Quantum scattering study of rotational energy-transfer in OH(A2Sigma+, v'=0) in collision with He(1S)

A quantum mechanical study of rotational energy transfer (RET) in OH(A 2?+, 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 ??J?=??N?=2, whereas a strong preference of the nearly isoenergetic transitions with ??J?=1 and ?N=0 was reported for OH+Ar.