The dyna~ics of t?e UV photochemistry of HBr on LiF (001) has been studied by angleresolve? tlI~e-~f-fhght mass spectrometry in ultra-high vacuum. Single-photon P?ot?dIS~oclatton o.f adsorbed HBr at 193 nm resulted in photofragment translational energy dlstnbutlOns that dIffered from those produced in the gas-phase photolysis. Angular distributions of the fast H-atom photofragments peaked at 55 + S° to the surface normal consi~tent wit~ a preferentially oriented adsorbate geometry. The angular distribution of atomIC H obtamed from photodissociation ofHBr(ad) using polarized light indicated that a substantial.frac~ion of the H atoms collided with the surface before leaving it. Two types of photoreacttons m the adsorbed state were observed. Molecular hydrogen was formed in the photoinitiated abstraction reaction, H + HBr(ad) -.H 2 (g) + Br, and its markedly non-Boltzmann translational energy distribution was found to have less energy than would be c?nsistent with g~-phase experiments (performed elsewhere). Photoproducts from the bm~olecular rea~tton 2~X (a.d) -+ H2 + X 2 , X = CI, Br were also observed in the present study.ThIS photoreaction, WhICh dId not depend on prior photodissociation of HX(ad), is thought to proceed through electronic excitation of an HX dimer in the adsorbed state.J. Chern. Phys. 95 (2). 15