Gilbert J. Mains, Lionel M. Raff and Samuel A. Abrash
J. Phys. Chem., 99, 3532 (1995)
Abstract
The decomposition dynamics of vinyl bromide upon single photon excitation at 193 nm have been investigated using classical trajectory methods on adiabatic excited-state potentials that have been obtained using empirical and ab initio CI methods. The excited-state potential surfaces are represented by a global analytic hypersurface previously developed for the vinyl bromide ground state with the C-Br bonding Morse-type potential replaced with one of the repulsive C-Br interactions obtained in the empirical or ab initio calculations. Energetic considerations suggest that the dissociation dynamics of vinyl bromide upon photolysis at 193 nm involves excitation to three or four repulsive C-Br states which include the Ì1A''(¹s*), b 3A''(¹s*) and c 3A'(ns*) potentials. The effects of a vertical excitation from the ground state to the Ì1A''(¹s*) and c 3A'(ns*) states have been determined by the computation of 300 or more trajectories in each case. The results show that the only products for these excitations are vinyl radicals and either Br(2P3/2) or Br(2P1/2) atoms. No HBr was observed. This result is consistent with the hypothesis advanced in our previous study of vinyl bromide dissociation on the ground-state surface where we suggested that the HBr formed in previously reported beam experiments [Israel J. Chem. 1989, 29, 383] is produced subsequent to internal conversion to the ground state. Combination of the trajectory results with the measured Br/HBr ratio of 1.28 indicates that the internal conversion probability lies in the range 0.44 - 0.64. The calculated time-of-flight (TOF) distribution for C2H3 radicals formed upon Br(2P3/2) atom dissociation from the Ì1A''(¹s*) state is peaked at a vinyl radical translational energy that is in excellent agreement with the observed maximum. However, the computed full-width at half-maximum for this and all other TOF distributions obtained from other surfaces are much smaller than that for the measured distribution when only Br(2P3/2) or Br(2P1/2) are the dissociation products. It is shown that an excellent fit to the measured TOF distribution can be obtained from a linear combination of the TOF distributions computed for excited states which dissociate into Br(2P3/2) and into Br(2P1/2). The values of the expansion coefficients yield a Br(2P3/2) / Br(2P1/2) ratio of 2.33 which is consistent with a statistical formation of Br(2P3/2) and Br(2P1/2).