State-resolved study of collisional energy transfer between A 2Π v=7 and X 2Σ+ v=11 rotational levels of CN

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Abstract
Collisional transfer from the A 2Π state of CN has been studied with initial and final state resolution by an optical–optical double resonance technique. Specific rotational levels in the v=7 vibrational manifold of the A state of CN in a flow of several Torr of argon are prepared by pulsed laser excitation in the A–X (7,2) band. After a short time delay, a second laser probes the populations of quantum levels in this vibrational manifold and in the nearly isoenergetic v=11 manifold of the X 2Σ+ state by fluorescence excitation in the overlapped B–A (8,7) and B–X (8,11) bands. The interelectronic A→X transfer rate is found to be comparable to that for purely rotational collisional transitions within the A state for all incident levels studied, regardless of whether or not they possess significant X state character, because of isolated molecule non‐Born–Oppenheimer mixing. Reflecting the near homonuclear character of the CN–Ar interaction potentials, the final X state populations exhibited a significant even–odd alternation as a function of the final rotational angular momentum quantum number. These populations could be adequately fit by the sudden scaling relationship for Π→Σ cross sections derived by Alexander and Corey [J. Chem. Phys. 8 4, 100 (1986)].