The Revolution in Actinide Spectroscopy Driven by X-Ray Emission

The advent of new, powerful, highly efficient, multi-component, X-ray monochromators used in the detection of tender x-rays has revolutionized spectroscopic investigations of the 5f electronic structure. All of the new experiments are, in essence, variants of X-ray Emission Spectroscopy (XES), where the improved monochromatized detection plays a key role.<br/><b>A</b> In HERFD (High Energy Resolution Fluorescence Detection), the monochromatized XES detection allows the performance of a scattering experiment that devolves into a higher resolution version of X-Ray Absorption Spectroscopy (XAS). It has been shown that the M₄ and M₅ spectra are essentially direct measurements of the j-specific (5f_5/2 and 5f_7/2) Unoccupied Density of States (UDOS), which can be directly correlated with the UDOS from Inverse Photoelectron Spectroscopy (IPES) and Bremsstrahlung Isochromat Spectroscopy (BIS). [1,2] In this case, ligand field density functional theory calculations of the dioxides of thorium, uranium, and plutonium have been combined with high-energy-resolution fluorescence detection (HERFD) in x-ray absorption spectroscopy and inverse photoelectron spectroscopy (IPES) measurements to provide powerful insight into the underlying composition of the unoccupied 5f electronic structure in these 5f localized systems. Fine structure in the 5f5/2 transitions in HERFD can be directly correlated with the fine structure in the leading edge of the IPES. The shapes, intensities, and systematics in HERFD and IPES are explained in a consistent and rigorous fashion in terms of the j-specific 5f electronic structure.<br/><b>B</b> Similarly, Resonant XES has been demonstrated to be Raman in nature, with a 5f-5f transition, not a simple charge transfer transition (ligand 2p to actinide 5f). [3] In this experiment, X-ray resonant Raman spectroscopy (XRRS), a variant of resonant inelastic x-ray scattering, has been used to investigate the two prototype systems, UF4 and UO2. Both are U5f2 and each is an example of 5f localized, ionic behavior and 5f localized, covalent behavior, respectively. From the M5 XRRS measurements, the 5f band gap in each can be directly determined and, moreover, a clear and powerful sensitivity to 5f covalency emerges.<br/><b>C</b> Finally, the 5f delocalization in U metal has been quantified with the combined 6p & 5f M_4,5 non-resonant XES, which exhibits strong angular momentum coupling effects. [4] Historically delocalization of the 5f states in the early actinides in general and U metal in particular is significantly important and yet poorly understood. Extant spectroscopic techniques have failed to resolve the situation. Here, it will be shown that x-ray emission spectroscopy of the M4,5 levels can provide the needed information, with a distinct difference between the delocalized U metal and localized uranium dioxide and uranium tetrafluoride cases. A peak ratio model, built upon electric dipole selection rules, has been developed and utilized, with quantitative agreement between experiment and theory. Possible expansion to other types of 5f mixing systems will be discussed.<br/>1. J.G. Tobin et al., Phys. Rev. B 105, 125129 (2022).<br/>2. J.G. Tobin et al., J. Electron Spect. Rel. Phen. 232, 100 (2019).<br/>3. J.G. Tobin et al., J. Phys. Cond. Matter 34, 505601 (2022).<br/>4. J.G. Tobin et al., MRS Bulletin 47, 1078–1083 (2022), Invited Impact Article.