Elimination mechanisms of Br2+ and Br+ in photodissociation of 1,1-and 1,2-dibromoethylenes using velocity imaging technique

Elimination pathways of the Br2+ and Br+ ionic fragments in photodissociation of 1,2- and 1,1-dibromoethylenes (C2H2Br2) at 233 nm are investigated using time-of-flight mass spectrometer equipped with velocity ion imaging. The Br2+ fragments are verified not to stem from ionization of neutral Br-2, that is a dissociation channel of dibromoethylenes reported previously. Instead, they are produced from dissociative ionization of dibromoethylene isomers. That is, C2H2Br2 is first ionized by absorbing two photons, followed by the dissociation scheme, C2H2Br2+ + hv -> Br-2(+) + C2H2. 1,2-C2H2Br2 gives rise to a bright Br2+ image with anisotropy parameter of -0.5 +/- 0.1; the fragment may recoil at an angle of similar to 66 degrees with respect to the C = C bond axis. However, this channel is relatively slow in 1,1-C2H2Br2 such that a weak Br2+ image is acquired with anisotropy parameter equal to zero, indicative of an isotropic recoil fragment distribution. It is more complicated to understand the formation mechanisms of Br+. Three routes are proposed for dissociation of 1,2-C2H2Br2, including (a) ionization of Br that is eliminated from C2H2Br2 by absorbing one photon, (b) dissociation from C2H2Br2+ by absorbing two more photons, and (c) dissociation of Br2+. Each pathway requires four photons to release one Br+, in contrast to the Br2+ formation that involves a three-photon process. As for 1,1-C2H2Br2, the first two pathways are the same, but the third one is too weak to be detected. (C) 2011 American Institute of Physics. [doi:10.1063/1.3584178]