Spectroscopy actinide

Figure 2. Schematic algebraic energy matrix of = 0 Actinide Spectroscopy...

Historically, the correlation between actinide chemistry and spectroscopy was anticipated before much experimental information was available in either field. There was therefore interest in actinide spectroscopy as an aid to predicting actinide chemistry, in the expectation that smaller quantities of these elements would be required. In practice, the chemistry developed first as soon as sufficient amounts were produced, while the spectroscopy encountered difficulties because the complexities were underestimated. The difficulty was not so much the enormous total number of levels, which could be counted readily, but the extent to which the levels interacted so as to preclude simplification. 

The Actinides. A Case Where Spectroscopy Triumphs over Chemistry ... 

Chemistry and spectroscopy of/-element organometallics Part II, the actinides. T. J. Marks, Prog. Inorg. Chem, 1979, 25, 223-333 (410). 

The organoactinide surface complexes exhibited catalytic activities comparable to Pt supported on sihca [at 100% propylene conversion at —63°C, >0.47s (U) and >0.40 s (Th)], despite there being only a few active sites (circa 4% for Th, as determined by CO poisoning experiments and NMR spectroscopy) [92]. Cationic organoactinide surface complexes [Cp An(CH3 ) ] were proposed as catalytic sites. This hypothesis could be corroborated by the use of alkoxo/hydrido instead of alkyl/hydrido surface ligands, which led to a marked decrease of the catalytic activity, owing to the oxophilic nature of the early actinides [203, 204]. Thermal activation of the immobihzed complexes, support effects, different metal/ligand environments and different olefins were also studied. The initial rate of propylene conversion was increased two-fold when the activation temperature of the surface complexes under H2 was raised from 0 to 150°C (for Th 0.58 0.92 s" ). 

Molecular Interactions Reaction Kinetics Basic Spectroscopy X-ray Crystallography Lanthanide and Actinide Elements Maths for Chemists Bioinorganic Chemistry Chemistry of Solid Surfaces Biology for Chemists Multi-element NMR... 

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). 

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. 

In Chap. E, photoelectron spectroscopic methods, in recent times more and more employed to the study of actinide solids, are reviewed. Results on metals and on oxides, which are representative of two types of bonds, the metallic and ionic, opposite with respect to the problem itineracy vs. localization of 5f states, are discussed. In metals photoemission gives a photographic picture of the Mott transition between Pu and Am. In oxides, the use of photoelectron spectroscopy (direct and inverse photoemission) permits a measurement of the intra-atomic Coulomb interaction energy Uh. 

Actinide metal samples are characterized by chemical and structure analysis. Multielement analysis by spark source mass spectrometry (SSMS) or inductively coupled argon plasma (ICAP) emission spectroscopy have lowered the detection limit for metallic impurities by 10 within the last two decades. The analysis of O, N, H by vacuum fusion requires large sample, but does not distinguish between bulk and surface of the material. Advanced techniques for surface analysis are being adapted for investigation of radioactive samples (Fig. 11) ... 

For actinides, the ideally trivalent Eb vs. Z curve is then simulated by passing a smooth curve parallel to the lanthanides one through the only truly trivalent value known at the time of the pubUcation" Cm, with AH = 89 4 Kcal/mol. I(III) s, also unknown, are taken from atomic spectroscopy and calculations ... 

Mdssbauer spectroscopy yields important information about the magnetism of actinide compounds and has been used with success concurrently with other techniques. 

Localization versus itineracy and the degree of hybridization of 5 f states with orbitals of the actinide atom (especially 6 d) as well as with those of the ligand in compounds are central questions for the understanding of bonding in actinide solids. Photoelectron spectroscopy provides answers to these questions. In narrow band solids, like the actinides, the interpretation of results requires the use of band calculations in the itinerant picture, as well as models of final state emission in the atomie picture. 

After a survey of the basic theory and some experimental aspects of photoelectron spectroscopy which are relevant to actinide solids, two systems are illustrated elemental actinide metals, in which the Mott transition between plutonium and americium is evidenced in a photographic way by photoemission, and strongly ionic oxides, in which the 5f localized behaviour is clearly seen, and indications of f-p or d-p covalent mixing are investigated. 

