Quantum Controlled Cold Scattering Challenges Theory.

Our previous rotationally inelastic cold scattering experiments between state prepared D 2 ( v = 2, j = 2, m = 0) and He disagreed with theory, raising serious concerns about either our understanding of the anisotropic potential or the accuracy of the measurement. To further interrogate interactions between molecular hydrogen and atomic helium, we study the Δ j = 1and Δ j = 2 rotational relaxation of HD ( v = 2, j = 2, m = 0) by collision with He. The two rotational transitions probe different anisotropic components of the van der Waals potential. Our state resolved scattering study shows that these two transitions are mediated by two different shape resonances l = 1 for Δ j = 1 and l = 2 for Δ j = 2. The strong l = 1 resonance dominates the Δ j = 1 scattering, agreeing with theory. However, the dominance of the weaker l = 2 resonance in the Δ j = 2 transition, which matches our earlier D 2 -He result, contradicts theoretical calculations. The continued contradiction, when we expect one-to-one correspondence between our stereodynamically controlled scattering experiment and theoretical calculations, makes us question the accuracy of the weaker anisotropic part of the H 2 -He interaction potential.