Electron Binding Energies of Uranium

Soft-x-ray appearance-potential spectroscopy has been used to determine the binding energies of the $N_4-N_7$, $O_1-O_5$, and $P_1-P_3$ levels of clean uranium surfaces. This is the first direct determination of most of these levels. The measured energies of the $N$ and $O$ levels are consistently a few electron volts lower than tabulated x-ray binding energies based on photoelectron spectroscopy of oxidized uranium. Exposure of clean uranium to oxygen, however, did not appreciably shift the appearance potentials. It is suggested that the discrepancy is in part a consequence of a "surface chemical shift." The density of $5f$ states above the Fermi energy was found to be double peaked with a peak separation of 2.5 eV.