Study of surface oxidation of rare-earth metals by photoelectron spectroscopy

Surface oxidation of La, Ce, Sm and Tb metals has been investigated by He(II) ultraviolet photoelectron spectroscopy (u.p.s.) and X-ray photoelectron spectroscopy (X.p.s.). Oxidation of La gives rise to La<SUB>2</SUB>O<SUB>3</SUB> on the surface. While Ce<SUB>2</SUB>O<SUB>3</SUB> appears to be the stable oxide on the surface, we find evidence for formation of CeO<SUB>2</SUB> at high oxygen exposure. Valence band of Sm clearly shows the presence of both divalent and trivalent states due to interconfigurational fluctuation. Exposure of Sm to oxygen first depletes the divalent Sm at the surface. While Sm<SUB>2</SUB>O<SUB>3</SUB> is the stable oxide on the surface of Sm, Tb<SUB>2</SUB>O<SUB>3</SUB> is the stable oxide on the surface of Tb (and not any of the higher oxides)