (2018).