B. Photo-adsorption, photo-desorption and photo-reactions at surfaces. Oxidation of CO and desorption of oxygen by ultra-violet irradiation of ZnO single crystals under ultra-high vacuum conditions
When the clean surface of a ZnO single crystal, mounted in an UHV flow system, is heated to temperatures of 400 or 450°C and, in the presence of 10–8 Torr O2 and CO, is illuminated periodically by ultra-violet light from a high pressure mercury arc, rapid changes of the partial pressures due to photo catalytic reaction to CO2 or photo desorption of oxygen are monitored by a quadrupole mass spectrometer. In the presence of O2 at pressures of 10–8 Torr or below, atomic oxygen is produced in a quantum process and, in the absence of a partner apt to reaction, emitted into the vacuum. The photo generation of O is accompanied by thermo-desorption of O2. In the presence of CO at pressures of 10–8 Torr or below, no atomic oxygen may be detected, instead, CO2 is produced by photo reaction. Atomic O is generated by photo dissociation of a ZnO bond, 4 eV being the upper limit of the energy necessary for this process.