PhD defense of Wilken Aldair Misael

ALDAIR MISAEL Wilken, Laboratoire PhLAM - UMR8523 - Equipe PCMT

Titre: Shedding x-rays on molecules through the lenses of relativistic electronic structure theory

Jury: A. SEVERO PEREIRA GOMES (PhLAM), D. GUILLAUMONT (Université de Montpellier), E. DONOVAN HEDEGARD (University of Southern Denmark), V. VALLET (PhLAM), J. TOULOUSE (Sorbonne Université), P. SAINCTAVIT (Sorbonne Université)

Summary:

This thesis aims to investigate the electronic structure of actinides by means of ab initio relativistic quantum chemistry methods, with a specific emphasis on the spectroscopic observables of the uranyl moiety (UO₂²⁺). Considering the pivotal role of this unit in the solid-state and solution chemistry of uranium, one of the most abundant and stable actinides on earth, as well as recent advancements in synchrotron radiation facilities, our investigation relies on evaluating the interaction of x-ray photons with the uranyl unit in varying degrees of complexity, ranging from molecules to crystalline solids.

First, we showcase how the resonant-convergent formulation of response theory can be employed to investigate the X-ray absorption fine structure (XAFS) of actinides. 4-component damped-response time-dependent density functional theory (4c-DR-TD-DFT) simulations for the uranyl tetrachloride dianion ([UO₂Cl₄]^2-) were found to be consistent with previous data for angle-resolved near edge x- ray absorption spectroscopy (NEXAFS) at the oxygen K-edge and high energy resolution fluorescence detected (HERFD) at the uranium M₄- and L₃-edges of the dicesium uranyl tetrachloride crystal (Cs₂UO₂Cl₄), a prototype system for actinide electronic structure investigations. We then present the results of collaborative work with the Rossendorf Beamline at the European Synchrotron Radiation Facility (ESRF). 2-component TD-DFT simulations within the Tamm-Dancoff approximation (2c-TDA) and HERFD measurements uranyl systems within different structural motifs highlight the role of charge transfer states in determining the spectral features at the uranium M4-edge.

The role of orbital correlation and relaxation in the core-ionization energies of heavy elements was investigated using the recently developed core-valence-separation equation-of-motion coupled- cluster method (CVS-EOM-CC). We also evaluated the performance of various 4- and 2-component Hamiltonians for calculating these properties. The results of this investigation highlight the importance of computing two-electron interaction beyond the zeroth order truncation, i.e., the Coulomb term, when working at the tender (1 keV – 5 keV) and hard x-ray (5 keV – 200 keV) ranges. We also evaluated the performance of quantum-chemical embedding methods to account for environmental effects. Specifically, we employed the frozen density embedding (FDE) method, which allowed us to gain valuable insights into how the equatorial ligands of the uranyl ion influence its spectroscopic properties. Notably, this method successfully addressed the role of such interactions in binding energies in the soft X-ray range and in the peak splittings observed in the emission spectra at the U M₄-edge. The latter is particularly significant as it has been instrumental in addressing a long- standing problem in actinide science: the role of 5f orbitals in actinyl bonding.

In summary, this thesis presents fundamental research work that aims to push the boundaries of ab initio quantum chemical methods when addressing spectroscopic observables toward the bottom of the periodic table, and the findings of this work capture how these approaches can provide further insights into state-of-the-art experiments.