Photoemission study of CuO andCu2O single crystals

We present results of resonant-photoemission and high-resolution x-ray-photoemission-spectroscopy (XPS) studies on single-crystalline CuO, which confirms most of the earlier data obtained from polycrystalline samples. However, some minor differences with earlier results were also observed, which are concentrated on a mainly oxygen-related feature. The important first ionization state is much better resolved in our high-resolution XPS data, and is found to have both copper and oxygen orbital character. This is consistent (though not a proof) with the theoretical prediction that the first ionization state is a ${}_{}{}^1{}_{}{}^{}$ ${\mathit{A}}_{1\mathit{g}}$ singlet with holes on both oxygen and copper sites. The 16-eV satellite is found to show a Cu antiresonance, and is assigned to a ${}_{}{}^1{}_{}{}^{}$ ${\mathit{A}}_{1\mathit{g}}$ singlet with both holes on the copper sites.