Core-level actinide

Fig. 7 a-c. Schematic representation of final state screening models for lanthanide and d-metal core level responses (a) and c)) (c.b. means conduction band). In part b), the possible situations for light and heavy actinides (before and after the Mott-Hubbard transition) are also represented... 

In the case of the most common X-ray somces used for XPS (Mg and Al), and of the most widely observed core levels of the actinides (4 f), the photoelectron kinetic energy is of several hundreds of eV. This ensures an analysis depth large enough to reveal the bulk properties. 

However, the 4fy/2 core level response of U metal does not show a poorly screened satellite, but only a pronounced asymmetry. This is somewhat in contrast with the greater localization predicted by theory ° when proceeding across the actinide series. 

The XPS valence band spectra for the dioxides of the transuranium elements (from Np to Bk) have been presented in an extensive and pioneering work that also includes core level spectra and has been for a long time the only photoemission study on highly radioactive compounds. High resolution XPS spectra (AE = 0.55 eV) were recorded on oxidized thin metal films (30 A) deposited on platinum substrates with an isotope separator. (The oxide films for Pu and the heavier actinides may contain some oxides with lower stoichiometry, since starting with Pu, the sesquioxides of the heavier actinides begin to form in high vacuum conditions.)... 

Indications of Hybridization From the Valence Band Spectra and Core Levels in Actinide Oxides... 

Common features of the 4 f core level spectra of actinide dioxides are the symmetry of the main lines and the appearance of a satellite at about 7 eV in their high binding energy side (Fig. 30). Similar satellites have also been found for UF4, for which compound the intensity is even higher than for the dioxides. It is perhaps interesting to report some analysis of these features, on the basis of final state models. 

Fig. 30. 4 f core levels spectra of actinide dioxides measured by XPS, from Xh02 to Cf02 (from Ref. 15)... 

It is worthwhile to mention the ample use of screening final states models in understanding core levels as well as valence band spectra of the oxides. The two-hole models, for instance, which have been described here, are certainly of relevance. Interpretational difference exists, for instance, on the attribution of the 10 eV valence band peak (encountered in other actinide dioxides as well), whether due to the non-screened 5f final state, or to a 2p-type characteristics of the ligand, or simply to surface stoichiometry effects. Although resonance experiments seem to exclude the first interpretation, it remains a question as to what extent a resonance behaviour other than expected within an atomic picture is exhibited by a 5 f contribution in the valence band region, and to what extent a possible d contribution may modify it. In fact, it has been shown that, for less localized states (as, e.g., the 3d states in transition metals) the resonant enhancement of the response is less pronounced than expected. 

For further references and discussion of core-level spectra in the actinides we refer to Kowalczyk10, Krause and Nestor63, Sham and Wending65 and Boring et al.214. ... 

As its name implies, XPS is a technique that relies on the ionization of the species of interest and that is particularly useful to investigate smface properties, and as such has been widely used to investigate actinide oxides, in particular those of uranium (given its predominant role in nuclear fuels) in different forms such as solids or thin films. There, the core levels associated with the 4/ electrons are particularly interesting, first due to their distinctive positions and the magnitude of the splitting of the 4/5/2 and 4fj/2 due to spin-orbit coupling (about 10 eV in the UO2 crystal [97]). 

Because of their basic resemblance to porphyrins, it was initially expected that the sapphyrins would mimic, at least on some level, the rich coordination chemistry displayed by the porphyrins. However, the larger core size ca. 5.5 A inner N-N diameter vs. ca. 4.0 A for porphyrins), the greater number of potentially chelating heteroatom centers, and the fact that pentaazasapphyrins when fully deprotonated are potentially trianionic ligands made sapphyrin a likely candidate for large metal chelation, particularly as a potential ligand for the trivalent lanthanides and actinides. Unfortunately, in spite of extensive effort, this hope remains largely unrealized. Nonetheless, some metal complexes of sapphyrins and heterosapphyrins have been successfully prepared and characterized. Their preparation and properties are reviewed in this section. 

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