The electronic structure of copper oxides has been investigated by photoelectron (x-ray photoemission, ultraviolet photoemission), Auger electron, and bremsstrahlung isochromat spectroscopies. The experimental results are compared with one-electron band-structure calculations as well as with a cluster configuration interaction model. It is demonstrated that the results for ${\mathrm{Cu}}_2$O agree well with band theory, whereas those for CuO clearly show strong deviations which we argue are due to electron-correlation effects in the open-shell $d$ bands. From the comparison to cluster calculations we extract values for the $\mathrm{Cu} d-d$ and $\mathrm{O} p-p$ Coulomb interactions, the O to Cu charge transfer energy, and the degree of $\mathrm{Cu} d-\mathrm{O} 2p$ hybridization. From this we demonstrate that CuO is a charge-transfer gap insulator.

Using soft x-ray absorption spectroscopy and magnetic circular dichroism at the Co-$L_{2,3}$ edge, we reveal that the spin state transition in ${\mathrm{LaCoO}}_3$ can be well described by a low-spin ground state and a triply degenerate high-spin first excited state. From the temperature dependence of the spectral line shapes, we find that ${\mathrm{LaCoO}}_3$ at finite temperatures is an inhomogeneous mixed-spin state system. It is crucial that the magnetic circular dichroism signal in the paramagnetic state carries a large orbital momentum. This directly shows that the currently accepted low- or intermediate-spin picture is at variance. Parameters derived from these spectroscopies fully explain existing magnetic susceptibility, electron spin resonance, and inelastic neutron data.

We found direct experimental evidence for an orbital switching in the V $3d$ states across the metal-insulator transition in ${\mathrm{VO}}_2$. We have used soft-x-ray absorption spectroscopy at the V $L_{2,3}$ edges as a sensitive local probe and have determined quantitatively the orbital polarizations. These results strongly suggest that, in going from the metallic to the insulating state, the orbital occupation changes in a manner that charge fluctuations and effective bandwidths are reduced, that the system becomes more one dimensional and more susceptible to a Peierls-like transition, and that the required massive orbital switching can only be made if the system is close to a Mott insulating regime.

Bulk-sensitive, polarization-dependent oxygen K- and copper L-edge absorption spectra of single-domain ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Sr}}_{\mathit{x}}$ ${\mathrm{CuO}}_4$ with ${\mathrm{CuO}}_2$ planes tilted off the sample surface were measured. A novel experimental configuration was utilized to achieve complete polarization geometry and eliminate optical path variations. Contrary to previous measurements, our data show for both intrinsic and doped holes a significant amount of O2${\mathit{p}}_{\mathit{z}}$ character, likely associated with apical O, but a very weak Cu3${\mathit{d}}_{3\mathit{z}}^2$-${\mathit{r}}^2$ character. The quantitative results impose strong contraints on superconductivity models invoking out-of-plane orbitals, and suggest that the apical O 2${\mathit{p}}_{\mathit{z}}$ orbital may play an important role in high-${\mathit{T}}_{\mathit{c}}$ phenomena.

We present soft-x-ray-absorption spectra (XAS) of ${\mathrm{LaCoO}}_3$ taken at different temperatures (80–630 K). The shape of the multiplets in the Co 2p XAS spectra conveys information on the symmetry and spin of the ground state. The O 1s XAS spectra are related to unoccupied metal bands through covalent mixing. The changes in the spectra taken at different temperatures provide information on the spin-state transition in this compound. At low temperature, 80 and 300 K, the material is in a highly covalent low-spin state. The main contribution to the ground state in this case is given by ${\mathit{t}}_{2\mathit{g}}^6$ ${(}^1$ ${\mathit{A}}_1$) with an occupancy of 0.56. At higher temperature, 550 and 630 K, the results indicate a gradual transition to a mixed-spin state. The main contribution to the high-spin part of the mixture is given by ${\mathit{t}}_{2\mathit{g}}^4$ ${\mathit{e}}_{\mathit{g}}^2$ ${(}^5$ ${\mathit{T}}_2$) with an occupancy of 0.71. There is no evidence of charge disproportionation in the temperature range 80–630 K. The O 1s XAS spectra reflect important changes in the unoccupied Co 3d bands across the spin-state transition.

The electronic structure of ${\mathrm{Ag}}_2$O has been investigated by photoelectron, Auger-electron, and bremsstrahlung isochromat spectroscopy. The experimental results are compared with model one-particle band-structure and two-particle cluster calculations. We extract values for the on-site Ag 4d and O 2p energies, the Ag 4d–O 2p transfer integrals, as well as the on-site Coulomb Ag 4d-4d and O 2p-2p interactions. A comparison with ${\mathrm{Cu}}_2$O and CuO is made, and the differences are discussed in view of high-$T_c$ superconductivity and the instability of AgO.