Mixed valency versus covalency in rare-earth core-electron spectroscopy

A comparison of photoemission from shallow core levels with deep core-level spectroscopies allows a distinction between mixed valency and covalency in rare-earth systems. The determinative parameter distinguishing these spectroscopies is the Coulomb interaction between the core hole and the 4f shell. This approach confirms that the compound CeF$_4$ is tetravalent, but does not deny the covalent admixture of 4f character into the valence band