Photodissociation of 1,2-C2H2Br2 at 248 nm: Competition between three-body formation Br+Br+C2H2 and molecular Br2 elimination

Abstract

The photodissociation of 1,2-C2H2Br2 at 248 nm has been studied by product translational spectroscopy. The results show that the molecule dissociates exclusively into the products (1) Br2+C2H2 and (2) Br (fast)+Br (slow)+C2H2 with a branching ratio ∼0.2:0.8. While the cleavages of the C–Br bonds are not symmetric, producing the Br atoms at unequal velocities, the anisotropy of the products indicates that both reactions occur in a fraction of a rotational period. Following an asynchronous concerted reaction, the triple products were simulated with the P(Et) distributions coupled by asymmetric angular distributions. A mechanism consistent with the measured results is proposed that the Br2 elimination is a result of a fast intersystem crossing from the ππ* pumped state while the triple products occur via a simultaneous asymmetric scission of the C–Br bonds along the nσ* state.