Differential Cross Sections for Pair-Correlated Rotational Energy Transfer in NO(A 2 Σ + ) + N 2 , CO, and O 2 : 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(A 2 Σ + , v = 0, j = 0.5 f 1 ) with N 2 , CO, and O 2 , 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 N 2 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 N 2 having the highest internal excitation. In contrast, the DCSs for collisions with O 2 are essentially only forward scattered, with little rotational excitation of the O 2 . 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) + O 2 results. This is consistent with the removal of these short-range scattering trajectories via facile electronic quenching of NO(A) by O 2 , in agreement with the literature determination of the coupled NO-O 2 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.