C-Br bond rupture in 193 nm photodissociation of vinyl bromide

Photofragment ion imaging has been applied to 193 nm photodissociation of vinyl bromide to measure the speed and angular distributions of Br atoms. Br atoms were observed in both spin-orbit states (P-2(J); J = 1/2 or 3/2) with the branching ratio, [Br*]/[Br], of 0.06 +/- 0.03. Both Br(P-2(J)) distributions were dominated by anisotropic high translational energy components, which are ascribed to C-Br bond rupture via surface crossing between the optically-excited (1)(pi, pi*) state and (1)(n(Br) or pi(Br), sigma *(C-Br)) repulsive state(s) The anisotropy parameters of the high translational energy components, 1.3 (Br) and 1.2 (Br*), provide the direction of the transition dipole moment for the pi* <-- pi transition to be 26 +/- 3 degrees from the C=C bond axis, in good agreement with ab initio calculations by Yamashita. Despite that the available energy is 10.5 kcal mol-'(1) smaller in the Br* channel, the peak energies of P(E-tau) in the Br* and Br channels were quite similar, indicating that the fine structure branching occurs in the molecular region. The low translational energy components of Br and Br* observed are ascribed to the dissociation of VBr fi-om the ground state, although contribution from the secondary dissociation of C2H2Br radical is also suggested for the Br channel. The anisotropy of the low energy component implies that the lifetime of the ground state VBr is shorter than its rotational period (4.7 mu s). (C) 1998 Elsevier Science B.V. All rights reserved.