Photostimulated formation of radicals on oxide surfaces

We present results ofin situ EPR investigations of the mechanism of photostimulated processes resulting in radical and ion-radical particle formation on the surfaces of oxide dielectrics (magnesium, calcium, aluminum oxides, zeolites). Three types of reactions are discussed: On the basis of investigation of the spectral relationships and the properties of surface active centre, it is concluded that light is absorbed by coordinatively unsaturated surface sites in the first system, whereas in the other processes, electron donor-acceptor (EDA) complexes between adsorbed molecules and surface active sites are supposed to be key intermediates. These EDA complexes are shown to incorporate donor solvent molecules as well. In this case the energetic characteristics of the photoprocesses are substantially determined by the ionization potential of solvent molecules. Mechanisms of photo- and thermostimulated processes are compared and possible similarities are discussed for all the reactions studied.