RESONANT PHOTOELECTRON EMISSION FROM LA2C UO4-CRYSTALS(X SINGLE)

Photoelectron spectra of Le(2)CuO(4+x) single crystals with various ox ygen contents (0.005 less than or equal to x less than or equal to 0.0 3)are measured with varying photon energy h omega from 15 to 33 eV. A resonant increase of the emission from the electron states with a bind ing energy or similar to 6.2 eV is observed for h omega similar to 17 eV, which is close to the excitation threshold of the La5p electron st ates. Similar investigations were performed for polycrystal metallic L a with increasing degree of surface oxidation. The resonance was not o bserved for a clear La surface, and it was very weak for a heavy oxidi zed surface. The resonance was most pronounced only for a slightly oxi dized surface (oxygen dose of about 0.5 Lengmurs). Common for the La2C uO4 and LaOx structures is the existence of the La-O bonds. Hence, for the resonance to manifest itself, it seems likely that the La and O a toms should be in a certain mutual arrangement which is realized in th e La2CuO4 structure or in slightly oxidized La.