Valence state at the surface of rare-earth metals

The valence state of a rare-earth metal surface is investigated by using general properties of the surface tension of metals. Thereby it is concluded that samarium is likely to have a divalent or partly divalent surface on top of its trivalent bulk phase, which agrees with recent spectroscopic observations. Also californium metal is discussed from this point of view. Finally, the position of the $4f$ level in a bulk gold atom as compared to its position in a surface gold atom is briefly discussed.