Energy-dependent cross sections and nonadiabatic reaction dynamics in F(2P3/2, 2P1/2)+n-H2→HF(v,J)+H

High-sensitivity direct IR laser absorption methods are exploited to investigate quantum state-resolved reactive scattering dynamics of F+n-H-2 (j = 0,1) --> HF(v, J) + H in low-density crossed supersonic jets under single collision conditions. Nascent rotational state distributions and relative cross sections for reactive scattering into the energetically highest HF (v = 3,J) vibrational manifold are obtained as a function of center-of-mass collision energies from E-com =2.4 kcal/mole down to 0.3 kcal/mole. This energy range extends substantially below the theoretically predicted transition state barrier [E-barrier approximate to 1.9 kcal/mole; K. Stark and H. Werner, J. Chem. Phys. 104, 6515 (1996)] for the lowest adiabatic F(P-2(3/2)) + H-2 potential energy surface, therefore preferentially enhancing nonadiabatic channels due to spin-orbit excited F*( P-2(1/2)) (Delta Espin-orbit = 1.15 kcal/mole) in the discharge source. The HF (v = 3,J) cross sections decrease gradually from 2.4 kcal/ mole down to the lowest energies investigated (E-com approximate to 0.3 kcal/ mole), in contrast with exact adiabatic quantum calculations that predict a rapid decrease below E-com approximate to 1.9 kcal/mole and vanishing reaction probability by E-com approximate to 0.7 kcal/mol. Further evidence for a nonadiabatic F*(P-2(1/2)) reaction channel is provided by nascent rotational state distributions in HF (v = 3,J), which are >2-3-fold hotter than predicted by purely adiabatic calculations. Most dramatically, the nascent product distributions reveal multiple HF (v = 3,J) rovibrational states that would be energetically inaccessible from ground state F(P-2(3/2)) atom reactions. These quantum state resolved reactive scattering studies provide the first evidence for finite nonadiabatic dynamics involving multiple potential energy surfaces in this well-studied "benchmark'' F+H-2 reaction system. (C) 1999 American Institute of Physics. [S0021-9606(99)00242-1].