Bulk and surface valence states in rare-earth metal monosulfides

We have studied the 4f-photoelectron spectra of SmS, EuS, DyS, TmS, and YbS using high-resolution (E150 meV) photoemission with synchrotron radiation. By means of a systematic treatment of the 4f-ionization energies, the bulk and surface valence states of these and several other rare-earth metal sulfides are determined by interpolation. In particular, we find in TmS that the trivalent state is stable by only 0.2 eV, and we show that, owing to this low stability, the surface in TmS is divalent. © 1982 The American Physical Society.