Hidden electronic state of CuO revealed by resonant inelastic x-ray scattering

High-resolution resonant inelastic x-ray scattering (RIXS) spectra from CuO in the $1s2p$ emission region were measured. A surprisingly complicated set of spectra was observed. Detailed analysis of these spectra reveals the existence of an excited state, which is hidden in the Cu-$K$ x-ray absorption spectrum of CuO. Furthermore, the energy of this excited state corresponds well to a $\mathrm{Cu}{4p}_z$-like state predicted by a real-space multiple-scattering technique, but has not been observed experimentally up to now. This result demonstrates the potential use of RIXS to complement x-ray absorption spectroscopy.