Theoretical insight into the magnetic circular dichroism of uranium N6,7-edge X-ray absorption

We use ligand-field density functional theory to determine the electronic structure and to model magnetic circular dichroism in the X-ray absorption spectroscopy (XAS) of uranium compounds. This study extends earlier work on tetravalent uranium ion, in which a model Hamiltonian was set up in order to study electronic structure with three nonequivalent 4f, 5f, and 6d electrons. In the earlier work, the model Hamiltonian took into consideration the interelectron repulsion, spin-orbit coupling interaction, and ligand-field splitting. Uranium N_6,7-edge XAS spectra were calculated on the basis of the 5f² → 4f¹³5f²6d¹ electron transition, showing spectral profiles that were mainly dominated by 4f electron spin-orbit coupling, as well as 6d ligand-field splitting. Fine structures were also observed due to the interelectronic repulsion between 4f-5f, 4f-6d, and 5f-6d electrons. Here, the theoretical study is extended to take into consideration the presence of an external magnetic field, incorporating into the model Hamiltonian for three-open-shell electron configuration a term for Zeeman interaction. Therefore, we are able to model spectra with a left-circularly and right-circularly polarized X-ray, demonstrating evidence of X-ray magnetic circular dichroism (XMCD) for a tetravalent U⁴⁺ ion in the molecular (U(η⁸-C₈H₈)₂) complex. The XMCD originates from a ground-state electronic structure with open-shell 5f electrons. Furthermore, the present calculation of uranium N_6,7-edge XAS and XMCD spectra also enables the ligand-field bonding analysis of the coordination compound.