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 UCl$_3$ and UF$_4$, representing mainly the difference in Coulomb interaction between a 2p3/2 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 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 5d$^9$5f$^1$ and 5d$^9$5f$^3$ 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 M$_4$,5 and O$_4$,5 spectra of Th metal, ThO$_2$, UO$_2$, and UF$_4$ at energies from ≃15 to 38 eV above threshold, which are assigned to multiple-scattering resonances