A crossed molecular beam study has been performed on the nonadiabatic reactions F(2P3/2) lsqb;F(2P1/2)rsqb;+HBr(DBr)rarr;HF(DF)+Br(2P3/2) lsqb;Br(2P1/2)rsqb;. Atomic F came from a seeded supersonic jet, so that the cross sectionsSr(Br) andSr(Br) could be measured as a function of collision energyET= 1ndash;11 kcal/mol. The reagent ratio lsqb;Frsqb;/lsqb;Frsqb; was varied by means of a variable temperature F atom source. Products Br and Br were detected by tunable vacuum ultraviolet laserhyphen;induced fluorescence (VUV LIF). The reaction cross sections showed no threshold, but a steep decline with increasingET; the barrier to reaction on the FHBr surface is 1 kcal/mol. The ratio lsqb;Brrsqb;/lsqb;Brrsqb; showed no correlation with lsqb;Frsqb;/lsqb;Frsqb; in the reagents, indicating that the source of Br was not the adiabatic process F+HBrrarr;HF+Br, but a nonadiabatic process F+HBrrarr;HF+Br, Br. The results at high collision energy indicate that there is a substantial barrier to the reaction F+HBrrarr;HF+Br. The cross section ratio for the two branches of F+HBr, i.e.,Sr(Br)/Sr(Br), is 0.056plusmn;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) = 0.0101plusmn;0.0016. The velocity dependence and isotope effect of the branching ratio are discussed in terms of an rsquo;rsquo;energy exchangersquo;rsquo; occurring well along the exit valley of the ground electronic state potentialhyphen;energy surface, in whichVndash;Etransfer between nascent HF and Br results in a hop onto the upper potentialhyphen;energy surface.
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