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 delta(s)ea = -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 delta(s)er = -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 E(F) is assigned to an image-potential surface state.