Giant Cu 2p Resonances in CuO Valence-Band Photoemission

We report the observation of a giant resonance in the Cu 2p resonant-photoemission spectra of CuO. The study allows the unambiguous identification of the local Cu 3d8 configuration in the valence-band photoemission spectrum, providing conclusive evidence for the charge-transfer nature of the insulating gap in the electronic structure of CuO. A comparison with 3p resonant-photoemission measurements is also presented to emphasize the strength of 2p resonant photoemission as a tool for studying the electronic structure of highly correlated 3d transition-metal systems.