Electronic structure of Cu₂O and CuO

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 Cu2O 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 Cu d−d and O p−p Coulomb interactions, the O to Cu charge transfer energy, and the degree of Cu d−O 2p hybridization. From this we demonstrate that CuO is a charge-transfer gap insulator