An investigation of the F+H-2 reaction based on a full ab initio description of the open-shell character of the F(P-2) atom

Expanding on an earlier Communication [M. H. Alexander, H.-J. Werner, and D . E. Manolopoulos, J. Chem. Phys. 109, 5710 (1998)], we present here the fu ll framework for the quantum treatment of reactions of the fluorine atom wi th molecular hydrogen. This involves four potential energy surfaces (PESs) and two, coordinate-dependent spin-orbit interaction terms, all of which we re fitted to the results of ab initio calculations. Quantum scattering calc ulations, based on a time-independent method formulated in hyperspherical c oordinates, were carried out to determine initial and final state-resolved reactive cross sections, for reaction of F in its ground (P-2(3/2)) and exc ited (P-2(1/2)) spin-orbit state with H-2 in j=0 and j=2(pH(2)) and j=1(oH( 2)). The overall reactivity of the excited state of F, which can occur only through nonadiabatic transitions, is found to be small, at most 25% of the reactivity of the ground spin-orbit state, which is adiabatically allowed. In addition, when compared with results of earlier calculations, based on a single, electronically adiabatic, PES, our calculations show that even fi ne details of the dynamics of the F+H-2 reaction will be well described by calculations on a single PES. The contribution of the excited spin-orbit st ate can be seen most clearly in the formation of HF products in the v=3 vib rational manifold, which are nearly thermoneutral (or even slightly endoerg ic) in the reaction of ground-state F atoms. The cross section for the near resonant electronic-rotational process [F*+H-2(j=0)-->F+H-2(j=2)] is found to be large, in confirmation of earlier work. (C) 2000 American Institute of Physics. [S0021-9606(00)01648-2].