Experimental and theoretical study of rotationally inelastic collisions of highly rotationally excited CN(A 2Π) with Ar

A collaborative experimental and theoretical study of rotationally inelastic collisions of CN(A 2Π,v=3,N=60) fine-structure Λ-doublet levels with argon 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 argon at a total pressure of ∼0.9 Torr. Specific levels of CN(A 2Π,v=3,N=60) were prepared by excitation with a pulsed dye laser on various rotational lines in the A 2Π–X 2Σ+(3,0) band, and collisionally populated levels were probed after a short delay by laser fluorescence excitation in the B 2Σ+–A 2Π(3,3) band. Final state distributions (relative state-to-state rate constants) are reported. To calibrate their magnitude, absolute total removal rate constants and the large state-to-state rate constants for ΔN=−1 fine-structure conserving, Λ-doublet symmetry-conserving transitions were determined. The measured rate constants were compared with theoretical rate constants computed in a quantum scattering treatment of the dynamics with ab initio CN(A 2Π)–Ar potential energy surfaces. The agreement of measured and computed rate constants is very good. The rate constants display dramatic Λ-doublet propensities which depend upon the reflection symmetry of the initial level. From examination of coupled-state partial cross sections as a function of the projection quantum number specifying the angle of approach of the Ar atom to the CN plane of rotation, these propensities are shown to arise from a “helicopter” approach orientation which facilitates curve crossings between effective potential energy curves correlating with different rotational asymptotes.