Experimental and Theoretical Study of State-Resolved Electronically Inelastic Collisions of Highly Rotationally Excited CN(A2Π) with Argon and Helium: The Role of Gateway Levels

Abstract

A collaborative study of A → X electronic transitions from CN(A 2 Π,v=3,N=60-63) fine-structure A-doublet levels induced by collisions with argon and helium is presented. Experimental state-to-state rate constants were determined with an optical-optical double resonance technique. Specific levels of CN(A 2 Π,v=3,N=60-63) were prepared by excitation of the photolytically generated radical with a pulsed dye laser on various rotational lines in the A 2 Π-X 2 Σ + (3,0) band, and collisionally populated levels in the v A = 3 and the nearly isoenergetic v X = 7 vibronic manifolds were probed after a short delay by laser fluorescence excitation in the B-X (3,7) and B-A (3,3) bands. Final state distributions (relative state-to-state rate constants) are reported for CN(A)-Ar collisions; the rate constants for transitions induced by He were considerably smaller. Absolute total removal rate constants were also determined. A crossing of the A 2 Π v = 3 F 1 f rotational/fine-structure manifold with the X 2 Σ + v = 7 F 2 levels occurs at J = 62.5. The dependence of A → X rate constants and the total removal rate constants on the initial level demonstrates the importance of this gateway in facilitating collisions between these manifolds. The experimental CN(A)-Ar rate constants have been compared with theoretical rate constants computed in a quantum scattering treatment of the dynamics based on ab initio CN(A,X)-Ar potential energy surfaces. The small non-Born-Oppenheimer mixing of the A and X states in the isolated CN molecule was also included in the calculations. The computed total removal rates show an enhanced value for the perturbed N = 62 F,f initial level, in agreement with experiment, but the computed state-to-state rate constants do not agree well with the experimentally determined values.