Nature of Ce 4f electronic states from 4d excitations in metals and insulators

We have investigated a variety of metallic and nonmetallic Ce materials by soft-x-ray-absorption spectroscopy at the ${N}_{4,5}$ thresholds employing the total-electron-yield method. All metallic systems, irrespective of their $\ensuremath{\gamma}$-like or $\ensuremath{\alpha}$-like properties, exhibit similar multiplet structures in the prethreshold region, corresponding to $4{d}^{9}4{f}^{2}$ final states. In contrast, $4{d}^{9}4{f}^{1}$ multiplets are observed for Ce${\mathrm{F}}_{4}$ and Ce${\mathrm{O}}_{2}$, as in La${\mathrm{F}}_{3}$, in accordance with the absence of localized $4f$ electrons in the ground states of these compounds. We conclude that tetravalent Ce compounds occur among insulators, but do not exist as metals.