Mixed Valence and Hybridization Effects in the Early Rare Earths as Studied by X-Ray Absorption and Photoelectron Spectroscopies

The general motivation for this work was to better understand the anomalous behavior of Ce-based systems by applying XPS, resonant photoemission and X-ray absorption spectroscopies. L(,III) absorption measurements on a number of Ce-based systems together with their La and/or Pr isomorphs, show that the double peaked Ce-edge is not caused by final state shakeup processes. Shakedown satellites have been identified in the absorption edges of LaNi(,5) and LaPd(,3) by comparing to isomorphs of heavier rare earths. The observed smallness of final state effects implies that the method of L(,III) absorption is both a convenient and reliable method for measuring the mixed valent quotient. XPS and L(,III) absorption measurements have been performed in a system where the electronic bandstructure can be varied continuously; namely Ce(Pd(,1-x)T(,x))(,3) where T = Rh or Ag. The shakedown intensities in the Ce 3d core-levels have been correlated with the mixed valent behavior in this system. The two peaks in the 4f-derived photoemission in Ce-based systems have been a major puzzle in the understanding of these mixed valent effects. Several theories have tried to explain this phenomenon. Resonant valence-band photoelectron measurements have been performed on the systems REAl(,2), RERu(,2) and REPd(,3) (RE = La, Ce, Pr, and Nd). One observes the bimodal 4f spectrum in PrAl(,2), PrPd(,3) and NdPd(,3) as well as in the Ce-isomorphs. This lends support to the theory of Liu and Ho and especially to the generalization of this theory to include hybridization effects by Riseborough, which envisions two different screening channels in the final state; one itinerant and one in which the screening is provided by a state pulled off the bottom of the conduction band.