Colour centers at the surface of alkali-hearth oxides. A new hypothesis on the location of the surface electron traps

Irradiation of highly dehydrated MgO by UV light in the presence of surface adsorbed hydrogen leads to the formation of particular types of surface colour centres indicated with F+S (H) (one-electron, paramagnetic) and FS(H) (two electrons, diamagnetic). FS centres are the surface counterparts of the well-known F colour centres formed in the bulk of ionic solids by high-energy irradiation or metal addition. In the particular case of F+S (H), the unpaired electron is in magnetic interaction with a nearby proton belonging to a hydroxyl group deriving from H2 heterolytic dissociative chemisorption (H2 H++H−) and consequent H+ stabilization on a surface oxide ion. The joint use of EPR, FT-IR and DR-UV–vis spectroscopies has allowed clarification of the mechanism of formation of these centres, which is based on the ionization of adsorbed hydryde groups by UV light and stabilization of the ionized electron into suitable positively charged surface electron traps. A fraction of these traps coincides with the site capable of stabilizing the hydride ion (in the form of bridged Mg3H), which is built up by an array of three Mg2+ ions reproducing a (111) facelet of the oxide. The same site can be also seen as a O2− 3c vacancy (3-coordinated surface anionic vacancy). Ab-initio quantum chemical calculations confirm the proposed assignment, which goes beyond the original model of 5-coordinated surface anion vacancies at the flat (100) MgO plane, which is thus left in favour of a new model that describes the surface electron traps as being localized in less coordinated regions of the surface.