Surface shift of the unoccupied 4fstate in La metal

Using inverse photoemission with variable photon energy (17≤hν≤25 eV), the existence of a surface shift of the unoccupied 4f state in La metal, analogous to the well-known surface core-level shift in photoemission from occupied levels, is demonstrated. In the outermost surface layer, the 4${\mathit{f}}^1$ electron-addition state shifts by ${\mathrm{\delta }}_{\mathit{s}}$=-0.65±0.10 eV relative to the bulk, i.e., towards the Fermi level, in quantitative agreement with the prediction of a thermochemical model. This observation opens new opportunities in the study of surface phenomena in rare-earth materials by inverse photoemission.