Two-channel competitive photodecomposition reaction of ethyl bromide at 174.3 – 174.5 nm

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The vacuum UV photolysis of ethyl bromide at 174.3 – 174.5 nm was studied over the pressure range 0.2 – 31.0 Torr at room temperature using a nitrogen atom resonance lamp. The pressure effect with and without helium as an additive was investigated. A scavenger effect of the reaction was also observed by adding NO as a radical scavenger. The principal reaction products were C 2H 4, C 2H 6 and CH 3CHBr 2. The product quantum yields φ of C 2H 4 and CH 3CHBr 2 were constant with increasing pressure of C 2H 5Br, while that of C 2H 6 was found to have a weak positive pressure dependence at low pressure but became constant on increasing the pressure. When the helium pressure was varied at a constant C 2H 5Br pressure, however, the quantum yields of all products were found to be pressure independent. The formation of C 2H 6 and CH 3CHBr 2 was completely suppressed by adding NO and that of C 2H 4 was reduced. These results were interpreted in terms of two-channel competition between molecular elimination processes and radical formation. The decomposition modes were 20% molecular elimination and 80% radical reactions. The above results suggest that two electronically excited states exist, and that a reactant molecule which absorbs a photon proceeds to one of two different electronically excited states via one of two pathways. One of the electronically excited states plays a role as the main source of the molecular elimination products and the other state is related to the radical decomposition mode.