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 ν= 20 000 cm^–1 to ν= 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 ν 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 ν side (except for H₂O vapour which reacts in depth). The absorption bands at low ν are ascribed to excitons bound at surface ions and those at high ν 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.