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 ($\Delta E\le 150$ 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.