Photolysis of HBr at 77 °K

Abstract

The photolysis of HBr has been studied at 77 °K. The photodecomposition of solid state HBr is an inefficient process which is strongly inhibited by a reaction product, probably Br atoms, or by added Br2 or trans-butene-2. The results suggest H2 is formed by abstraction from HBr by thermal H atoms, the activation energy for the abstraction being ~350 cal mole−1. The photolysis of both pure HBr and HBr with added Br2 or trans-butene-2 can be described by an absorber complex mechanism similar to that inferred by Hughes and Purnell in the photolysis of HI. The absorption tail of solid HBr extends beyond the gas phase absorption into the wavelength region in which trapped radicals are generated in HBr–olefin mixtures but where there is no H2 evolution. The solid state absorption spectrum of Br2 is contrasted with that of gaseous Br2; an absorption peak is observed at 2725 Å which may be associated with (Br2)n aglomerates.