The ultraviolet photodissociation dynamics of hydrogen bromide

The technique of H Rydberg atom photofragment translational spectroscopy has been applied to investigate the ultraviolet photodissociation dynamics of hydrogen bromide. Branching fractions between the channels forming ground Br(P-2(3/2)) and spin-orbit excited Br(P-2(1/2)) atoms have been determined at 15 independent wavelengths in the range 201-253 nm, and photofragment recoil anisotropies for these two channels have been characterized at six different wavelengths within the same wavelength range. The channel forming ground state products, H+Br(P-2(3/2)), is observed to arise solely from a perpendicular (i.e., Delta Omega=1) transition at all excitation energies, whereas the channel to formation of excited state products, H+Br(P-2(1/2)), has a marked wavelength dependence: at long wavelengths (lambda=243 nm), the photofragments are produced by a parallel (i.e., Delta Omega=0) photodissociation mechanism, which becomes more perpendicular in character as the photolysis energy is increased. Within the wavelength range studied, the branching fractions indicate that Br(P-2(3/2)) products are formed in preference to Br(P-2(1/2)) products, with propensities that are relatively invariant to excitation wavelength, although a small, yet pronounced, cusp appears at lambda similar to 235 nm. The observations are discussed with reference to the known behavior of the other hydrogen halides and highlight the influence of spin-orbit interactions in the photofragmentation dynamics of this series of molecules. (C) 1999 American Institute of Physics. [S0021-9606(99)01601-3].