A vacuum ultraviolet photolysis of C,H,Br at 147 nm was studied over a pressure range of 0.5-50 torr a t 298 K. The effects of additives He and NO were also investigated.
The principal reaction products were found to be C,H, and C,H,, with lesser yields of CH, and C,H,. With increasing pressure the product quantum yields 4z of C,H, , CH,, and CH,H, remained constant, while that of C,H, decreased from 0.03 to almost 0. The effect of He as an additive was found to be extremely small on the quantum yields of the major products. Addition of NO completely suppresses the formation of CH,, C2H2, and C2Hs, and reduces partially the production of C,H,. The primary processes appear to involve two electronically excited states. One state mainly yields C,H, by molecular elimination of HBr and is thought to be due to a Rydberg transition. The other state decomposes to C2H, and Br radicals by C-Br bond fission. These two competitive reaction modes contribute to the photodecomposition in proportions of 50% and 50%. The extinction coefficient for C,H,Br at 147 nm and at 298 K has been determined as E = (1/PL) ln(Io/IJ = 712 2 7 atm-' . cm-'.