The electronic structure of actinyl ions

Polarized high resolution absorption spectra at 4.2 K are reported for single crystals of Cs2U(Np)O2Cl4 and CsU(Np)O2(NO3)3. Five transitions of the charge-transfer type are observed in both complexes. Zeeman effect measurements give values of g(parallel) in every case. MCD measurements in the nitrate complex permit the unambiguous assignment of a number of excited state symmetries. It is shown by evaluation of the electron-electron repulsion integrals that the excited states are best described as originating from σsub(u)γγ', as opposed to πsub(u)3γγ', configurations. These configurations are analogous to those describing the charge-transfer states of the uranyl(VI) ion and in some cases the parentage of the neptunyl transitions can be recognized from the polarization of their vibronic structure. (author)