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 $L_3$, $M_3$, $M_4$,5, $N_4$,5, and $O_4$,5, thresholds. At $L_3$ and $M_3$, similar spectra with analogous information are observed. While in the metallic U systems, the white lines at $L_3$ and $M_3$ show only minor variations in position and relative peak height, a shift of ≃4 eV is observed between ${\mathrm{UCl}}_3$ and ${\mathrm{UF}}_4$, representing mainly the difference in Coulomb interaction between a 2$p_{3/2}$ core hole and the 5f configurations. The spectra at the $M_4$,5 and $N_4$,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 $M_4$,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 $O_4$,5 thresholds, on the other hand, show well-resolved structures, which are atomiclike for ${\mathrm{ThF}}_4$ and ${\mathrm{UF}}_4$ and may be described by spin-orbit and exchange splitting of the 5$d^9$5$f^1$ and 5$d^9$5$f^3$ final states, respectively. In metallic U systems, the $O_4$,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 $M_4$,5 and $O_4$,5 spectra of Th metal, ${\mathrm{ThO}}_2$, ${\mathrm{UO}}_2$, and ${\mathrm{UF}}_4$ at energies from ≃15 to 38 eV above threshold, which are assigned to multiple-scattering resonances.