Studies of the reactions HBr(HI)+e to or from Br-(I-)+H using the FALP and SIFT techniques

The authors have determined the rate coefficients beta for the dissociative attachment reactions of electrons with HBr and HI molecules in their VT-FALP apparatus, and in a parallel study they have determined the rate coefficients k for the exact reverse of these reactions, namely the associative detachment reactions of Br - and I - (and additionally F - and Cl - ) with H atoms using their VT-SIFT apparatus. The ratios of beta /k for these reactions yield values for the free energy changes in the reactions which, combined with their calculated entropy changes, provide values for the enthalpy changes in the reactions. The results obtained indicate, in accordance with expectations, that the HI reaction is closely thermoneutral and that the dissociative attachment reaction of electrons with HBr is endothermic. The derived bond energies in HI and HBr are within error bounds equal to those previously determined using thermochemical data. Some aspects of the kinetics of the reactions are discussed.