CopperL2,3near-edge structure inCu2O

We report on high-resolution total-yield measurements of the Cu $L_{2,3}$ absorption edge in ${\mathrm{Cu}}_2$O using the JUMBO monochromator at the Stanford Synchrotron Radiation Laboratory. Care was taken to prepare the surface of a well-characterized single crystal, rather than use oxidized copper. A comparison was also made with electron-energy-loss spectra (transmission of a thin ${\mathrm{Cu}}_2$O crystal). Prominent white lines or core excitons appear at the spin-orbit-split Cu $L_3$ (931 eV) and $L_2$ (951 eV) thresholds. These edge features are like those observed in CuO (even though the Cu $d$ states are filled in ${\mathrm{Cu}}_2$O) but are asymmetrically broadened on the high-energy side, probably due to more than one component. The core binding energy is measured by photoemission, and local states are compared with transitions to the continuum. We believe that the strong edge features in ${\mathrm{Cu}}_2$O can be understood in part as transitions to antibonding molecular-orbital states of the linear O-Cu-O molecular cluster peculiar to copper in the ${\mathrm{Cu}}_2$O crystal structure.