We present synchrotron-excited oxygen x-ray K-emission spectroscopy ͑O K␣ XES͒ and oxygen x-ray absorption spectroscopy ͑O 1s XAS͒ spectra of transition-metal ͑TM͒ oxides MnO, CoO, and NiO. The comparison of oxygen K-emission and absorption spectra to valence band photoemission and bremsstrahlung isochromat spectra measurements shows that O 1s XAS is not strongly influenced by the core hole effect, whereas the TM 2p XAS significantly shifts to a lower energy. New and effective methods for determining the band gap and anion-to-cation cation charge-transfer energies of the oxides from the measured spectra are presented and applied, and the combination of O XAS and XES is shown to agree well with the results of numerical electronic structure methods applied to strongly correlated oxides. For MnO, the charge-transfer energy is found to be 6.6 eV and the band gap is 4.1 eV; for CoO, the values are 6.1 and 2.6 eV and for NiO, the values are 5.4 and 4.0 eV.

The electronic structure of Zn 1−x Co x O ͑x = 0.02, 0.06, and 0.10͒ diluted magnetic semiconductors is investigated using soft x-ray emission spectroscopy and first-principles calculations. X-ray absorption and emission measurements reveal that most Co dopants are incorporated at the Zn sites and that free charge carriers are absent over a wide range of Co concentrations. The excess Co interstitials appear in the samples with high Co concentration ͑10 at. %͒ but are isolated without any direct exchange interaction with substitutional Co atoms. The lack of free charge carriers and the direct Co-Co interactions is responsible for the absence of ferromagnetism in the samples. First-principles calculations suggest that the exchange interaction between substitutional Co atoms induces only an antiferromagnetic coupling, and strong ferromagnetism in Co-doped ZnO requires not only free charge carriers but also the Co interstitials directly interacting with substitutional Co atoms.

BaFe 2 As 2 exhibits properties characteristic of the parent compounds of the newly discovered iron (Fe)-based high-T C superconductors. By combining the real space imaging of scanning tun-neling microscopy/spectroscopy (STM/S) with momentum space quantitative Low Energy Electron Diffraction (LEED) we have identified the surface plane of cleaved BaFe 2 As 2 crystals as the As terminated Fe-As layer-the plane where superconductivity occurs. LEED and STM/S data on the BaFe 2 As 2 (001) surface indicate an ordered arsenic (As)-terminated metallic surface without reconstruction or lattice distortion. It is surprising that the STM images the different Fe-As orbitals associated with the orthorhombic structure, not the As atoms in the surface plane.

The electronic structure in alkaline earth AeO (Ae = Be, Mg, Ca, Sr, Ba) and post-transition metal oxides MeO (Me = Zn, Cd, Hg) is probed with oxygen K -edge X-ray absorption and emission spectroscopy. The experimental data is compared with density functional theory electronic structure calculations. We use our experimental spectra of the oxygen K -edge to estimate the bandgaps of these materials, and compare our results to the range of values available in the literature. From the calculated partial DOS we conclude that the position of main O K -edge X-ray emission feature in BeO, SrO and BaO is defined by the position of the np-states of the cation while in the other oxides studied here the main O K -edge X-ray emission feature is defined by the position of the (n-1)d (for CaO) or nd-states of the cation.

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