Photoluminescence from surface states in MgO and CaO powders

Abstract

Photoexcitation spectra of luminescence from activated MgO and CaO powders have been obtained over the wavelength range 120–300 nm at sample temperatures down to 10 K. These spectra show broad features attributable to electronic transitions of O 2− ions in coordination states varying from 6-fold (bulk ions) to 3-fold. O 2− ions in 4 and 5-fold coordination contribute a broad inhomogeneously broadened spectral feature centered at 225 and $ ̃ 250 nm in MgO and CaO powders, respectively. This feature is interpreted as a surface exciton band well separated in energy from the bulk exciton band at 160 and 180 nm, respectively. An interaction between discrete bulk exciton levels and these surface bands yields dispersion-like structure superimposed on the broad surface photoexcitation bands.