The photoinduced reaction of digermane with the Si(111) surface using ultraviolet irradiation has been studied using temperature-programmed desorption (TPD) and Auger electron spectroscopy (AES). Hydrogen and germane desorption yields and relative Ge/Si AES signals are used to determine the reactivity of digermane. UV irradiation during or after dosing of the Si crystal surface at low temperatures (e120 K) enhances the reactivity of digermane compared to similar doses without UV irradiation. Adsorption and photoexcitation of digermane at 110 K lead to significantly more reaction that at 120 K. We also find that UV irradiation after dosing the Si(111) surface with digermane enhances the reactivity more than simultaneous UV irradiation at 110 K, but the opposite is true at 120 K. Evidence is also found for precursor-mediated adsorption, and this is used to rationalize the dramatic changes with temperature. Photoexcitation of digermane at low temperatures leads initially to deposition of a-Ge:H.