Covalency in the uranyl ion: A polarized x-ray spectroscopic study

Abstract

Polarized oxygen Kα x-ray absorption and emission spectra, near 530 eV, of a single crystal of Cs2UO2Cl4 are reported. With the aid of density functional theory calculations and the absorption data, the relative energies of the empty molecular orbitals having primarily uranium 5f and 6d character are assigned. The emission spectra give access to the energy of excitations to these orbitals from the various filled valence-shell orbitals. These energies support the conclusion from the optical spectra that valence excitations from the σu occupied valence orbitals occur at substantially lower energies than those from the σg, πg, and πu orbitals. This latter group of molecular orbitals have much larger oxygen-2p character. The participation of the pseudocore 6p shell in the covalent bonding is established directly by the presence of a charge-transfer transition in emission. With the aid of previous work on the polarized uranium L1- and L3-edge absorption spectra, the order of the empty metal-centered antibonding valence orbitals is established as 5fπ<5fσ∼6dδ∼6dσ<6dπ. The greater antibonding character of 6dπ compared with 6dσ can be explained by the relatively small overlap of the latter orbital with oxygen 2p. However, the relative energies of 5fπ and 5fσ cannot be due to differences in overlap and are attributed instead to a strong hybridization between 5fσ and 6pσ. This superposition is confirmed by the linear dichroism in the U L1-edge absorption spectrum.