Probing the electronic structures of low oxidation-state uranium fluoride molecules UF(x)- (x = 2-4).

We report the experimental observation of gaseous UF(x)(-) (x = 2-4) anions, which are investigated using photoelectron spectroscopy and relativistic quantum chemistry. Vibrationally resolved photoelectron spectra are obtained for all three species and the electron affinities of UF(x) (x = 2-4) are measured to be 1.16(3), 1.09(3), and 1.58(3) eV, respectively. Significant multi-electron transitions are observed in the photoelectron spectra of U(5f(3)7s(2))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 UF(x) (x = 2-4) neutrals. Theoretical calculations are performed to qualitatively understand the photoelectron spectra. The entire UF(x)(-) and UF(x) (x = 1-6) series are considered theoretically to examine the trends of U-F bonding and the electron affinities as a function of fluorine coordination. The increased U-F bond lengths and decreased bond orders from UF2(-) to UF4(-) indicate that the U-F bonding becomes weaker as the oxidation state of U increases from I to III.