Electronic structure of actinide compounds fromLIII-edge x-ray absorption

Double-peaked ${\mathrm{L}}_{\mathrm{III}}$-edge x-ray-absorption (XA) spectra are reported for various compounds of Np. Related high-pressure studies of ${\mathrm{UO}}_3$ support the idea that these peaks are due to core-ionized final states with different 5f occupancies reflecting the correlated electronic ground state. The weighted mean ${\mathrm{L}}_{\mathrm{III}}$-XA positions shift to higher energies with decreasing 5f occupancy and, for nonmetallic compounds, depend monotonically on the respective ${}_{}{}^{237}{}_{}{}^{}\mathrm{Np}$ Mössbauer isomer shifts. Deviations from this correlation are observed for metallic compounds due to conduction-electron contributions to the isomer shift.