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(²P_3/2) [F(²P_1/2)]+HBr(DBr)→HF(DF)+Br(²P_3/2) [Br(²P_1/2)]. Atomic F came from a seeded supersonic jet, so that the cross sections S_r(Br) and S_r(Br) could be measured as a function of collision energy E_T = 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 E_T; the barrier to reaction on the FHBr surface is S_r(Br)/S_r(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, S_r(Br)/S_r(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.