SURFACE SHIFTS OF 4F ELECTRON-ADDITION AND ELECTRON-REMOVAL STATES INGD(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-remova l 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 separatio n of the surface shifts into initial-state and final-state contributio ns; the latter reveal that the IPE and PE final states are better scre ened 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.