Highly resolved surface shifts in a mixed-valent system: EuPd2Si2

Synchrotron-radiation-excited photoelectron spectroscopy is employed to study the bulk and surface states of the mixed-valent system Eu${\mathrm{Pd}}_2$ ${\mathrm{Si}}_2$, which undergoes a valence transition from ${\mathrm{Eu}}^{2.2+}$ at room temperature to ${\mathrm{Eu}}^{2.9+}$ at low temperature (≲ 100 K). The high resolution employed, coupled with a relatively large bulk-to-surface energy shift to the ${\mathrm{Eu}}^{2+}$ emission, allows a definitive identification of the bulk and surface states of ${\mathrm{Eu}}^{2+}$. The ${\mathrm{Eu}}^{3+}$ emission is not observed at room temperature, but is readily apparent in the low-temperature spectra.