Dynamics of nonadiabatic reactions. I. F (2P3/2, 2P1/2)+HBr (DBr)→HF (DF)+Br (2P3/2, 2P1/2)

Abstract

A crossed molecular beam study has been performed on the nonadiabatic reactions F(2P3/2) [F(2P1/2)]+HBr(DBr)→HF(DF)+Br(2P3/2) [Br(2P1/2)]. Atomic F came from a seeded supersonic jet, so that the cross sections Sr(Br) and Sr(Br) could be measured as a function of collision energy ET = 1–11 kcal/mol. The reagent ratio [F]/[F] was varied by means of a variable temperature F atom source. Products Br and Br were detected by tunable vacuum ultraviolet laser-induced fluorescence (VUV LIF). The reaction cross sections showed no threshold, but a steep decline with increasing ET; the barrier to reaction on the FHBr surface is <1 kcal/mol. The ratio [Br]/[Br] showed no correlation with [F]/[F] in the reagents, indicating that the source of Br was not the adiabatic process F+HBr→HF+Br, but a nonadiabatic process F+HBr→HF+Br, Br. The results at high collision energy indicate that there is a substantial barrier to the reaction F+HBr→HF+Br. The cross section ratio for the two branches of F+HBr, i.e., Sr(Br)/Sr(Br), is 0.056±0.004 at room temperature. This ratio declines slightly with increasing collision energy, and is sensitive to isotopic substitution: for F+DBr at 300 K, Sr(Br)/Sr(Br) = 0.0101±0.0016. The velocity dependence and isotope effect of the branching ratio are discussed in terms of an ’’energy exchange’’ occurring well along the exit valley of the ground electronic state potential-energy surface, in which V–E transfer between nascent HF and Br results in a hop onto the upper potential-energy surface.