HD (v = 1, j = 2, m) orientation controls HD-He rotationally inelastic scattering near 1 K

To investigate how molecular orientations affect low energy scattering, we have studied the rotational relaxation of HD (v = 1, j = 2, m) → (v' = 1, j' = 0) by collision with ground-state He, where v, j, and m designate the vibrational, rotational, and magnetic quantum numbers, respectively. We experimentally probed different collision geometries by preparing three specific m sublevels, including an m entangled sublevel, belonging to a single rovibrational (v = 1, j = 2) energy level within the ground electronic state of HD using Stark-induced adiabatic Raman passage. Low collision energies (0-5 K) were achieved by coexpanding a 1:19 HD:He mixture in a highly collimated supersonic beam, which has defined the direction of the collision velocity and restricted the incoming orbital angular momentum states, defined by the quantum number l, to l ≤ 2. Partial wave analysis of experimental data shows that a single l = 2 input orbital dominates the scattered angular distribution, implying the presence of a collisional resonance. The differential scattering angular distribution exhibits a greater than fourfold stereodynamic preference for the m = 0 input state vs m = ±2, when the quantization axis is oriented parallel to the collision velocity.