The experimental information available from x-ray photoemission spectroscopy regarding the electron correlation energy $U$ of $3d$ and $4f$ electrons in solids is considered. It is found that $U$ is of the order of 7 eV for $4f$ electrons and ∼ 2 eV for $3d$ electrons.

• Received 8 January 1973

DOI:https://doi.org/10.1103/PhysRevB.7.5086