UNOCCUPIED ELECTRONIC STRUCTURE OF LI2CUO2

We have investigated the unoccupied electronic structure of quasi-one-dimensional ${\mathrm{Li}}_{2}{\mathrm{CuO}}_{2}$ using polarization-dependent x-ray absorption spectroscopy. Although an isotropic ``in-plane'' situation is expected for the O $1s$ absorption edge, due to the single oxygen site in the edge-sharing chain geometry, we observe a clear energy shift of peaks derived from the upper Hubbard band, dependent on the direction of the light polarization. For the possible explanation of this observed shift, alternative scenarios are discussed, based upon the electronic structure as obtained from calculations within the local-density approximation, the effects of coupling to phonons during the core-level excitation process, as well as on the polarization-dependent charge distribution in the final state resulting in varying contributions from intersite electron-electron interactions.