For rare-earth elements having two different $4f$ configurations in the same material, $3d$ and $4d$ core-level photoemission spectra will exhibit two sets of structures, or replicas, each corresponding to one of the $4f$ configurations. Our estimates of the positions of these structures agree with the available data and suggest the utility of replicate core levels as a useful probe of valence-fluctuation materials.

• Received 1 October 1979

DOI:https://doi.org/10.1103/PhysRevLett.43.1760