Experimental and theoretical study of rotationally inelastic collisions of CN(A2pi) with N2

Optical-optical double resonance was employed to study rotational energy transfer in collisions of selected rotational/fine-structure levels of CN(A2pi, v = 3) with N2. The CN radical was generated by 193 nm photolysis of BrCN in a slow flow of N2 at total pressures of 0.2-1.4 Torr. Specific fine-structure lambda-doublet levels of CN(A2pi, v = 3) were prepared by pulsed dye laser excitation on isolated lines in the CN A-X (3,0) band, while the initially excited and collisionally populated levels were observed after a short delay by laser-induced fluorescence in the B-A (3,3) band. Total removal rate constants for specified rotational/fine-structure levels involving total angular momentum J from 4.5 to 12.5 were determined. These rate constants decrease with increasing J, with no obvious dependence on the fine-structure/lambda-doublet label. State-to-state relative rate constants were determined for several initial levels and show a strikingly strong collisional propensity to conserve the fine-structure/lambda-doublet label. Comparison is made with the results of quantum scattering calculations based on potential energy surfaces averaged over the orientation of the N2 molecule. Reasonable agreement is found with experimentally determined total removal rate constants. However, the computed state-to-state rate constants show a stronger propensity for fine-structure and lambda-doublet changing transitions. These differences between experiment and theory could be due to the neglect of the N2 orientation and the correlation of the CN and N2 angular motions.