Stereodynamics of cold HD and D₂ collisions with He

Perreault et al. [1] have recently reported rotational quenching of vibrationally excited and aligned HD and D₂ in collisions with He using the Stark-induced adiabatic Raman passage (SARP) techniques. The SARP technique, combined with co-expansion of the colliding partners in the same molecular beam allows cold, controlled rotationally inelastic scattering near 1 kelvin. Here, we present a comprehensive quantum mechanical study of stereodynamic control of HD/D₂ + He collisions that uses highly accurate interaction potentials with diagonal Born-Oppenheimer corrections appropriate for the HD and D₂ collision partners. Results show that rotational quenching in both systems (Δj=-1 & -2 in HD and Δj=-2 in D₂) is dominated by an l=1 shape resonance below 1 K, regardless of the initial vibrational level of HD or D₂ for vibrational levels less than 4. Our analysis does not support the hypothesis of Perreault et al. [1] that a strong l=2 resonance near 1 K controls the angular distribution for Δj=-2 transition 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.

[1] W. E. Perreault, H. Zhou, N. Mukherjee, and R. N. Zare, Quantum controlled cold scattering challenges theory. J. Phys. Chem. Lett., 13, 10912 (2022).