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

Polarized oxygen Kalpha 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 sigma(u) occupied valence orbitals occur at substantially lower energies than those from the sigma(g), pi(g), and pi(u) 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 5f(pi)<5f(sigma)similar to6d(delta)similar to6d(sigma)<6d(pi). The greater antibonding character of 6d(pi) compared with 6d(sigma) can be explained by the relatively small overlap of the latter orbital with oxygen 2p. However, the relative energies of 5f(pi) and 5f(sigma) cannot be due to differences in overlap and are attributed instead to a strong hybridization between 5f(sigma) and 6p(sigma). This superposition is confirmed by the linear dichroism in the U L1-edge absorption spectrum. (C) 2002 American Institute of Physics.