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

Abstract

High-sensitivity direct IR laser absorption methods are exploited to investigate quantum state-resolved reactive scattering dynamics of F+n-H2(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 Ecom=2.4 kcal/mole down to 0.3 kcal/mole. This energy range extends substantially below the theoretically predicted transition state barrier [Ebarrier≈1.9 kcal/mole; K. Stark and H. Werner, J. Chem. Phys. 104, 6515 (1996)] for the lowest adiabatic F(2P3/2)+H2 potential energy surface, therefore preferentially enhancing nonadiabatic channels due to spin–orbit excited F*(2P1/2) (ΔEspin–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 (Ecom≈0.3 kcal/mole), in contrast with exact adiabatic quantum calculations that predict a rapid decrease below Ecom≈1.9 kcal/mole and vanishing reaction probability by Ecom≈0.7 kcal/mol. Further evidence for a nonadiabatic F*(2P1/2) 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(2P3/2) 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+H2 reaction system.