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.

Soft x-ray absorption spectroscopy in the M-edge region is explored as a tool for studying 4f instabilities. The quasiatomic MIV,V spectra, recorded by total-electron yield under UHV conditions, carry information on the initial-state 4f occupancy. For compounds of the heavy rare earths, even at high dilution (e.g., Tm0.05Y0.95Se), mean valences can be derived, which agree with those from lattice-constant systematics (e.g., for TmSe and Sm0.3Y0.7S). A moderate surface sensitivity allows the observation of surface-induced valence changes on EuPd3, TmS, and SmAl2. The MIV,V spectra studied for 10 Ce compounds fall in two categories: pure 3d94f1 final-state multiplets for γ-like compounds, and additional peaks at about 5 eV higher energies for α-like and intermediate systems. With increasing intensity of the satellites, which are assigned to 3d94f1 final states, the 3d94f2 multiplet structure gets washed out. An interpretation of the spectra with the recent Anderson-impurity theory of Gunnarsson and Schönhammer leads to 4f occupancies greater than 0.7, even in extreme α-like systems. In addition, MIII-edge spectra exhibit one peak in γ-like and two peaks in α-like and intermediate Ce systems, which are assigned to 3p54f1 and 3p54f0 final corestates. With increasing α-like character, the 4f1 peak is found to move to higher energies presumably due to a decrease in 4f screening with increasing 4f hybridization.

No abstract

No abstract

No abstract