Energy-dependent cross sections and nonadiabatic reaction dynamics in F(~2P_(3/2, ~2P_(1/2) + n - H_2 -> HF(v, J) + H

摘要

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) approx= 1.9 kcal/mole; K. Stark and H. Werner, J. Chem. Phys. 104, 6515 (1996)] for the lowest adiabatic F(~2P_(3/2)) + H_2 potential energy surface, therefore preferentially enhancing nonadiabatic channels due to spin-orbit excited F~*(~2P_(1/2)) (#DELTA#E_(spin-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) approx= 0.3 kcal/mole), in contrast with exact adiabatic quantum calculations that predict a rapid decrease below E_(com) approx= 1.9 kcal/mole and vanishing reaction probability by E_(com) approx= 0.7 kcal/mol. Further evidence for a nonadiabatic F~*(~2P_(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(~2P_(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.