The role of low energy resonances in the stereodynamics of cold He+D2 collisions

TL;DR

This study uses quantum mechanics to analyze low-energy resonances in cold He+D2 collisions, revealing a discrepancy with experimental results and suggesting the importance of energy-resolved measurements.

Contribution

The paper provides a first-principles quantum mechanical analysis of He+D2 collisions, identifying a dominant l=1 resonance and highlighting discrepancies with experimental observations.

Findings

Predicted a dominant l=1 resonance at low energies

Discrepancy between theoretical predictions and experimental angular distributions

Energy-resolved measurements could clarify the resonance effects

Abstract

In recent experiments using the Stark-induced Adiabatic Raman Passage (SARP) technique, Zhou et al. measured the product's angular distribution for the collisions between He and aligned D2 molecules at cold collision energies. The signatures of the angular distributions were attributed to a l=2 resonance that governs scattering at low energies. A first principles quantum mechanical treatment of this problem is presented here using a highly accurate interaction potential for the He-H2 system. Instead, our results predict a very intense l=1 resonance at low energies, leading to angular distributions that differ from those measured in the experiment. A good agreement with the experiment is achieved only when the l=1 resonance is artificially removed, for example, by excluding the lowest energies present in the experimental velocity distribution. Our analysis revealed that neither the position nor the intensity of the l=1 resonance significantly changes when the interaction potential is modified within its predicted uncertainties. Energy-resolved measurements may help to resolve the discrepancy.