Quantum scattering study of rotational energy transfer in OH(A 2Σ+, v′=0) in collisions with He(1S)

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Abstract
A quantum mechanical study of rotational energy transfer (RET) in OH(A 2Σ+, v’=0) in thermal collisions with He(1S) has been performed. The interaction potential of OH(A)+He was computed using the coupled electron pair approximation (CEPA) and a very large basis set. An analytical fit of the resulting OH–He potential was employed in close‐coupling (CC) and coupled states (CS) calculations of integral RET cross sections for collision energies up to 5000 cm−1. The cross sections were integrated over a Boltzmann energy distribution to yield thermally averaged rate coefficients. State‐to‐state RET coefficients for the lowest 11 fine structure levels of OH(A, v’=0) were calculated as a function of the temperature. The agreement between the theoretical and recently measured values at 300 K is very good. The data for the OH(A)+He system are compared to the results of a previous theoretical study of the OH(A)+Ar system [A. Degli Esposti and H.‐J. Werner, J. Chem. Phys. 93, 3351 (1990)]. The theoretical findings fully confirm the qualitatively different behavior of the OH–He and OH–Ar systems, which has been found experimentally by A. Jörg, U. Meier, and K. Kohse‐Höinghaus [J. Chem. Phys. 93, 6453 (1990)]. For rotationally inelastic collisions with He the calculations predict a strong propensity for conserving the Fi fine structure levels in OH. In contrast, only a weak propensity for Fi conservation was reported for OH+Ar. In addition, our calculations for OH+He show a preference for transitions with ‖ΔJ‖=‖ΔN‖=2, whereas a strong preference of the nearly isoenergetic transitions with ‖ΔJ‖=1 and ΔN=0 was reported for OH+Ar.