  Uranium and plutonium's 5f electrons are tenuously poised between strongly
bonding with ligand spd-states and residing close to the nucleus. The unusual
properties of these elements and their compounds (eg. the six different
allotropes of elemental plutonium) are widely believed to depend on the related
attributes of f-orbital occupancy and delocalization, for which a quantitative
measure is lacking. By employing resonant x-ray emission spectroscopy (RXES)
and x-ray absorption near-edge structure (XANES) spectroscopy and making
comparisons to specific heat measurements, we demonstrate the presence of
multiconfigurational f-orbital states in the actinide elements U and Pu, and in
a wide range of uranium and plutonium intermetallic compounds. These results
provide a robust experimental basis for a new framework for understanding the
strongly-correlated behavior of actinide materials.
