Surface effects on core-level binding energies and valence in thulium chalcogenides

Abstract

Vacuum-cleaved (100) surfaces of mixed-valent TmSe, divalent TmTe, and trivalent TmS were studied by high-resolution, angle-integrated photoelectron spectroscopy with the use of synchrotron radiation in the energy range 30h110 eV. In the topmost surface layers of TmSe and TmTe the 4f levels are found to be shifted to higher binding energies by 0.320.04 and 0.410.05 eV, respectively. In both TmSe and TmS the topmost surface layers are divalent. In the case of TmSe a separation of the Tm2+ 4f12 spectral feature into surface and bulk contributions allows a determination of the bulk mean valence v=2.550.05. While a shift of the Se 3d levels to lower binding energy is observed for Se atoms in the topmost surface layer of TmSe, no such shift can be resolved for the Te 4d levels of TmTe. The surface-derived divalent spectral features can be quenched completely in all three cases by exposure of the surfaces to submonolayer amounts of oxygen, resulting in the formation of trivalent surface oxides. Values for the electron mean free path l are derived from the observed surface- and bulk-spectral intensities, with l decreasing with decreasing electron kinetic energy down to 45 eV. Smaller singularity indices of the Doniach-Sunjić line shapes as well as smaller extrinsic losses are observed for photoemission from the divalent surface layers as compared to the bulk. © 1982 The American Physical Society.