Spin-orbit effects in quantum mechanical rate constant calculations for the F+H-2 -> HF+H reaction

Exact and approximate quantum mechanical calculations of reaction probabili ties and cumulative reaction probabilities have been carried out for the FH-2 reaction on the ab initio adiabatic potential energy surfaces by Stark and Werner (SW) and by Hartke, Stark, and Werner (HSW), the latter includin g spin-orbit corrections in the entrance channel. These data have been empl oyed to obtain thermal rate constants for the title reaction in the tempera ture range 200-700 K. The exact and approximate results have been compared with experimental determinations and previous theoretical predictions. In p articular, the reaction probabilities obtained on the HSW surface are found to be in very good agreement with recent calculations by Alexander [J. Che m. Phys. 109, 5710 (1998)] based on the exact treatment of spin-orbit and C oriolis coupling for this system. However, the rate constants calculated on the HSW PES are systematically lower than the experimental values, which i ndicates that the height of the adiabatic potential energy surface is too h igh. Furthermore, an estimate of cross sections from the reaction probabili ties calculated by Alexander shows that the contribution to the low tempera ture rate constants from spin-orbit excited F(P-2(1/2)) atoms through nonad iabatic channels is very small and, thus, nonadiabatic effects are not suff icient to bring the calculated rate constants to a better agreement with th e experimental measurements. (C) 1999 American Institute of Physics. [S0021 -9606(99)00533-4].