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

Abstract

For a number of metallic and nonmetallic U systems as well as a few Th compounds, the nearedge x-ray-absorption fine-structure spectra, taken by transmission and total-electron-yield techniques, are reported at the L3, M3, M4 5, X4 &, and 04 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 UC13 and UF4, representing mainly the difference in Coulomb interaction between a 2p3/2 core hole and the 5f configurations. The spectra at the M4s and N4s thresholds are dominated by intense absorption peaks due to 3d 5f and 4-d Sf transitions probing directly the unoccupied Sf states. No multiplet structure due to 3d 5f or 4d-5f exchange interaction is resolved, but the linewidths of the M4 q 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 Oqs thresholds, on the other hand, show well-resolved structures, which are atorniclike for ThF4 and UFq and may be described by spin-orbit and exchange splitting of the Sd Sf' and Sd 5f final states, respectively. In metallic U systems, the 045 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~and 04 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.

Comparative Study of f-Element Electronic Structure across a Series of Multimetallic Actinide and Lanthanoid-Actinide Complexes Possessing Redox-Active Bridging Ligands

“…The WL peak height is a measure of the density of unoccupied states, and since the final photoelectron state is a 6d state for all these spectra, the total area under that excitation should be constant; the spectral width of the final state accounts for differences in WL peak height. Normalized absorption 56,57 Although the overall energy differences on going from 12 → 5b → 16 are smaller than described for inorganic and metallic uranium species, the trend of ∆E WL (U III -U IV ) > ∆E WL (U IV -U V ) is observed in the organometallic systems reported here. This discussion has assumed that the data can be interpreted at face value, using the estimated errors on the "white-line" to directly relate to errors in estimating the valence, in spite of the previously mentioned distribution of E WL for given actinide valence species.…”

Covalency in oxidized uranium

“…There should be U4d5/2 XAS (about 736 eV) and U4d3/2 XAS (about 778 eV) peaks [11,12]. However, as suggested by the XES results, these peaks will be small relative to the F1s XAS.…”

“…However, both exhibit very similar U L3 extended x--ray absorption fine structure (EXAFS) behavior, indicative of quantitatively similar interatomic distances [10]. UF4 has been studied before with X--ray absorption [11], but these new measurements are complementary to the earlier study (e.g., FK(1s) XAS), and have been performed with improved resolution and over a more extensive energy range (e.g the U L3 (2p3/2) X--ray absorption fine structure, both XANES and EXAFS) [10,12]. The result of this comparative study is that UF4 exhibits continued 6d covalency but the almost complete loss of 5f covalency, while UO2 clearly displays both strong 5f and 6d covalencies.…”

Resonant x-ray spectroscopy of uranium intermetallics at the M4,5 edges of uranium

“…The most straightforward are based on absorption spectroscopy, and these were already performed in the 1980s at the most available of absorption edges, that of the 2p → 6d transitions (L 2,3 ) 19,20 This same group extended the absorption spectroscopy to the M 4,5 edges (transitions from the 3d core states to the partially filled 5f states) at the same time 21 . These measurements were useful, but limited in their resolution by the large intrinsic core-hole interaction at the different edges.…”