Evidence for the 5f localisation in thin Pu layers

High-purity ultrathin layers of Pu deposited on Mg were studied by X-ray photoelectron (XPS) and high-resolution valence band (UPS) spectroscopy. The itinerant character of the 5f states is gradually lost with reduced thickness, and the localised 5f states, which appear as a broad peak 1.6 eV below the Fermi level, were observed for one monolayer. At intermediate thickness three narrow peaks appear close to the Fermi level, which are identical to features observed in PuSe and other Pu materials. A comparative study of bulk alpha -Pu indicates a surface re-organisation yielding more localised f-electrons at conditions of thermodynamic equilibrium.