Comparative stereodynamics in molecule-atom and molecule-molecule rotational energy transfer: NO(A²Σ⁺) + He and D₂

We present a crossed molecular beam scattering study, using velocity-map ion-imaging detection, of state-to-state rotational energy transfer for NO(A²Σ⁺) in collisions with the kinematically identical colliders He and D₂. We report differential cross sections and angle-resolved rotational angular momentum polarization moments for transfer of NO(A, v = 0, N = 0, j = 0.5) to NO(A, v = 0, N' = 3, 5-12) in collisions with He and D₂ at respective average collision energies of 670 cm^-1 and 663 cm^-1. Quantum scattering calculations on a literature ab initio potential energy surface for NO(A)-He (Kłos et al., J Chem Phys 129, 244303 (2008)) yield near-quantitative agreement with the experimental differential scattering cross sections, and good agreement with the rotational polarization moments. This confirms that the Kłos et al. potential is accurate within the experimental collisional energy range. Comparison of the experimental results for NO(A) + D₂ and He collisions provides information on the hitherto unknown NO(A)-D₂ potential energy surface. The similarities in the measured scattering dynamics of NO(A) imply that the general form of the NO(A)-D₂ potential must be similar to that calculated for NO(A)-He. A consistent trend for the rotational rainbow maximum in the differential cross sections for NO(A) + D₂ to peak at more forward angles than those for NO(A) + He is consistent with the NO(A)-D₂ potential being more anisotropic with respect to NO(A) orientation. No evidence is found in the experimental measurements for coincident rotational excitation of the D₂, consistent with the potential having low anisotropy with respect to D₂. The NO(A) + He polarization moments deviate systematically from the predictions of a hard-shell, kinematic-apse scattering model, with larger deviations as N' increases, which we attribute to the shallow gradient of the anisotropic repulsive NO(A)-He potential energy surface.