Mixed valency of TmSe

Vacuum-cleaved single crystals of mixed valence TmSe were studied by x-ray photoemission. The divalent and trivalent part of the Tm $4f$ spectrum were fitted using optical multiplet energies and fractional parentage intensities. From the areas of the divalent and trivalent components we obtained the average valences for samples with four different stoichiometries. Comparison of these results with other estimates of the average valence suggest that the x-ray photoemission spectroscopy data are significantly affected by a divalent surface layer. However, attempts to demonstrate the reality of this layer by varying the take-off angle were inconclusive. It was verified that the ${\mathrm{Tm}}^{2+}$ ground state lies at the Fermi energy, giving a direct proof of the homogeneous character of the mixed valency. The location of this multiplet can therefore be used in general to distinguish between homogeneous and inhomogeneous mixed valency.