Experimental Proof for Coordination-Dependent Valence of Tm Metal

Photoemission measurements of Tm metal films (≃ 1000 Å thick) grown on a copper substrate at 13 K reveal a partial surface valence transition to the divalent $4f^{13}$ state. At room temperature, Tm films deposited on smooth substrates remain purely trivalent, whereas deposition on rough substrates yields small divalent components in the photoemission spectrum. This phenomenon is shown to occur for particularly low-coordinated atoms on rough surfaces, in quantitative agreement with thermochemical predictions.