O- radical ions on MgO as a tool to unravel structure and location of ionic vacancies at the surface of oxides: a coupled experimental and theoretical investigation

The present paper analyzes the results of experiments consisting in the ble aching of color centers (F-s(+)(H) or F-s(-)(D)) at the surface of polycrys talline MgO after exposure to N2O, and in their regeneration following admi ssion of hydrogen in the dark (no UV irradiation), The reactions have been followed by EPR, using systematically computer simulation for the interpret ation of the experimental results. After hydrogen exposure and irradiation, the spectra give evidence of the presence of trapped electrons close to a surface hydroxyl group, while after N2O reaction with F centers the dominan t species are a family of O- radical ions; in the latter case the spectra p resent well resolved features which permit the accurate determination of th e g factors of the different radical species. Different sites are character ized by different g values of O- ions which are trapped there after reactio n with N2O. Up to three cycles (bleaching - regeneration of F centers in th e dark) can be performed prior to final surface saturation, The alternation of hydrogen and deuterium admission has clarified that the F centers produ ced in the dark reaction are not the same as formed initially by UV irradia tion. The evidence collected suggests that the MgO sample presents a family of "hot sites" where heterolytic dissociation of molecular hydrogen can ta ke place, and a more abundant family of "cold sites", where the former reac tion is not possible, but atomic hydrogen can be ionized to generate an F c enter. An attempt to associate specific structural models with the various sites has been performed by means of quantum mechanical simulation of simpl e surface defects. The energy of several reactions (H, heterolysis, N2O dec omposition, H ionization, H, reaction with surface O-) has been calculated, and the g factor of O- surface ions in different environments has been est imated. This study seems to exclude that the isolated vacancy at the (0 0 1 ) face may act either as a hot or a cold site. (C) 2001 Elsevier Science B. V. All rights reserved.