Theoretical Modelling of High-Resolution X-Ray Absorption Spectra at Uranium M 4 Edge
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MRS Advances © 2018 Materials Research Society DOI: 10.1557/adv.2018.470
Theoretical Modelling of High-Resolution X-Ray Absorption Spectra at Uranium M4 Edge Jindřich Kolorenč1 and Kristina O. Kvashnina2,3 1
Institute of Physics, Czech Academy of Sciences, Na Slovance 2, 182 21 Praha, Czech Republic
2
Institute of Resource Ecology, Helmholtz-Zentrum Dresden-Rossendorf, 01314 Dresden, Germany
3
Rossendorf Beamline at The European Synchrotron (ESRF), CS40220, 38043 Grenoble Cedex 9, France
ABSTRACT
We investigate the origin of satellite features that appear in the high-resolution x-ray absorption spectra measured at the uranium M4 edge in compounds where the uranium atoms are in the U6+ oxidation state. We employ a material-specific Anderson impurity model derived from the electronic structure obtained by the density-functional theory.
INTRODUCTION The resolution of the x-ray absorption spectroscopy is fundamentally limited by the life-time broadening of the core hole created in the absorption event. For the actinide M edges (3d 5f transitions), this broadening is large due to a short life time of the deep 3d hole (⁓3.5 eV [1]). However, when the absorption is detected by monitoring a particular core-to-core process of filling the created hole (the 4f 3d transition in the case of the actinide M edge), the broadening is reduced and it is determined by a considerably longer life time of the shallower 4f hole (⁓0.3 eV [2]) [3]. This method is referred to as the high-energy-resolution fluorescence-detected x-ray absorption spectroscopy (HERFD-XAS) in the literature [4]. The spectra recorded in this way do not exactly coincide with the conventional x-ray absorption: XAS involves only one photon and can be calculated from the Fermi golden rule, whereas HERFD-XAS involves two photons and thus should be modelled by the Kramers–Heisenberg formula. Nevertheless, the Kramers–Heisenberg formula simplifies to an expression very similar to the conventional XAS formula in certain cases. One set of such simplifying assumptions was recently discussed in the context of the resonant x-ray emission spectroscopy (RXES) at the L edge of lanthanides [5]: If (i) the shape of the core orbitals can be neglected so that the core-valence Coulomb interaction is fully determined by just one Slater integral, (ii) if this core-valence Slater integral is the same in the intermediate state (3d hole) and in the final state (4f hole), and (iii) if the detector registers all polarizations of the emitted photon equally, then the HERFD-XAS essentially coincides with the conventional XAS,
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only the life-time broadening is described by a function that slightly differs from the Lorentzian. In the case of the L-edge RXES of praseodymium, this simplified model achieves a very good agreement with experiment [5].
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