Abstract
A theoretical overview of the core-to-core (3d-4f) resonant inelastic X-ray scattering (RIXS) spectra of actinide dioxides AnO2 (An = Th, U, Np, Pu, Am, Cu, Bk, Cf) is provided. The 3d-4f RIXS maps were calculated using crystal-field multiplet theory and turned out to be significantly different at the An M5 vs M4 edges, because of selection rules and final state effects. The results of the calculations allowed for a general analysis of so-called high-energy-resolution fluorescence-detected X-ray absorption (HERFD-XAS) spectra. The cuts of the calculated RIXS maps along the incident energy axis at the constant emitted energy, corresponding to the maximum of the RIXS intensity, represented the HERFD spectra and provided their comparison with calculated conventional X-ray absorption (XAS) spectra with a reduced core-hole lifetime broadening at the An M5 and M4 edges. Although the An M5 HERFD profiles were found to depart from the X-ray absorption cross-section, in terms of appearing additional transitions, the results of calculations for the An M4 edges demonstrate overall better agreement between the HERFD and XAS spectra for most dioxides, keeping in mind a restricted HERFD resolution, because of the core–hole lifetime broadening in the final state. The results confirm the utility of HERFD for the An chemical state determination and indicate the importance of calculating the entire RIXS process in order to interpret the HERFD data correctly.
Highlights
The application of the high-energy-resolution fluorescence detected X-ray absorption (HERFD-XAS) technique to actinide (An) compounds has led to a striking improvement in the resolution of the spectra at the An M4,5 edges (4−8 times higher, according to various estimates), because of a reduced core−hole lifetime broadening in the final state of the spectroscopic process.[1−3] This can be viewed as a real breakthrough in actinide research, since the enhanced sensitivity of the method allows for probing the An oxidation state, 5f occupancy, local symmetry,stoichiometry, oxygen/metal (O/M) ratio, etc. with much greater capability and efficiency
Considering the available experimental data, recorded with high resolution, we find a fairly good agreement in the spectral shape between calculated and measured HERFD spectra for the Th M4 edge of ThO2,3 for the U M5 and M4 edges of UO2,29 and for Np M5 edge of NpO2.30 all of the main structures of the experimental Pu M5 and M4 HERFD spectra of PuO25 are reproduced by the crystalfield multiplet calculations; the agreement between calculated and measured spectra becomes somewhat worse than that observed for dioxides of other early actinides
Both of the selection rules, when the 4f 7/2 and 4f5/2 components are involved in the 5f n → 3d95f n+1 → 4f135f n+1 excitation−de-excitation process at the M5 edge, whereas only the 4f5/2 component is involved at the M4 edge, and the final state effects of the RIXS process due to the 4f-5f interaction are contributing to the calculated differences
Summary
The application of the high-energy-resolution fluorescence detected X-ray absorption (HERFD-XAS) technique to actinide (An) compounds has led to a striking improvement in the resolution of the spectra at the An M4,5 edges (4−8 times higher, according to various estimates), because of a reduced core−hole lifetime broadening in the final state of the spectroscopic process.[1−3] This can be viewed as a real breakthrough in actinide research, since the enhanced sensitivity of the method allows for probing the An oxidation state, 5f occupancy, local symmetry,. (c) Comparison between the calculated conventional XAS spectrum (black curve) at the U M5 edge with a reduced core−hole lifetime broadening and a HERFD cut (red curve) of the 3d-to-4f RIXS map along the incidence energy axis at an emitted energy corresponding to the RIXS maximum This cut is indicated by a dashed line in panel (b). The population of states due to finite temperature was not considered
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