The Free Actinide Atom

Most results on the free actinide atom came from atomic spectroscopy and from atomic quantum calculations of wave functions and eigenvalues pf their outer electrons. This section cannot be an exhaustive review devoted to the theory and interpretation of the very complex spectra of the actinide atoms and ions. We shall recall briefly the theoretical approach used in atomic calculations and then give some of the numerous useful informations that derived from atomic studies for solid state physicists and chemists. 

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The results of atomic spectroscopy as well as atomic quantum calculations have made it possible to determine the ground state of the free actinide atoms. These results (see Table 1 ) (that will be reviewed in the next section of this Chapter) confirm the progressive filling of the 5f shell. From the point of view of the electronic structure of the free atom, therefore, question ii. is solved in the sense of actinides being a series in which the unsaturated 5 f shell is progressively filled (only one or two electrons being accomodated in the 6d shell). 

Actinide spectra reflect the characteristic features of the 5/ orbitals which can be considered as both containing the optically active electrons and belonging to the core of filled shells. The electronic transition spectra of actinide ions in solution are dominated by the structure of the / levels and transitions within the / shell. Free-atom spectra provide more information about the interactions between the 5/ and the valence electrons. The emission spectra of the free actinide atoms have an enormous number of lines. In the uranium spectrum, about 100,000 lines have been measured, from which about 2500 lines have been assigned. 

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