Bulk and surface electronic structure ofLi2O

We have used photoemission and electron-energy-loss spectroscopies to determine the surface and bulk electronic structure of a single-crystal sample of the alkali-metal oxide ${\mathrm{Li}}_2$O. The predominately O 2p valence band exhibits two main features in the photoemission spectrum, whose relative widths are consistent with results of an ab initio Hartree-Fock calculation of the bulk electronic structure. However, changes in the intensity of the two features as a function of the exciting photon energy are large and not understood. Energy-loss spectra show the presence of a strong surface exciton in the bulk band gap of ${\mathrm{Li}}_2$O; such excitonic surface states do not exist for the alkali halides. Its energy is about 2 eV less than the bulk band gap, similar to the surface excitonic shifts that have been observed in the alkaline-earth oxides. © 1996 The American Physical Society.