Surface Core Level Shifts of Yb Films in the Multilayer,Monolayer and Submonolayer Range

Surface core level shifts in Yb and other rare earth elements, as well as in their compounds, arise from a reduced band width of the 5d band, caused by the reduced coordination at surface sites, with the strength and direction of the shifts depending on the occupation of the 5d band. If the coordination, or more exactly, the local symmetry is considered to be the decisive factor for changing the band width, it is obvious to look for core level shifts even at the interface of two solids. In order to investigate such interface core level shifts, thin Yb films of thicknesses from the multilayer down to the submonolayer range, grown on a polycristalline W substrat, were investigated by photoelectronspectroscopy. It was found, that for decreasing film thicknesses, the ratio of the surface to bulk related 4f intensities increases systematically, and finally, for thicknesses of one up to two monolayers, only the broad, surface related part of the Yb 4f doublet can be observed. For Yb coverages fewer than one monolayer, there is a sudden shift of this remaining doublet towards the Fermi energy, which must result from a reduced occupation of the Yb 5d band due to an electron transfer from Yb to W at the interface.