Surface Shifts of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mn>4</mml:mn><mml:mi>f</mml:mi></mml:math>Electron-Addition and Electron-Removal States in Gd(0001)

Abstract

Using inverse photoemission (IPE), a surface shift of the $4{f}^{8}$ electron-addition state to lower energies by ${\ensuremath{\delta}}_{s}^{\mathrm{ea}}=\ensuremath{-}0.48\ifmmode\pm\else\textpm\fi{}0.04$ eV is observed for Gd(0001). The analogous shift of the $4{f}^{6}$ electron-removal state, as obtained by photoemission (PE), amounts to ${\ensuremath{\delta}}_{s}^{\mathrm{er}}=\ensuremath{-}0.29\ifmmode\pm\else\textpm\fi{}0.03$ 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.