State-resolved rotationally inelastic collisions of highly rotationally excited CN(A (2)Pi) with helium: Influence of the interaction potential

A collaborative experimental and theoretical study of rotationally inelasti c collisions of CN (A (2)Pi, v=3, N=60-62) fine-structure Lambda -doublet l evels with helium is presented. Experimental state-to-state rate constants were determined with an optical-optical double resonance technique. The CN radical was prepared by 193 nm photolysis of BrCN diluted in slowly flowing helium at a total pressure of similar to0.3 Torr. Specific levels of CN(A (2)Pi ,v=3,N=60-62) were prepared by excitation with a pulsed dye laser on various rotational lines in the A (2)Pi -X (2)Sigma (+) (3,0) band, and col lisionally populated levels were probed after a short delay by laser fluore scence excitation in the B (2)Sigma (+)-A (2)Pi (3,3) band. Final state dis tributions (relative state-to-state rate constants) are reported. To calibr ate their magnitude, absolute total removal rate constants were determined. The measured rate constants were compared with theoretical rate constants computed in a quantum scattering treatment of the dynamics based on ab init io CN(A (2)Pi)-He potential energy surfaces. The agreement of measured and computed rate constants is very good. The state-to-state rate constants dis play propensities for conservation of the Lambda -doublet and fine structur e labels for the range of initial levels studied. The significant differenc es in the final state distributions in CN(A (2)Pi)-He collisions and the pr eviously studied CN(A (2)Pi)-Ar system are shown to arise from differences in the interaction potentials for these two systems. (C) 2001 American Inst itute of Physics.