At low energies, the quantum wave–like nature of molecular interactions results in distinctive scattering behavior, ranging from the universal Wigner laws near 0 kelvin to the occurrence of scattering resonances at higher energies. It has proven challenging to experimentally probe the individual waves underlying these phenomena. We report measurements of state-to-state integral and differential cross sections for inelastic NO-He collisions in the 0.2 to 8.5 centimeter–1 range with 0.02 centimeter–1 resolution. We studied the onset of the resonance regime by probing the lowest-lying resonance dominated by s and p waves only. The highly structured differential cross sections directly reflect the increasing number of contributing waves as the energy is increased. Only with CCSDT(Q) level of theory was it possible to reproduce our measurements.

We present state-to-state differential cross sections for collisions of NO molecules (XΠ,j=1/2,f) with He atoms and ortho-D (j = 0) molecules as a function of collision energy. A high angular resolution obtained using the combination of Stark deceleration and velocity map imaging allows for the observation of diffraction oscillations in the angular scattering distributions. Differences in the differential cross sections and, in particular, differences in the angular spacing between individual diffraction peaks are observed. Since the masses of D and He are almost equal and since D(j = 0) may be considered as a pseudo-atom, these differences directly reflect the larger size of D as compared to He. The observations are in excellent agreement with the cross sections obtained from quantum close-coupling scattering calculations based on accurate ab initio NO-He and NO-D potential energy surfaces. For the latter, we calculated a new NO-D potential energy surface.

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