Resonant photoelectron emission in La2CuO4+x single crystals

Photoelectron spectra of La2CuO4+x single crystals with various oxygen contents (0.005 ⩽ x ⩽0.03) are measured at photon energy hω varying from 15 to 33 eV. A resonant increase in emission is observed from electron states belonging to the valence band and characterized by the binding energy ∼6.2eV for hω ∼ 17 eV, which is close to the excitation threshold for La5p electron states. Similar measurements have been made for metallic La with a gradually increasing degree of surface oxidation in order to clarify the origin of this effect. It is found that the resonance is not observed in the case of a pure La surface, is manifested weakly in the case of complete oxidation, and is very strong for an intermediate oxidation (with an oxygen dose ∼0.5 langmuirs). A common feature of the structures of La2CuO4 and LaOx is the presence of La–O bonds. Consequently, the resonance is manifested for a certain mutual arrangement of La and O atoms which is realized in a structure of the type of La2CuO4 or in the case of a slightly oxidized La surface.