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

Abstract

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 = 0.5f 1) 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.