Electronic spectra of the surfaces of alkaline earth oxides

Abstract

The surfaces of fine powder samples of MgO, CaO and SrO have been characterized by the diffuse reflectance technique over the wavelength range 200–2000 nm. Spectra due to F⁺_s centres (electron in surface anion vacancy), S_H centres (F⁺_s centre with nearby —OH group) and adsorbed O^–₂ species have been identified. In powders which have been freed from surface contaminants fluorescences exist which can be quenched by O₂, H₂O or CO₂. These fluorescences are interpreted as due to the presence of surface states which create an “effective” band gap at the oxide surfaces at significantly lower energies than the band gaps typical of the bulk oxides.