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

Abstract

A collaborative experimental and theoretical study of rotationally inelastic collisions of CN (A 2Π, v=3, N=60–62) fine-structure Λ-doublet levels 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 ∼0.3 Torr. Specific levels of CN(A 2Π,v=3,N=60–62) 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 were determined. The measured rate constants were compared with theoretical rate constants computed in a quantum scattering treatment of the dynamics based on ab initio CN(A 2Π)–He potential energy surfaces. The agreement of measured and computed rate constants is very good. The state-to-state rate constants display propensities for conservation of the Λ-doublet and fine structure labels for the range of initial levels studied. The significant differences in the final state distributions in CN(A 2Π)–He collisions and the previously studied CN(A 2Π)–Ar system are shown to arise from differences in the interaction potentials for these two systems.