Three dimensional potential energy surfaces for the collision systems OH(X 2Π)+He and OH(X 2Π)+Ar have been calculated using the coupled electron pair approximation (CEPA) and large basis sets. The asymptotically degenerate 2Πx and 2Πy states split into two states of 2A′ and 2A″ symmetry, respectively, when the C∞v symmetry is lifted by the approach of the noble gas atom. The average and half difference of the calculated points on the A″ and A′ potential energy surfaces were fitted to analytical functions, which were then vibrationally averaged. These potential energy surfaces have been used in quantum scattering calculations of cross sections for collision induced rotationally inelastic transitions. Test calculations showed that the cross sections obtained from exact close-coupling calculations (CC) and within the coupled states approximation (CS) are in close agreement for these systems, and therefore the CS approximation has been used in all further calculations. Rotational transitions with Λ doublet resolution show, within the same spin–orbit manifold and at low collision energies, a propensity to populate preferentially the e final levels in the F1(2Π3/2) state and an e/f conserving propensity in the F2(2Π1/2) state, while transitions between the two spin–orbit manifolds show a parity conserving propensity. For the v=2 vibrational level kinetic rate coefficients were calculated for a large range of temperatures. The calculated cross sections are in excellent agreement with recent measurements of Schreel, Schleipen, Epping, and ter Meulen.
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.
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