# Potential Energy Curves of Core-Excited States of the U M5 Absorption Edge Manifold of UO2 2+

## TL;DR

This paper studies the electronic structure of uranium dioxide using X-ray absorption data and theoretical models to understand bonding and spectroscopy.

## Contribution

The study introduces two novel theoretical methods to assess the covalent character of the U–O bond.

## Key findings

- The simulated U M5 XANES spectrum matches the experimental data, validating the theoretical model.
- The orbital projections and ⟨z²⟩ values reveal insights into the electronic structure and covalent bonding in UO2²⁺.
- The study links X-ray spectroscopy data with chemical bonding characteristics through potential energy curves.

## Abstract

The X-ray Absorption Near Edge Structure of the U M5 X-ray absorption edge of UO2
2+ is analyzed using the potential energy curves
of an isolated UO2
2+ obtained from rigorous, multiconfigurational all-electron ab initio wave functions for the ground and core-excited
configurations. The spectroscopic parameters for the potential energy
curves are reported. Two novel theoretical methods have been used
as measures of the covalent character of the U–O bond: (1)
The projection of the U­(5f) and U­(6d) orbitals of the isolated U6+ cation on the orbitals of the ground and the core-excited
states of UO2
2+ and (2) the size of the orbital charge distributions given by the
⟨z
2⟩ expectation values.
This gives direct insight into the variation of the electronic structure
of the bond as the U–O bond length is changed. The excellent
agreement of the simulated spectrum with the experimental U M5 XANES spectrum proves the validity of our theoretical model.
The potential energy curves and the simulated XANES spectra for varying
bond lengths together with the information from the two measures about
the covalent character allow us to establish a link between X-ray
spectroscopy and chemical bonding.