Bandlike and excitonic states of oxygen inCuGeO3:Observation using polarized resonant soft-x-ray emission spectroscopy

Polarized resonant x-ray fluorescence spectroscopy at O $1s$ resonances was used to study ${\mathrm{CuGeO}}_3.$ Two distinct spectral contributions are identified, i.e., an intense “main band” of soft-x-ray fluorescence, reflecting primarily bandlike $\mathrm{O}\hspace{0.5em}2p$ states, and resonant inelastic x-ray scattering (RIXS) characterized by spectral features that disperse linearly with excitation energy and resonate at the $\mathrm{O}\hspace{0.5em}1s$ pre-edge peak. RIXS reflects excitoniclike (“optical”) final states and has previously only been observed for metal-ion core edges. Moreover, analysis of the excitation energy and polarization dependence of the main band allows the identification of features deriving from the site-specific partial density of states as well as strong evidence for the observation of a two-hole state (Zhang-Rice singlet).