Pu M4- and M5-edge Resonant Inelastic X-ray Scattering of PuO2

Resonant inelastic X-ray scattering (RIXS) is a valuable tool and can lead to a deeper understanding of the electronic structure of materials as shown e.g. for transition metal compounds.1 For uranium, the first M5-edge core-to-core 3d4f RIXS spectra were measured in 19962 , but for plutonium the first M5-edge RIXS was published only in 2017.3 This technique holds a high potential for actinide spectroscopy, which is not yet thoroughly explored. The aim of this work is an enhanced understanding of the Pu 3d4f RIXS maps measured at the Pu M4 and M5 absorption edges.
The Pu M4,5-edge core-to-core 3d4f RIXS consists of electronic transitions from 3d to 5f (3d→5f) states followed by X-ray emission via 4f→3d transitions fulfilling the dipole selection rule (∆J = 0, ±1). As unoccupied 5f valence states of Pu are directly probed, the Pu oxidation state and its chemical bonding have a large effect on the RIXS maps. In the PuO2 3d4f RIXS in Figure 1 it is visible that the maximum of the resonant X-ray emission intensity (line A) is shifted to a higher emission energy in comparison to the normal emission (line B). For uranium this shift appears to be correlated to a lower electronic density around the metal, corresponding to a higher uranium oxidation state.3 In addition, previous results suggest that the energy shift between lines A and B increases for more localised 5f states.3
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This study focuses on the analysis of how different parameters affect the 3d4f RIXS map of PuO2. To understand the origin of the different spectral features, calculations based on ab initio multiplet calculations (MOLCAS and CLIPS) and semi-empirical multiplet approaches (Quanty) are applied. The effect of different contributions to the experimental resolution is studied and the impact of the physical state of PuO2 and its contact with water on the RIXS maps are discussed.
X-ray absorption spectroscopy experiments were performed with powdered 239PuO2. To investigate potential size effects, PuO2 thin film samples of 65–100 nm thickness where prepared by direct-current (dc) sputter deposition from a 242Pu metal target with Ar/O2 mixtures as sputter gas on SiN/Si wafers.4 An experimental setup where 242PuO2 thin films are in contact with H2O under anaerobic conditions was designed. In addition, a colloidal Pu suspension (5 mmol/L) at pH 1.5 was investigated. It was obtained by aging of amorphous, hydrated 242PuO2 precipitates in HClO4 at pH 1.5 and Eh=0.78 V for 9 months. The XAS experiments were performed at the INE and CAT-ACT beamlines at the KIT Light Source in Karlsruhe Germany.5,6
3d4f RIXS spectra for the PuO2 powder measured with various experimental resolutions, revealed that different broadening effects have a large influence on the shape of the spectra and the energy position of the main resonant feature. Theoretical calculations and simulation of different broadening effects will be compared to the experimental results. For the Pu colloids and PuO2 thin films similar spectra are obtained. In comparison to the PuO2 powder sample the resonant emission is shifted by 1.8 eV to higher emission energies. The RIXS spectrum of the PuO2 thin film in contact with water also changes compared to the as prepared dry film. It has been a long-standing discussion if and at which conditions PuO2+x is stable.7–9 The spectral differences will be discussed as a potential presence of PuO2+x and/or changes in the geometric structure or differences in the 5f electron localization.10
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