Dynamics of nonadiabatic reactions. I. F (2P3/2, 2P1/2)+HBr (DBr)→HF (DF)+Br (2P 3/2, 2P1/2)

J. W. Hepburn*, K. Liu, R. G. Macdonald, F. J. Northrup, J. C. Polanyi

*Corresponding author for this work

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45 Scopus citations


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.

Original languageEnglish (US)
Pages (from-to)3353-3364
Number of pages12
JournalThe Journal of Chemical Physics
Issue number7
StatePublished - 1981

ASJC Scopus subject areas

  • Physics and Astronomy(all)
  • Physical and Theoretical Chemistry


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