$M$-Edge X-Ray Absorption Spectroscopy: A New Tool for Dilute Mixed-Valent Materials

The valence of Tm compounds is derived from $M_{\mathrm{V}}$ x-ray absorption spectra recorded by total electron yield under ultra-high-vacuum conditions. For mixed-valent systems the spectra are superpositions of ${\mathrm{Tm}}^{3+}$ (three lines) and ${\mathrm{Tm}}^{2+}$ (one line) components, providing accurate mean valence values even in highly dilute systems, such as ${\mathrm{Tm}}_x{\mathrm{Y}}_{1-x}\mathrm{Se}$, which agree well with those from lattice-constant systematics. A surface valence change on TmS(100) is identified as an initial-state effect.