Photodissociation of vibrationally excited hydrogen bromide

A two-step laser photodissociation technique was used to measure the cross section for photodissociation of HBr(v=1) at 258.9 nm. The cross section was found to be (5.5±1.1) ×10−20 cm2/molecule and is 34±7 times larger than the cross section for photodissociation of HBr(v=0) at the same wavelength. A theoretical estimate of the ratio of cross sections, based on a ’’reflection approximation’’ treatment of the UV absorption spectrum of HBr, gave poor agreement with the experimental result. The limitations of simple approximations to the photodissociation process in the threshold region of a photodissociative transition are discussed.