$3d$ core-level x-ray photoelectron spectrum (XPS) of Eu in Eu${\mathrm{Cu}}_2$${\mathrm{Si}}_2$ exhibits two sets of simple spin-orbit split structures. We have assigned these structures to two different valence states of Eu (${\mathrm{Eu}}^{2+}$ and ${\mathrm{Eu}}^{3+}$) in Eu${\mathrm{Cu}}_2$${\mathrm{Si}}_2$. The relative intensities of 2+ and 3+ peaks are strong functions of temperature. XPS in the $3d$ region of Eu has been employed for the first time to study the phenomenon of valence fluctuation. The relative advantage of $3d$ core-level spectroscopy of Eu over $4d$ and $4f$ spectral studies is brought out.

• Received 11 December 1980

DOI:https://doi.org/10.1103/PhysRevB.23.4283