Spectral dependencies of the quantum yield of photochemical processes on the surface of wide band gap solids. 3. Gas/solid systems

The spectral dependencies of quantum yields of photostimulated adsorption of oxygen on KBr and ZrO2 specimens were assessed during photoexcitation into the F centers produced by preirradiation with UV light. The spectral limits of this adsorption were shifted to wavelengths longer than the absorption band edge of the specimens. However, of great import are the spectral dependencies of the quantum yields of photochemical processes for the original state(s) of metal oxides irradiated in the intrinsic and extrinsic absorption bands. Such dependencies for the photoadsorption of donor (H-2 and CH4) and acceptor (O-2) molecules in gas/solid systems are reported for a variety of metal oxides of varying band gaps (E-g), namely TiO2 (rutile form, E-g = 3.0 eV), ZnO (3.0 eV), ZrO2 (5.0), Sc-2 O-3 (5.4 eV), MgO (8.7 eV), and CaO (7.5 eV) as well as for Bi-2 O-3 (3.2 eV), CeO2 (3.4 eV), Ga-2 O-3 (4.4 eV), In-2 O-3 (3-1 eV), Nb-2 O-5 (3.3 eV), and Y-2 O-3 (5.5 eV). The approach taken to model and compare with experimental results was reported earlier (ref 4). For the first six metal oxides, the dependencies are rationalized on the basis of the absorption spectra of the specimens for which the origins of the extrinsic light absorption are known. The corresponding dependencies for the remaining oxides in the series are nonetheless also reported albeit they await characterization of the origins of the spectral features for a complete understanding to be achieved.