QUANTUM SCATTERING STUDY OF ROTATIONAL ENERGY-TRANSFER IN OH(A2-SIGMA+,V'=0) IN COLLISIONS WITH HE(1S)

A quantum mechanical study of rotational energy transfer (RET) in OH(A 2-SIGMA+, upsilon' = 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, upsilon' = 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. Jorg, U. Meier, and K. Kohse-Hoinghaus [J. Chem. Phys. 93, 6453 (1990)]. For rotationally inelastic collisions with He the calculations predict a strong propensity for conserving the F(i) fine structure levels in OH. In contrast, only a weak propensity for F(i) conservation was reported for OH + Ar. In addition, our calculations for OH + He show a preference for transitions with \DELTA-J\ = \DELTA-N\ = 2, whereas a strong preference of the nearly isoenergetic transitions with \DELTA-J\ = 1 and DELTA-N = 0 was reported for OH + Ar.