A model for luminescent emission from surface states in MgO and CaO powders

Abstract

An analysis of photoluminescence and photoexcitation spectra of MgO and CaO powders at 77 K shows that luminescent emission may be associated with low-coordination oxygen ions adjacent to defect sites. In this model luminescent emission involves a charge transfer transition from an electronically excited O^2- state to the ground state of the F⁺ _s centre. The corresponding charge transfer transition from ground state O^2- ions to F⁺ _s centres is analogous to the α bands observed in the alkali halides, and is responsible for the broad absorption bands near 2 eV observed on γ-irradiation of MgO and CaO powders in vacuo. Involvement of the F⁺ _s (2p) state in quenching of luminescent emission in CaO has been detected. The effect of H₂ adsorption in quenching emission has been investigated and can be understood in terms of the derived model for the emission complex.