Differential Cross Sections for Pair-Correlated Rotational Energy Transfer in NO(A²Σ⁺) + N₂, CO, and O₂: Signatures of Quenching Dynamics

A crossed molecular beam, velocity-map ion-imaging apparatus has been used to determine differential cross sections (DCSs), as a function of collider final internal energy, for rotationally inelastic scattering of NO(A²Σ⁺, v = 0, j = 0.5f₁) with N₂, CO, and O₂, at average collision energies close to 800 cm–1. DCSs are strongly forward scattered for all three colliders for all observed NO(A) final rotational states, N′. For collisions with N₂ and CO, the fraction of NO(A) that is scattered sideways and backward increases with increasing N′, as does the internal rotational excitation of the colliders, with N₂ having the highest internal excitation. In contrast, the DCSs for collisions with O₂ are essentially only forward scattered, with little rotational excitation of the O₂. The sideways and backward scattering expected from low-impact-parameter collisions, and the rotational excitation expected from the orientational dependence of published van der Waals potential energy surfaces (PESs), are absent in the observed NO(A) + O₂ results. This is consistent with the removal of these short-range scattering trajectories via facile electronic quenching of NO(A) by O₂, in agreement with the literature determination of the coupled NO-O₂ PESs and the associated conical intersections. In contrast, collisions at high-impact parameter that predominately sample the attractive van der Waals minimum do not experience quenching and are inelastically forward scattered with low rotational excitation.