Br*(2P1/2)+H2(v=0,1): Laser studies of the competition between reactive pathways and inelastic energy transfer channels

A systematic experimental study has been made on inelastic and reactive processes in the Br*(2P1/2)+H2(v=0,1) system. Laser photolysis and time-resolved infrared detection techniques allow direct observation of the previously unreported E–V equilibrium: Br*+H2(v=0)⇄kVEkEVBr +H2(v=1). A nonlinear kinetic analysis is presented that permits an estimate of the quantum efficiencies for the forward and reverse E–V processes, and provides an explanation for previous discrepancies in measured total Br* quenching rates with H2. In this work the quenching rate constant for Br*by H2 is determined at very low Br* concentrations to be 6.3(±1)×10−12 cm3 molecule−1 sec−1. At higher concentrations of Br*, second order processes involving collisions of Br*with H2(v=1) have been investigated. A substantial fraction of the Br*+H2(v=1) collisions react to form HBr(v=0)+H(2S1/2). It is suggested that the resonant E–V transfer pathway in the entrance channel Br*+H2(v=1) → Br+H2(v=2) may provide an efficient mechanism for potential surface crossing, thereby enhancing the reaction probability.