Near-edge x-ray-absorption fine structure in uranium compounds

For a number of metallic and nonmetallic U systems as well as a few Th compounds, the near-edge x-ray-absorption fine-structure spectra, taken by transmission and total-electron-yield techniques, are reported at the L₃, M₃, M₄,5, N₄,5, and O₄,5, thresholds. At L₃ and M₃, similar spectra with analogous information are observed. While in the metallic U systems, the white lines at L₃ and M₃ show only minor variations in position and relative peak height, a shift of {}4 eV is observed between UCl₃ and UF₄, representing mainly the difference in Coulomb interaction between a 2p3/2 core hole and the 5f configurations. The spectra at the M₄,5 and N₄,5 thresholds are dominated by intense absorption peaks due to 3d-5f and 4d-5f transitions probing directly the unoccupied 5f states. No multiplet structure due to 3d-5f or 4d-5f exchange interaction is resolved, but the linewidths of the M₄,5 peaks in the metallic U systems are found to decrease by {}20% with increasing U-U distance, reflecting a narrowing of the unoccupied 5f bands. The spectra at the O₄,5 thresholds, on the other hand, show well-resolved structures, which are atomiclike for ThF₄ and UF₄ and may be described by spin-orbit and exchange splitting of the 5d⁹5f¹ and 5d⁹5f³ final states, respectively. In metallic U systems, the O₄,5 structures are broadened and partly lost, presumably due to the more itinerant nature of the 5f states. Additional relatively weak peaks are observed in the M₄,5 and O₄,5 spectra of Th metal, ThO₂, UO₂, and UF₄ at energies from {}15 to 38 eV above threshold, which are assigned to multiple-scattering resonances.