Multiconfigurational Simulations of X-ray Absorption Spectroscopy : Insights into Actinyl Covalency

Stanistreet-Welsh, Kurtis and Kerridge, Andrew (2024) Multiconfigurational Simulations of X-ray Absorption Spectroscopy : Insights into Actinyl Covalency. PhD thesis, Lancaster University.

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Abstract

Multiconfigurational Restricted Active Space Self-Consistent Field (RASSCF) theory calculations are employed to simulate oxygen K-edge and actinide M4/5-edge XANES spectra of the actinyls from uranyl to plutonyl. Both XANES techniques are routinely used to determine ground-state (GS) covalency in actinide systems, but relies on the assumption that ground- and probed core-excited state (CES) bonding orbitals do not undergo substantial orbital relaxation. This assumption is addressed in each stage of the thesis through a combination of Quantum Theory of Atoms in Molecules (QTAIM) and orbital composition analysis. The influence of actinyl model on the accuracy of simulated spectra and orbital relaxation, is investigated for uranyl O K-edge XANES (chapter 3). The ability of the RASSCF methodology to manage systems with unpaired electrons was investigated through simulations of actinyl(VI) O K-edge and An M4/5-edge spectra (chapter 4). Finally, the ability of RASSCF simulations to correctly capture the shift behavior of An M4/5-edge spectra due to a change in oxidation state was investigated through simulations of actinyls in both the +6 and +5 oxidation states. Further investigation sought to establish a relationship between the energy separation of An M4/5-edge peaks and axial covalency. Changes in bonding orbitals between the GS and CESs in each chapter are quantified and rationalized in the context of QTAIM analysis. The RASSCF methodology detailed in this thesis lays the foundation for future actinide XANES studies or adaption to other types of spectroscopy that access the core-state. The results of this thesis represent a notable contribution to the field, with the O K-edge simulations being the first such RASSCF simulations of this edge to be reported for the actinides. Similarly, the final results chapter demonstrates the ability of RASSCF simulations to capture the correct shift behavior for the actinides due to a change in oxidation state for the first time.