Energy levels actinides

In what follows we briefly review some of the previous attempts to analyze the available spectra of plutonium (6). In addition, we estimate energy level parameters that identify at least the gross features characteristic of the spectra of plutonium in various valence states in the lower energy range where in most cases, several isolated absorption bands can be discerned. The method used was based on our interpretation of trivalent actinide and lanthanide spectra, and the generalized model referred to earlier in the discussion of free-ion spectra. 

The atomic spectra of the actinides are very complex and it is difficult to identify levels in terms of quantum numbers and configurations (6). The chemical behaviour of the elements is dictated by the configurations of the electrons around the nucleus and in the case of the actinides it is the competition between the 5/ 1 7 s2 and the 5 /n 1 6 d 7 s2 levels that dictates these chemical properties. A comparison of the /-energy levels of the lanthanides and the actinides shows that less energy is required for the promotion of the 5 / -> 6 d levels than for the 4/ -> 5 d levels in the lanthanides. As a result of this lower energy requirement by the actinides they have the tendency to display higher valences since the bonding electrons are more readily available. It is only at the commencement of the second half of the actinides that there is commencement of properties which echo those of the lanthanides. 

A better description of the energy level structure of the actinide atoms is obtained by adding to (3) terms of relativistic origin, which, in fact, represent the magnetic interaction of orbital and spin momenta of the electrons. (They are of particular importance for actinides since they depend on the fourth power of the atomic number Z) ... 

This competition, manifested in the structure of the actinide atomic spectra, also appears as a competition between configurations there is an accumulation of many configurations, both even and odd, at about the same height, leading to a high density of energy levels (in fact, we had arrived at the same conclusion by inspecting Fig. 1 in Sect. A.I.2). 

The energy level values of the lowest spectroscopic term of the electronic configuration of lanthanide as well as actinide atoms, were tabulated by Brewer. Such tables are very useful for phenomenological correlations concerning actinide metals (see Chap. C). From these tables one can obtain Table 1 giving the ground state and the first excited level of the actinide atoms as well as of the lanthanide atoms for comparison ... 

Pursuing a systematic interpretation of the electronic energy level structure of lanthanides and actinides within the framework of crystal-field theory led to the most important accomplishments in Bill s scientific career. In the 1980s, powerful computer programs were devel-... 

Electronic Energy Level and Intensity Correlations in the Spectra of the Trivalent Actinide Aquo Ions. I. Es3+, W.T. Camall, D. Cohen, P.R. Fields, R.K. Sjoblom, and R.F. Bames, J. Chem. Phys. 59, 1785-1789 (1973). 

Also electrons of other inner orbitals than d may of course give rise to covalent bonding, provided they are on suitable energy levels. It is thus very likely that the abnormal stability of certain actinide complexes is due to partially covalent bonds involving 5/electrons (5). On the other hand there are no signs that the 4/electrons of the lanthanides are able to participate in such bonds evidently these are situated too deep within the atom. 

Figure 1. Electronic energy level diagram for gas phase actinide hexafluorides. The regions in which a given hexafluoride exhibits continuous absorption are shown shaded with diagonal lines. 5f electron states are shown as short horizontal lines. The thermodynamic dissociation limits and resultant gas phase products are shown to the right of the energy level diagram for each hexafluoride. UF (g), a 5f system, has no low-lying electronic levels and is thermodynamically more stable than NpF (g) or PuF Cg). For these reasons UF is unlikely to be a good model compound for transuranic hexafluoride photochemistry studies.

It will be noted that the transitions are often broader than those found in the spectra of lanthanide complexes - and indeed the later actinides, see Section 12.2.4. The 5f energy levels are more sensitive to coordination number than are the corresponding levels in the lanthanides since there are bigger crystal-field effects, one sees pronounced differences between the spectra of 6-coordinate [UCle] and of U" +(aq) (Figure 12.5), leading to the conclusion that the uranium(iv) aqua ion was not six coordinate (most recent EX-AFS results suggest a value of 9 or 10, see Table 11.1). Figure 12.6 displays another example of the difference in spectra between similar complexes of different coordination number. 

The electron configurations of the actinides in the gas phase are listed in Table 14.3. Whereas in the case of the lanthanides only up to two f electrons are available for chemical bonding, in the case of the actinides more than two f electrons may be engaged in chemical bonds (e.g. all the electrons in compounds of FT(VI) and Np(VII)). This is due to the relatively low differences in the energy levels of the 5f and 6d electrons up to Z 95 (Am). However, these differences increase with Z and the chemistry of elements with Z > 96 becomes similar to that of the lanthanides. The special properties of the actinides are evident from their oxidation states, plotted in Fig. 14.12 as a function of the atomic number. In contrast to the lanthanides, a tendency to form lower oxidation states is observed with the heavier actinides. The... 

---