Surface effects in Eu intermetallics: A resonant photoemission study

The Eu---transition-metal compounds EuAg₅, EuRh₂, EuPt₅, and EuPd₅ were studied by resonant 4d→4f photoemission techniques. In divalent EuAg₅, a surface core-level shift of the final-state 4f⁶ multiplet components of 0.75 eV is found. For the trivalent Eu compounds EuPt₅ and EuPd₅ as well as for the mixed-valence compound EuRh₂, a surface valence change to the lower-valence state is observed. For EuPt₅, an inhomogeneously mixed-valence surface is noticed, pointing to a strong coordination dependence of the surface valence transition. The energy position of the final-state $4f$ screening orbital is discussed on the basis of $4d$ photoemission and photoabsorption threshold energies.