Mixed valence in Sm overlayers on Al(001)

Samarium is found to form ordered overlayers when deposited on a clean Al(001) substrate. Annealing is necessary to produce the crystallographic order. The x-ray photoemission spectrum from the $3d$ level of Sm in such layers shows that the occurrence of the divalent initial state is dependent on the presence of structural disorder or large separations between Sm atoms. In annealed structurally ordered films we find an average valence of 2.95 for the samarium atom. Spectroscopic data for the $3d$ levels are presented.