Surface Shifts of 4f Electron-Addition and Electron-Removal States in Gd(0001)

Using inverse photoemission (IPE), a surface shift of the 4f⁸ electron-addition state to lower energies by δₛᵉᵃ=-0.48±0.04 eV is observed for Gd(0001). The analogous shift of the 4f⁶ electron-removal state, as obtained by photoemission (PE), amounts to δₛᵉʳ=-0.29±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 EF is assigned to an image-potential surface state.