Hydrogen Rydberg-atom time-of-flight spectroscopy was used to study the photolysis of HX (X=Cl, Br, I) adsorbed on LiF(001). Adsorption characteristics were investigated by temperature programmed desorption. While HCl and HBr appeared to form stable but perforated monolayers on LiF(001), HI did not wet the surface under equilibrium conditions. This behavior of HI was ascribed to the mismatch of its van der Waals diameter and the spacing of the adsorption sites on the substrate lattice. The photolysis experiments were performed with 193 nm excimer laser radiation incident on submonolayers at low temperatures (30-60 K). The H-atom translational energy distributions, P (E’(T)), showed a marked decrease in the fraction of H-atoms scattered inelastically in going from HCl(ad) to HBr(ad), followed by a similar increase in going from HBr(ad) to HI(ad). These nonmonotonic changes in P-INEL down the series HX(ad) (X=CL, Br, I) were thought to be due to the opposed effects of decreasing adsorbate density (which decreases P-INEL by reducing the probability that recoiling (H) over right arrow encounters a neighboring HX(ad)), and increasing collision energy (which increases P-INEL) At H-atom energies of E(T) approximate to 3.3 eV vibrational structure in the H-atom translational energy distribution, P(E’(T)), following photolysis of HI(ad) gave evidence of efficient T-->V energy transfer to neighboring adsorbate molecules.