Cold quantum-controlled rotationally inelastic scattering of HD with H2 and D2 reveals collisional partner reorientation

Abstract

Molecular interactions are best probed by scattering experiments. Interpretation of these studies has been limited by lack of control over the quantum states of the incoming collision partners. We report here the rotationally inelastic collisions of quantum-state prepared deuterium hydride (HD) with H and D using a method that provides an improved control over the input states. HD was coexpanded with its partner in a single supersonic beam, which reduced the collision temperature to 0-5 K, and thereby restricted the involved incoming partial waves to s and p. By preparing HD with its bond axis preferentially aligned parallel and perpendicular to the relative velocity of the colliding partners, we observed that the rotational relaxation of HD depends strongly on the initial bond-axis orientation. We developed a partial-wave analysis that conclusively demonstrates that the scattering mechanism involves the exchange of internal angular momentum between the colliding partners. The striking differences between H/HD and D/HD scattering suggest the presence of anisotropically sensitive resonances.

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

“…Using quantum state selected scattering of NO with He, Vogels et al previously demonstrated that in the presence of collisional resonance the resonant partial wave dominates the scattering angular distribution. 5 The nearly complete control achieved in the present study is in contrast with our earlier work on the rotationally inelastic collisions of HD with H 2 and D 2 molecules, 6,7 where the partner molecule was not state-selected and so the input channel remained only partially defined.…”

“…(7) results from the interference of the input orbital states which are scattered (diffracted) by the interaction potential, and so, the complex coefficients c l ′ m l ′ that appear in Eq. (7) are not independent of one another. The scattering matrix, which is mathematically defined by Ψ outgoing = SΨ incoming , [39][40][41] gives the transition probabilities connecting specific input and output asymptotic states.…”

“…Because the scattering center-of-mass frame coincides with the moving frame of reference of the molecular beam, the scattering angular distribution can then be easily extracted from the velocity distribution using the final speed of the scattered HD molecules calculated from conservation of energy. 6,7…”

“…While fitting the experimental H-SARP and V-SARP angular distributions, the complex coefficients appearing in Eq. (7) in the partial wave expansion must be related to the same set of scattering matrix elements to ensure that the complex coefficients are internally consistent. As discussed in the supplementary material, we were able to reduce drastically the number of terms in the partial wave expansion because the input and output quantum states in our experiments were highly defined.…”

“…The relative values of the complex amplitudes c l ′ m l ′ of the possible outgoing orbital states for H-SARP and V-SARP scattering in Eq. (7) and their expressions in terms of the relevant scattering matrix elements S J (jl;j ′ l ′ ).…”