Thermal energy reactions of OH − + Cl 2, Br 2: rate coefficients, product branching fractions and OH product vibrational populations

Abstract

The rate coefficients, branching fractions, and OH product vibrational distributions are measured in a flowing afterglow apparatus for the OH − + Cl 2 and OH − + Br 2 reactions at thermal energy. Both reactions proceed at the Langevin collision rate, with k = 1.33(±0.13) × 10 −9 cm 3 molecule −1 s −1 for OH − Cl 2 and k = 1.60(±0.16) × 10 −9 cm 3 molecule −1 s −1 for OH − + Br 2. The branching fractions for OH − + Cl 2 are 0.75(±0.10) Cl − 2 + OH, 0.24(±0.10) Cl − + HOCl and a discernible ⩽0.01 HOCl − + Cl. For OH − + Br 2, the branching fractions are 0.75(±0.10) Br − 2 + OH, 0.15(±0.05) Br − + HOBr and 0.10(±0.05) HOBr − + Br. The vibrational populations of the OH product from the charge transfer channel are measured by laser-induced fluorescence detection and found to be OH( v = 1)/OH( v = 0) = 0.01(±0.01) for both reactions. While the OH( v) states are consistent with a Franck—Condon mechanism, arguments for a long-lived intermediate complex are made on the basis of substantial branching fractions into Cl − + HOCl (Br − + HOBr) and HOCl − + Cl (HOBr − + Br) channels and the extreme difference in bond length for Cl 2 and Cl − 2 (Br 2 and Br − 2).