A crossed molecular beam study has been performed on the nonadiabatic reactions F(2P3/2) [F*(P1/2)] + HBr(DBr)→HF(DF) + Br(2P3/2) [Br*( 2P1/2)]. Atomic F came from a seeded supersonic jet, so that the cross sections S,(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., 5r(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, 5r(Br*)/5r(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.
ASJC Scopus subject areas
- Physics and Astronomy(all)
- Physical and Theoretical Chemistry