High Resolution Soft X-Ray Absorption Measurements of La sub (2-x) Sr sub x CuO sub 4

High resolution and bulk sensitive soft x-ray absorption spectra of doped and undoped La sub 2-x Sr sub x CuO sub 4 samples have been measured using the Dragon beamline with a novel soft x-ray fluorescence detector. Near the oxygen K-shell absorption threshold, a sharp peak is observed in both doped and undoped samples. THis observation is in clear contrast to previous measurements, and to the general belief that this spectral feature is due to the doping-induced O 2p holes. It is evident that the existence of such a peak does not depend on doping, and that the previous interpretation of this absorption feature as a direct measurement of unoccupied O 2p density of states needs to be reconsidered. The core-excitonic effect on O K-edge absorption spectra will be emphasized by presenting the measurements on various oxides.