Photoelectron spectroscopy has been presented in many review articles and books. Therefore here only a brief description is given with special emphasis on actinides. For further reading, some selected literature is recommended " . ... 

The apparent binding energies measured in photoelectron spectroscopy are affected by other many-body effects, which are not treated here °. Only a few of them, relevant for open shell systems, hence for actinides, will be treated in Chapt. II. 3. 

In the chapter, we have illustrated some results of photoelectron spectroscopy on two classes of actinide materials, elemental metals and oxides, which we thought particularly relevant as they represent metallic and almost completely ionic bonding. Our interest having been focused on the localization vs. itineracy problem of the 5 f states, as well as on their hybridization with other electron states, we have particularly concentrated on those results which could throw light on these two aspects. 

Photoelectron spectroscopy has long be considered as to be able to provide a photographic picture of the one-electron density of state of solids. In reality, the spectra of actinide solids (as of other narrow band solids) need very often more than this naive interpretation. In the case of 5 f response, final state effects are found to provide useful information even in the case of metals, as illustrated in this chapter. The general conclusion that the photoelectron spectroscopic response depends on many-electron excited final states as much as it depends on the initial states, when narrow bands are involved, must be emphasized. This points to the necessity both of better final state models and of band calculations giving reliable pictures of conduction bands. 

On the basis of the known electronic properties of actinides (which have been discussed elsewhere in this book), theoreticians had distinguished the 5f itinerant behaviour of light actinide metals from the 5 f localized behaviour of heavy actinide metals from Am on. The crossover, presented often as a Mott transition, had been predicted to occur between Pu and Am metal, due to the localized character of the 5f state in the latter. Photoemission spectroscopy demonstrates this phenomenon directly with the observation of a 5 f multiplet away from the Fermi level. The detailed description of this peak is certainly complicated, as often happens for response of localized states in photoemission on the other hand (Fig. 17) the contrast to the emission of Pu metal is convincing. 

Two final remarks which concern predictable future developments of photoelectron spectroscopy with regard to actinide solids, should be added. 

As we have seen, the most advanced photoelectron techniques, especially those which necessitate the use of synchrotron radiation sources, have been applied until now only to U and Th systems. Application on Pu and Am systems as well as to heavier actinides is to be expected in the future. The same development is likely to occur as for neutron experiments, where more and more these hazardous actinides are investigated at high levels of instrumental sophistication. Difficulties arising from handling and protection problems are, of course, much greater for photoelectron spectroscopy. 

Mixed valence phenomena, such as studied by photoelectron spectroscopy in lanthanide systems, are expected to become important especially (but not only) in the second half of the actinide series. It is to be expected that much of the photoelectron spectroscopic effort will be in the future devoted to the study of these phenomena in actinides, especially as soon as measurements on hazardous actinides will become more feasible. 

Chapters C, D and E, discuss thermodynamic and structural properties, magnetism, and photoelectron spectroscopy of actinide systems in their relation to bonding. 

As already mentioned, UV-Vis spectroscopy is an effective tool to study metal complexes. For different actinide(IV) compounds in ILs (Np(IV), Pu(IV), U(IV) as [C4CiIm]2[AnCy complexes in [C4CiIm][Tf2N]) a similarity with solid complexes having an octahedral An(IV) environment was estimated [14,15]. In other ILs, for example, uranyl ions dissolved in [C4QIm] [NfO], [U02] may be present as a bare cation [16]. 

Fig. 7. Potential mechanisms of actinide (represented by Cm(ni)) interaction with colloids as interpreted from laser fluorescence spectroscopy (TRLFS) experiments. Spectra are taken from Stumpf et al. (2001o, b) and Chung et al. (1998).

Stumpf, Th. Fanghanel, Th. 2002. A time-resolved laser fluorescence spectroscopy (TRLFS) study of the interaction of trivalent actinides (Cm(III)) with calcite. Journal of Colloid and Interface Science, 249, 119-122. 

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