Electronic Structure of the Principal Uranium Centre in Sodium Fluoride

Abstract

It has long been known1 that uranium in sodium fluoride provides a complex fluorescence spectrum due to the presence of several centres. These centres are believed to be different uranium-oxygen complexes. The principal centres have C4v symmetry and have been attributed to UO5 groups2. Charge compensation is obtained if the hexavalent uranium ion substitutes for a sodium ion and the divalent oxygen ions substitute for five neighbouring fluorine ions. The fluorescence transition has a magnetic dipole character between non-degenerate states. Hence if the ground state has A1 symmetry the lowest excited state has A2 symmetry. The resonant fluorescent transition occurs at 563.6nm (17743cm−1). A theoretical model has been constructed along the lines of the calculations for the uranyl ion3. In this model excitation consists of an electron in a 2p oxygen ligand orbital being transferred to a uranium 5f orbital. There are a number of possibilities to consider as the electron may come from either the axial oxygen or the equatorial oxygens and in either case it can come from a σ orbital or a π orbital. Consider first the case of an electron coming from the axial oxygen ion. The determinantal wave functions for the excited states are of the form |σ f| or |π3 f|. The Hamiltonian includes one-electron, crystal field, spin-orbit, Coulomb and exchange terms. The ω - ω coupling scheme was used throughout the calculation. Matrices were constructed for the different representations of C4v symmetry. In the of model there are 3A1, 3A2, 4B2 and 7 pairs of E states. The spin-orbit coupling constant for the uranium 5f electron was taken to be 1950cm−1. Starting values for the exchange parameters were calculated from atomic basis functions using the computer program system MOLECULE. The crystal field, exchange and one-electron parameters were treated as variables. The matrices were diagonalized for different sets of parameters, and in the case of the E states g-values were calculated. Numerous calculations were made for different sets of parameters and a least squares fitting routine was used to get a fit between experimental and theoretical values of energy levels and g-values for the lowest seven excited states. It was not found possible to get a good fit on a σf model and the calculations were extended to the π3 f axial model and the equatorial model. For the equatorial model linear combinations of the various σ orbitals are formed to provide orbitals transforming as representations of C4v. A good fit has been obtained for the energy levels and g-values in the b2f model. This solution will be assessed critically for applicability to uranium centres in sodium fluoride.