Irradiation with 1.7 and 1.97 eV photons of O2 physisorbed on GaAs(110) at 25 K induced strong surface oxidation. The oxidation rate was dependent on photon energy as well as substrate doping type. The 1.97 eV photons induced reaction approximately 30 times faster than did the 1.7 eV photons. For fixed photon energy, reaction on p-type substrates was approximately 6 times faster than on n-type substrates. These results stand in constrast from those of room temperature experiments where reaction rate was independent of doping type and the dependence on photon energy reflected only the substrate photon absorption coefficient. We show that photoexcited hot electrons are responsible for photo-induced reactions at low temperature. Coupling between hot electrons and physisorbed O2 is via resonant tunneling involving the O2 electron affinity level.
