Spectra and Electronic Structures of Free Actinide Atoms and Ions

Abstract

This chapter deals with the electronic properties of isolated actinide atoms and ions, observed in the vapor phase at low density. The free atoms have all or most of the valence electrons present, and the spectra are 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, where the spectra are largely due to transitions within the 5f shell. In both cases, the energy level structure is dominated by the structure of the 5f shell, but in different ways. 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 field has high symmetry and, moreover, the 5f electrons are shielded from it by the 6s and 6p electrons. The result is a mild perturbation in which each 5f level is split into a number of close components. In free atoms, on the other hand, the valence electrons, which now have to be considered, interact strongly with the 5f electrons and also with each other. Hence ech 5f level gives rise to many daughter levels, which 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 deduced more directly from crystal spectra but the properties of the valence electrons (in particular, the chemical properties) must be deduced from the free-atom spectra.