Reflectance spectra of surface states in strontium oxide and barium oxide

Abstract

Reflectance spectra of SrO and BaO powders outgassed at temperatures up to 1073 K have been measured over the range from text-decoration:overlineν= 20 000 cm–1 to text-decoration:overlineν= 52 000 cm–1. Spectral features attributable to exciton states are described, and the effects of O2, N2O, CO2 and H2O on the spectra are reported. Fluorescence occurs when spectra are measured in vacuo. SrO-coated MgO and BaO-coated MgO have also been studied. The spectra show two regions separated by an absorption edge which is centred at 44 000 cm–1 for SrO and 31 500 cm–1 for BaO. On the low text-decoration:overlineν side of the edge the spectra are sensitive to the adsorption of the gases, but exposure to gases causes little or no change in the spectra on the high text-decoration:overlineν side (except for H2O vapour which reacts in depth). The absorption bands at low text-decoration:overlineν are ascribed to excitons bound at surface ions and those at high text-decoration:overlineν to excitons bound at ions in the bulk.The chemical interaction of the gases with the surface states increases in intensity in the order O2 < N2O < CO2 < H2O for SrO and N2O < O2 < CO2 < H2O for BaO. The results are correlated with those for MgO and CaO reported previously.