The electronic structures of actinide systems are extremely complicated and pose considerable challenges both experimentally and theoretically because of strong electron correlation and relativistic effects. We report the investigation of the electronic structure and chemical bonding of a series of uranium oxide (UO2-) and fluorides (UFx -, x = 2-4) using photoelectron spectroscopy and relativistic quantum chemistry. The electron affinities of the corresponding neutral molecules are accurately measured. For UO2-, intense photoelectron bands are observed from its low-lying (7ssg)2(5ffu)1 orbitals and the more deeply-bound O2p-based molecular orbitals with a large energy gap. Surprisingly, numerous weak photodetachment transitions are also observed in the gap region due to extensive two-electron transitions, suggesting strong electron correlations among the (7ssg)2(5ffu)1 electrons in UO2-. The experimental data are interpreted using multi-reference ab initio calculations with inclusion of spin-orbit coupling. Vibrationally resolved photoelectron spectra are obtained for the three fluoride anions. Significant multi-electron transitions, similar to those of UO2-, are observed in the case of only U(5f37s2)F2-, as a result of strong electron correlation effects of the two 7s electrons. The U-F symmetric stretching vibrational modes are resolved for the ground states of all UFx (x = 2-4) neutrals. Theoretical calculations are performed to qualitatively understand the photoelectron spectra. The current data provide experimental electronic structure information to test various relativistic quantum chemistry methods aimed at strongly correlated electron systems.

Probing the electronic structures of low oxidation-state uranium molecules

247th National Meeting (2014)

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