An experimental and theoretical evaluation of surface states in MgO and other alkaline earth oxides

Ultra-violet (u.v.) diffuse reflectance spectra of microcrystalline MgO, CaO, SrO and BaO reveal bands at energies below those of bulk excitonic transitions. Correlation of the spectra permit three distinctive absorptions (I, II, III) to be identified, each with similar characteristics in the respective oxides. The absorptions are ascribed to surface states, and the transitions have the characteristics of excitons. Excitons II and III obey the Mollwo–Ivey relation typical of bound excitons, but excition I with the highest energy of transition behaves similarly to a free exciton of the bulk. The four oxides are isostructural and ionic. The Levine–Mark theory of the surface states of ionic insulators, not hitherto tested for oxides, is examined. The numerical values of the parameters appropriate to the oxides are evaluated by using experimental data on bulk excitons. It is shown that the theory accounts satisfactorily for the energies of exciton I in MgO, CaO and SrO on the basis that it is an excitation of a surface anion located on a cube face of the crystal (fivefold coordination). Excitons II and III can only be tentatively assigned, but they are attributed to surface ions in fourfold and threefold coordination, respectively, the ions being at steps, edges or kink sites rather than on low-index faces, and thereby having a similar character to bulk excitons bound at internal lattice imperfections. The increased covalency shown by ions in low coordination prevents the application of the Levine–Mark theory.