Valence fluctuations in thin films and the α and δ phases of Pu metal determined by 4f core-level photoemission calculations

The average number of 5f electrons making up the valence state in plutonium metal together with the electronic fluctuations on each metal site has been a recent subject of debate. For the phase of Pu, where compared to the phase increased localization more atomiclike character leads to decreased overlap and volume increase, an f count close to either 5 or 6 has been proposed depending on the type of electronic structure calculation. In order to resolve the controversy, we analyze the Pu 4f photoemission spectrum, which displays well screened and poorly screened peaks that can be used as a measure for the degree of localization. A simple analytical two-level model already shows on general grounds that the f count for Pu must be between 5 and 5.5. Furthermore, we present detailed Anderson impurity model calculations including the full multiplet structure for Pu 4f photoemission, which are compared to previous experimental results obtained from 1 to 9 monolayers thin films of Pu on Mg and from Pu metal in the and phases. The trend in the satellite to main peak intensity ratio as a function of the Pu layer thickness gives a clear indication that Pu metal has an 5f5 like ground state. For the Pu allotropes and thicker films an f count of 5.22 is obtained with a Coulomb interaction U=4 eV. The 5f fluctuations in Pu metal are very prominent and strongly material dependent. The calculations give a ground state with 9.6% f4, 58.8% f5, and 31.6% f6 for the phase and 5.7% f4, 66.4% f5, and 27.8% f6 for the phase while for the thin films the amount of f5 and the localization strongly increase with reduced thickness. The obtained findings are in agreement with recent electronic structure calculations for Pu using local-density approximation with dynamical mean-field theory and with the branching-ratio analysis of the Pu N4,5 edge in electron-energy-loss spectroscopy.