PNNL

Abstract

The UF5 - and UF6 - anions are produced using electrospray ionization and investigated by photoelectron spectroscopy and relativistic quantum chemistry. An extensive vibrational progression is observed in the spectra of UF5 -, indicating significant geometry changes between the anion and neutral ground state. Franck-Condon factor simulations of the observed vibrational progression yield an adiabatic electron detachment energy of 3.82 ± 0.05 eV for UF5 -. Relativistic quantum calculations using density functional and ab initio theories are performed on UF5 - and UF6 - and their neutrals. The ground states of UF5 - and UF5 are found to have C4v symmetry, but with a large U-F bond length change. The ground state of UF5 - is a triplet state (3B2) with the two 5f electrons occupying a 5fz3-based 8a1 highest occupied molecular orbital (HOMO) and the 5fxyz-based 2b2 HOMO-1 orbital. The detachment cross section from the 5fxyz orbital is observed to be extremely small and the detachment transition from the 2b2 orbital is more than ten times weaker than that from the 8a1 orbital at the photon energies available. The UF6 - anion is found to be octahedral, similar to neutral UF6 with the extra electron occupying the 5fxyz-based a2u orbital. Surprisingly, no photoelectron spectrum could be observed for UF6 - due to the extremely low detachment cross section from the 5fxyz-based HOMO of UF6 -.