Surface valence changes in rare earths monitored by giant-resonance electron energy losses

Electron energy loss spectroscopy of 4d giant resonances in the rare earths is shown to be a sensitive monitor of valence changes in surface reactions with O₂ and S. Yb is found to be divalent on exposure to S but forms a trivalent oxide, while Gd is consistently trivalent. Eu forms a monosulfide, and an oxide of possibly mixed divalent and trivalent stoichiometry, while for Sm mixture of divalent and trivalent spectral features is consistently observed.