193 nm photolysis of vinyl bromide: Nascent product distribution of the C2H3Br→C2H2 (vinylidene)+HBr channel

The internal energy content of the photofragments HBr and C2H2 from the minor channel of the photolysis of C2H3Br with 193 nm radiation has been measured using time-resolved infrared Fourier transform IR emission spectroscopy with 0.5 μs resolution. Vibrational level population and the rotational population of the HBr fragment are determined from 1.0 μs following the photolysis until complete HBr relaxation. The nascent distribution of HBr is extrapolated, from a collision quenching model with a Boltzmann distribution, to be at 8690 and 7000 K for the vibration and rotation respectively. The product vibrational energy distribution supports a reaction mechanism based on the 3-centered HBr elimination process yielding vinylidene and HBr. The nascent internal energy of vinylidene is deduced to be 24 kcal/mol. Vinylidene isomerizes to acetylene and the acetylene emission bands, ν3, ν4+ν5 and ν5, are detected.