Bandlike and excitonic states of oxygen in CuGeO₃: Observation using polarized resonant soft-x-ray emission spectroscopy

Polarized resonant x-ray fluorescence spectroscopy at O 1s resonances was used to study CuGeO₃. Two distinct spectral contributions are identified, i.e., an intense "main band" of soft-x-ray fluorescence, reflecting primarily bandlike O 2p states, and resonant inelastic x-ray scattering (RIXS) characterized by spectral features that disperse linearly with excitation energy and resonate at the O 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).