Spectra and Electronic Structures of Free Actinide Atoms and Ions

This chapter reviews the spectra and the deduced electronic properties of isolated actinide atoms and ions observed in the vapor phase at low density. The free atoms or ions have all or most of the valence electrons present, and the observed spectra can be assigned to transitions due essentially to changes in the quantum numbers of the valence electrons. This is in contrast to the spectra of actinides in crystals or in solution (dealt with in depth in Chapter 18), where the observed spectra are largely due to transitions within the 5f shell. In crystals, the actinide ions are exposed to the electric field of the surrounding ions, which produces a Stark effect on the levels. The magnitude of the effect is relatively small because the 5f electrons are shielded from the crystal field by the 6s and 6p electrons. The result is a small perturbation in which each 5f level is split into a number of close components. In free atoms, the valence electrons interact strongly with the 5f electrons and also with each other. Hence each 5f level gives rise to many daughter levels that are more widely split than the parent separations and have large angular momentum contributions from the parent. The result in this case is a great number of levels whose structure is not simply related to the structure of the 5f levels or to the structure of the valence-electron levels by themselves. It is evident that the 5f level structure can be obtained more directly from crystal spectra but the properties of the valence electrons (in particular, implications for the chemical properties) must be deduced from the free-atom spectra.