Nature of Ce4felectronic states from4dexcitations 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 $\gamma$-like or $\alpha$-like properties, exhibit similar multiplet structures in the prethreshold region, corresponding to $4d^{9}4f^2$ final states. In contrast, $4d^{9}4f^1$ 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.