Photodissociation and Theory to Investigate Uranium Oxide Cluster Cations

Uranium oxide cluster cations of the form U_nO_m⁺ are produced by laser vaporization of a depleted uranium rod in a pulsed supersonic expansion. Ions are mass-analyzed and mass-selected with a time-of-flight spectrometer and studied with UV laser multiphoton dissociation. Cations of the stoichiometry UO₂(UO₃)_n⁺ were observed as photofragments from all photodissociated cluster cations. (UO₃)_n⁺ clusters were also observed to result from dissociation of larger (UO₃)_n⁺ clusters, with UO₃ neutral as a common leaving group. Electronic structure calculations were used to investigate the stability of the prominent uranium oxide cluster cations using density functional theory (DFT) with the hybrid B3LYP exchange-correlation functional and at the CCSD­(T) level with cc-pVnZ-PP basis sets (n = D,T), including diffuse orbitals as computational expense and availability permitted. Clustering energies, relative energies and dissociation energies of the cations are reported. The lowest energy neutral (UO₃)_n clusters up to n = 3 are rings, n = 4 and 5 are chains with very low energy rings, and n = 6 is 3D. The lowest energy structures for UO₂(UO₃)_n⁺ are composed of uranyl-like UO₂⁺ units bound by bridging oxygens to other UO₂²⁺ units for n = 2 and 3, and for n = 4 a more complex 3D structure is predicted.