Resonant inelastic x-ray scattering at the actinide M4,5 -edges

Understanding the electronic structure of actinide materials is crucial for both fundamental research and nuclear applications. The partially filled <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mn>5</a:mn><a:mi>f</a:mi></a:mrow></a:math> 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 <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:msub><b:mi>M</b:mi><b:mrow><b:mn>4</b:mn><b:mo>,</b:mo><b:mn>5</b:mn></b:mrow></b:msub></b:math> edges (3551 and 3725 eV). We present data of <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:msub><c:mi>UO</c:mi><c:mn>2</c:mn></c:msub></c:math>, a well-established model actinide compound. VB-RIXS is particularly well suited for probing the <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:mrow><d:mn>5</d:mn><d:mi>f</d:mi></d:mrow></d:math>-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 <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mo>(</e:mo><e:msub><e:mi>M</e:mi><e:mn>5</e:mn></e:msub><e:mo>)</e:mo></e:math> and 90 meV <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"><f:mo>(</f:mo><f:msub><f:mi>M</f:mi><f:mn>4</f:mn></f:msub><f:mo>)</f:mo></f:math>, enabling the resolution of multiplet excitations and crystal-field effects, as well as charge-transfer and fluorescencelike 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.