Quantum simulation of alignment dependent differential cross sections in co-propagating molecular beams at cold collision energies

Cold collisions can be achieved experimentally by co-propagating colliding partners of similar mass within a single molecular beam. This technique, combined with Stark-induced adiabatic Raman passage (SARP), makes it possible to measure the angular distributions for different molecular axis alignments of the incoming molecules, thus probing the stereodynamics of the collisions at very low energies. Reproducing SARP experiments is a very stringent test for theory, even though the systems under study involve closed-shell atoms and molecules with very few electrons for which exact quantum scattering calculations on highly accurate potential energy surfaces are computationally feasible. While simulations of some experimental results using first-principles theoretical calculations have been satisfactory, theory has been unable to reproduce the experimental angular distributions for He + D2 inelastic collisions. Furthermore, an ℓ = 1 partial-wave resonance predicted by theory came at variance with the ℓ = 2 resonance obtained by fitting the experimental results. Here, we demonstrate how theory and experiment can be reconciled by the explicit consideration of the divergence of the molecular beam. While the effect of the divergence is almost irrelevant for collision energies higher than 0.5 K, at lower collision energies, it can produce significantly different results. Our simulations show that signatures of an ℓ = 2 resonance obtained through fitting of the experimental angular distribution ignoring beam divergence is, in fact, an ℓ = 1 resonance observed in the scattering calculations. These results indicate that the combination of theory and experiment is necessary for the analysis and interpretation of complex molecular beam experiments.