Actinide photoelectron spectroscopy

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. 

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. 

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. 

Spectroscopy. The application of optical and photoelectron spectroscopy to elucidate electron energy states of pure actinide metals is still in the initial stages (46). Reflectivity measurements on Th samples (mechanically polished, electropolished, or as grown from the vapour phase) demonstrate the importance of sample and surface preparation (47), and explain reasons for discrepancies in published results (48, 49). Preliminary measurements of the optical reflectivity of Am films evaporated on different window materials (50) seem to indicate that the 5f levels are lying more than about 6 eV below the FERMI level, thus supporting the interpretation of the electrical resistivity results... 

In this paper, we shall present a brief overview of the application of photoelectron spectroscopy to the study of actinide materials. Some phenomenology will be discussed as will studies of specific materials. Only illustrative examples will be presented (4). 

Beeching, L. J., Dyke, J. M., Morris, A., Ogden, J. S., Study of the electronic stracture of the actinide tetrabromides ThBr4 and UBr4 using ultraviolet photoelectron spectroscopy and density functional calculations, J. Chem. Phys., 114, (2001), 9832-9839. Cited on pages 252,663. 

Gas Phase He-I and He-II Photoelectron Spectroscopy of Volatile Lanthanide and Actinide Complexes Jennifer C. GREEN... 

Low symmetry complexes, photoelectron spectroscopy Biscylcopentadlenyl actinide complexes, photoelectron spec tiros copy... 

I. Fragala, G. Condorelh, P. Zanella, E. Tondello, Photoelectron spectroscopy of actinide organometaUic compounds I. Bis(cyclooctatetraene)actinide(fV) complexes, J. Organomet. Chem., 122, 357-363 (1976). 

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