An analysis was made of the bleaching of color centers, Fs+(H) or Fs+(D), at the surface of polycrystalline material after exposure to N2O, and their regeneration after admitting H in the dark: no ultra-violet irradiation. The reactions were monitored by means of electron paramagnetic resonance, using computer simulations to interpret the experimental results. After H exposure and irradiation, the spectra revealed the presence of trapped electrons close to a surface hydroxyl group. After N2O reaction with F centers, the predominant species were a family of O- radical ions. In the latter case, the spectra exhibited well-resolved features which permitted the accurate determination of the g-factors of the various radical species. The different sites were characterized by differing g-values of the O- ions which were trapped there after reaction with N2O. Up to 3 cycles of bleaching and regeneration of F centers in the dark could be performed before final surface saturation. The alternation of H and D admission revealed that the F centers produced in the dark reaction were not the same as those formed initially by ultra-violet irradiation. The evidence suggested that the sample offered a family of so-called hot sites where heterolytic dissociation of molecular H could take place, plus a more abundant family of so-called cold sites where the former reaction was not possible but atomic H could be ionized to generate an F center. An attempt was made to associate specific structural models with the various sites by means of quantum-mechanical simulations of simple surface defects. The energies of several reactions (H2 heterolysis, N2O decomposition, H ionization, H2 reaction with surface O-) were calculated, and the g-factor of O- surface ions in various environments was estimated. The results seemed to bar the isolated vacancy at the (001) face from acting either as a hot or a cold site.

O- Radical Ions on MgO as a Tool to Unravel Structure and Location of Ionic Vacancies at the Surface of Oxides. G.Pinarello, C.Pisani, A.D’Ercole, M.Chiesa, M.C.Paganini, E.Giamello, O.Diwald: Surface Science, 2001, 494[2], 95-110