Surface Shifts of4fElectron-Addition and Electron-Removal States in Gd(0001)

Using inverse photoemission (IPE), a surface shift of the $4f^8$ electron-addition state to lower energies by ${\delta }_s^{\mathrm{ea}}=-0.48\mathrm{}\pm 0.04$ eV is observed for Gd(0001). The analogous shift of the $4f^6$ electron-removal state, as obtained by photoemission (PE), amounts to ${\delta }_s^{\mathrm{er}}=-0.29\mathrm{}\pm 0.03$ eV, i.e., also to lower energies. This allows a separation of the surface shifts into initial-state and final-state contributions; the latter reveal that the IPE and PE final states are better screened at the surface than in the bulk. An IPE peak at 3.15 eV above $E_F$ is assigned to an image-potential surface state.