C-Cl bond rupture in ultraviolet photodissociation of vinyl chloride

Cl atomic elimination in ultraviolet photodissociation of vinyl chloride at 193 and 210 nm has been studied by photofragment ion imaging. The translational energy and angular distributions of the Cl fragments were measured for the two spin-orbit states of P-2(J) (J = 1/2, 3/2). The translational energy distributions of both of the P-2(J) fragments consist of two components; one is a Gaussian-Like translational energy distribution with high anisotropy, beta = 1.0 +/- 0.2, while the other is a statistical distribution with low anisotropy, beta = 0.3 +/- 0.2. These features were almost identical for the two photoexcitation wavelengths. The anisotropy parameter, beta, of the Gaussian translational energy component is in good agreement with the value predicted for the pi* <-- pi transition by ab initio calculations. The high translational energy component of the Cl fragment is assigned to a prompt dissociation induced by the electronic relaxation from the (pi,pi*) to (pi(Cl),sigma*(C-Cl)) repulsive state, while the low translational energy component is assigned to a slow dissociation induced by the relaxation to the ground state. The [Cl*]/[Cl] fine structure branching has been found to be almost identical for the two dissociation pathways despite he large difference of the recoil velocities of the Cl atoms, suggesting an important role of the motion along the coordinate perpendicular to the C-Cl stretching in the nonadiabatic transition.