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High-resolution valence band RIXS at the actinide M$_{4,5}$-edges

Martin Sundermann, Henrik Hahn, Denise S. Christovam, Maurits W. Haverkort, Roberto Caciuffo, Bernhard Keimer, Liu Hao Tjeng, Andrea Severing, Hlynur Gretarsson

TL;DR

This work addresses the challenge of resolving actinide $5f$ electronic structure by developing high-resolution valence-band RIXS (VB-RIXS) at the U $M_{4,5}$ edges and applying it to the model compound UO$_2$. Using the IRIXS tender-x-ray endstation, the authors achieve energy resolutions of $50$ meV at the $M_{5}$ edge and $90$ meV at the $M_{4}$ edge, enabling the separation of crystal-field multiplets up to ~2 eV and high-energy charge-transfer and fluorescence-like features up to ~$10$ eV. The spectra reveal a ground-state multiplet $^3H_4$ with a $oldsymbol{ ext{Γ}}_5$ triplet in cubic symmetry and show that high-energy features require incorporating covalency beyond a simple point-charge model. Because VB-RIXS probes the initial state with the same $ ext{Δ}$ and $U$ in both initial and final states, it provides direct, quantitative insights into crystal-field splitting and U–ligand covalency, offering a powerful framework for modeling actinide materials and extending to intermetallic and mixed-valent systems.

Abstract

Understanding the electronic structure of actinide materials is crucial for both fundamental research and nuclear applications. The partially filled 5f shells exhibit complex behavior due to strong correlations and ligand hybridization, requiring advanced spectroscopic techniques. Here, we report on the development and application of high-resolution valence-band resonant inelastic x-ray spectroscopy (VB-RIXS) experiments at the uranium M$_{4,5}$ edges (3551 and 3725\,eV). We present data of UO$_2$, a well-established model actinide compound. VB-RIXS is particularly well suited for probing the 5f-shell electronic structure, as it probes, in contrast to core-to-core RIXS, excitations without leaving a high-energy core hole in the final state. In VB-RIXS, we achieve energy resolutions of 50\,meV (M$_5$) and 90\,meV (M$_4$), enabling the resolution of multiplet excitations and crystal-field effects, as well as charge-transfer and fluorescence-like features with unprecedented clarity. As such, high resolution VB-RIXS offers direct insights into both low-energy, near ground-state properties and high-energy hybridization and covalency effects. Our results demonstrate the power of VB-RIXS as a versatile and powerful tool for probing the strongly correlated electronic structure of actinide materials, providing essential input for quantitative modeling and the validation of theoretical concepts.

High-resolution valence band RIXS at the actinide M$_{4,5}$-edges

TL;DR

This work addresses the challenge of resolving actinide electronic structure by developing high-resolution valence-band RIXS (VB-RIXS) at the U edges and applying it to the model compound UO. Using the IRIXS tender-x-ray endstation, the authors achieve energy resolutions of meV at the edge and meV at the edge, enabling the separation of crystal-field multiplets up to ~2 eV and high-energy charge-transfer and fluorescence-like features up to ~ eV. The spectra reveal a ground-state multiplet with a triplet in cubic symmetry and show that high-energy features require incorporating covalency beyond a simple point-charge model. Because VB-RIXS probes the initial state with the same and in both initial and final states, it provides direct, quantitative insights into crystal-field splitting and U–ligand covalency, offering a powerful framework for modeling actinide materials and extending to intermetallic and mixed-valent systems.

Abstract

Understanding the electronic structure of actinide materials is crucial for both fundamental research and nuclear applications. The partially filled 5f shells exhibit complex behavior due to strong correlations and ligand hybridization, requiring advanced spectroscopic techniques. Here, we report on the development and application of high-resolution valence-band resonant inelastic x-ray spectroscopy (VB-RIXS) experiments at the uranium M edges (3551 and 3725\,eV). We present data of UO, a well-established model actinide compound. VB-RIXS is particularly well suited for probing the 5f-shell electronic structure, as it probes, in contrast to core-to-core RIXS, excitations without leaving a high-energy core hole in the final state. In VB-RIXS, we achieve energy resolutions of 50\,meV (M) and 90\,meV (M), enabling the resolution of multiplet excitations and crystal-field effects, as well as charge-transfer and fluorescence-like features with unprecedented clarity. As such, high resolution VB-RIXS offers direct insights into both low-energy, near ground-state properties and high-energy hybridization and covalency effects. Our results demonstrate the power of VB-RIXS as a versatile and powerful tool for probing the strongly correlated electronic structure of actinide materials, providing essential input for quantitative modeling and the validation of theoretical concepts.

Paper Structure

This paper contains 6 sections, 6 figures, 1 table.

Table of Contents

  1. Introduction
  2. Methods: core-to-core RIXS versus VB-RIXS
  3. Experiment and Analysis
  4. Results and Discussion
  5. Summary
  6. Acknowledgment

Figures (6)

  • Figure 1: (a) Scattering channels in core-to-core RIXS and VB-RIXS. (b) Energy-level diagram of VB-RIXS final state. (c) Energy-level diagrams of the VB-RIXS and core-to-core RIXS initial states as well as core-to-core RIXS final state. (d) M4-edge XAS (black line) compared to HERFD spectra in M4-edge core-to-core RIXS (green line). (e) Cartoonlike VB-RIXS spectrum with multiplet and crystal-field excitations.
  • Figure 2: (a), (b) XAS spectra at the U M$_5$- (red dots) and M$_4$-edges (blues dots). The vertical lines refer to incident energies used for the RIXS maps. (c) Experimental U M$_5$- (red) and M$_4$-edge (blue) VB-RIXS spectra measured at T = 15 K. The intensity scales are directly comparable. Insets show elastic signals from the carbon tape at the U M$_5$- (red) and M$_4$-edges, respectively, fitted with a Lorentzian lineshape as resolution function. (d) Full multiplet VB-RIXS calculations for the 5f$^2$2 configuration with spherical symmetry, and 50 and 90 meV broadening, respectively.
  • Figure 3: U M$_5$-edge (red) data in panel (a) and U M$_4$-edge (blue) data in panel (b) are the same as in Fig. 2, shown with expanded intensity scale for the energy transfer range -0.25 to 2.25eV. The inset in the U M$_4$-edge panel shows the region due to $^3$H$_4$ multiplet scattering divided by 10. LS term symbols are given for orientation. The gray lines correspond to a crystal-field calculation based on a point-charge model (see Sec. III).
  • Figure 4: (a) Temperature dependence of M$_5$-edge VB-RIXS data with E$_\text{in}$ = E$_\text{res}$, measured below and above $T_N$ = 30.8 K, for energy transfer range [-0.25;2.25]eV. The arrow shows the time sequence of the measurements. (b) Zoom to the three crystal-field excitations of the ground state multiplet $^3$H$_4$ in the energy range [0.13;0.23]eV. (c) Empirical Lorentzian fit with equal widths (FWHM 0.58 meV) to crystal-field split $^3$F$_2$ (2 lines) and $^3$H$_4$ multiples (3 lines) of M$_5$-edge VB-RIXS data at 15 K.
  • Figure 5: Intensity zoom to high energy excitations of VB RIXS data measured at M$_5$ (red) and M$_4$ edge (blue) with E$_\text{in}$ = E$_\text{res}$. Multiplet exciatitons are denoted with ff, fluorescence with Fl and charge transfer with CT.
  • ...and 1 more figures