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

Abstract

Adiabatic and diabatic potential energy surfaces (PES’s) for the interaction of Ar(1S0) with the CN molecule in its ground (X 2Σ+) and first excited (A 2Π) 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 2Σ+ and A 2Π states of CN. The appropriate diabatic PES’s are then used in the study of rotationally inelastic scattering of CN(A 2Π) 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 ∼0.5 Torr. Specific fine-structure Λ-doublet levels of CN(A 2Π,v=3) 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. 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 Λ-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 2Π)–Ar PES’s. The even–odd oscillation in final state populations is ascribed to the near homonuclear character of the PES’s.

Elastic depolarization and polarization transfer in CN(A2Π,v= 4)+Ar collisions

“…In contrast, elastic depolarization is expected to be sensitive to longer ranges extending into the attractive well. 23,36 As was discussed in the introduction, recent experiment and theory have shown considerably deeper attractive minima in both the V sum and V Σ surfaces than originally calculated by Alexander and co-workers, 57 which will also have an as yet undetermined effect on the magnitude of the V 1 coupling PES. 50,52 It is therefore an interesting and open question to what extent the V 1 and V Σ PESs may also contribute to the elastic depolarization of CN(A), which could be addressed by further QM scattering calculations.…”

“…The total population removal rate constants shown in figure 5 Alexander et al 57 but compares favourably to the average range of the attractive well.…”

“…83 The V diff PES for CN(A)-Ar, like that of OH(X)-Ar, has a significant magnitude at the range of the V sum attractive minimum, and the cross-sections for spin-orbit changing collisions are similar to those for conserving transitions. 57 We therefore speculate that the V diff potential may well play an important role in the elastic depolarization of CN(A) by Ar. However, a significant additional difference for CN(A) + Ar is the possibility of EET to the X 2 Σ + state, mediated by the V 1 coupling PES.…”

“…These nevertheless agreed well with the corresponding experimental relative rate constants. 57 RET is usually assumed to be primarily determined by the repulsive wall of the PES, suggesting that the electrostatic coupling is unimportant in this region. In contrast, elastic depolarization is expected to be sensitive to longer ranges extending into the attractive well.…”

“…Dagdigian and co-workers' experiments on collisions of CN (A 2 Π, v = 3, j = 6.5, F 1 , f) with Ar found EET cross-sections to be very similar to those observed for RET within the A state. 75 However, on the reasonable basis that the V 1 coupling 57 were not included in the accompanying scattering calculations of CN(A) + Ar RET rate constants. These nevertheless agreed well with the corresponding experimental relative rate constants.…”