Ultraviolet (UV) irradiation (lambda < 400 nm) of H2O co-adsorbed with alkali metal atoms (AM=K, Cs) on the graphite (0001) surface at 90 K leads to the formation of a range of desorbing products (H-2, CH4, CO, and CO2); and to the formation of oxygen-rich AM-O-C complexes on the surface. The reaction proceeds via hydrated AM complexes [AM-(H2O)(n)], which represent the main photo-reactive center in the co-adsorption systems. The proposed mechanism is initiated by the formation of hot electron-hole pairs in the substrate under UV irradiation. Attachment of a hot electron to an AM-(H2O)(n) complex can lead to selective dissociation of the associated water molecules. The resultant release of reactive species (e.g., excited H; OH) initiates a range of branched/chain reactions yielding the observed desorbing species.