The technique of H Rydberg atom photofragment translational spectroscopy ha
s 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 r
ecoil anisotropies for these two channels have been characterized at six di
fferent wavelengths within the same wavelength range. The channel forming g
round state products, H+Br(P-2(3/2)), is observed to arise solely from a pe
rpendicular (i.e., Delta Omega=1) transition at all excitation energies, wh
ereas the channel to formation of excited state products, H+Br(P-2(1/2)), h
as a marked wavelength dependence: at long wavelengths (lambda=243 nm), the
photofragments are produced by a parallel (i.e., Delta Omega=0) photodisso
ciation mechanism, which becomes more perpendicular in character as the pho
tolysis energy is increased. Within the wavelength range studied, the branc
hing 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 influen
ce of spin-orbit interactions in the photofragmentation dynamics of this se
ries of molecules. (C) 1999 American Institute of Physics. [S0021-9606(99)0
1601-3].