We have used low-energy (hν=17–45 eV) photoemission to study thin films of Sm evaporated on Nb. We find that Sm evaporated onto a room-temperature Nb substrate is initially entirely divalent. On a time scale of the order of 50 min at room temperature a valence transition to mixed-valent or trivalent Sm occurs. This is accompanied by dramatic changes in the photoemission spectra, including a large increase in the total yield of photoelectrons and the appearance of a series of new constant-kinetic-energy peaks. The photon-energy dependence of these peaks indicates that they are due to Auger recombination of the Sm 5p level. We also observe a 5p→5d resonance in Sm for the first time. The underlying cause of these effects appears to be an ordering transition in the Sm film, which changes the Sm 5d occupancy and increases the resonance amplitude. This mechanism is evidently a very sensitive indicator of a Sm valence change.
Physical Review B (Condensed Matter) 37(12), pp.6605-6610