Potential energy surfaces for the CN(X 2Σ+,A 2Π)Ar system and inelastic scattering within the A state

Adiabatic and diabatic potential energy surfaces (PES's) for the interaction of Ar(S-1(0)) with the CN molecule in its ground (X (2)Sigma(+)) and first excited (A (2)Pi) electronic states were determined using multireference configuration-interaction calculations. The three electronically adiabatic potential energy surfaces (PES's 2A' and 1A') are transformed to three diabatic PES's plus one additional PES which describes the coupling between the two diabatic PES's of A' symmetry which correlate asymptotically with the X (2)Sigma(+) and A (2)Pi states of CN. The appropriate diabatic PES's are then used in the study of rotationally inelastic scattering of CN(A (2)Pi) in collisions with Ar. Experimental state-to-state rate constants were measured 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 similar to 0.5 Torr. Specific fine-structure Lambda-doublet levels of CN(A (2)Pi,v=3) 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 collisionally populated levels were probed after a short delay by laser fluorescence excitation in the B (2)Sigma(+)-A (2)Pi(3,3) band. State-to-state rate constants, both relative and absolute, were determined for several rotational levels with J=6.5 and 7.5. The final state distributions displayed an even-odd alternation as a function of the final angular momentum J for scattering into certain fine-structure Lambda-doublet manifolds. The measured state-to-state rate constants agreed quite well with rate constants derived from quantum scattering calculations with the ab initio CN(A (2)Pi)-Ar PES's. The even-odd oscillation in final state populations is ascribed to the near homonuclear character of the PES's. (C) 2000 American Institute of Physics. [S0021-9606(00)00602-4].