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

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. 84, 100 (1986)].