Photochemistry actinide

Exclted states, primary processes Lanthanides, electronic structure Lanthanides, excited states Lanthanides, photochemistry Actinides, electronic structure ActlnldCs, excited states Uranyl ion, photochemistry Uranyl complexes, photochemistry Uranyl Ion, luminescence quenching Photochemistry, actinide alkyls... 

Studies of actinide photochemistry are always dominated by the reactions that photochemically reduce the uranyl, U(VI), species. Almost any UV-visible light will excite the uranyl species such that the long-lived, 10-lt seconds, excited-state species will react with most reductants, and the quantum yield for this reduction of UQ22+ to U02+ is very near unity (8). Because of the continued high level of interest in uranyl photochemistry and the similarities in the actinyl species, one wonders why aqueous plutonium photochemistry was not investigated earlier. 

Isotope photoseparation techniques for actinides probably will include only gaseous systems, hexafluorides and metal vapors. Hence, aqueous actinide photochemistry is not likely to influence isotope separations. However, the intense interest in laser separation techniques for the gaseous systems promotes interest in the aqueous systems. 

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.

Prior to the advent of the laser, photo-induced reactions sat, for the most part, in the chemical background. Photochemists of that time typically gave little thought to actinide elements other than uranium which was known for years to be a very excellent chemical actinometer when present as the uranyl ion. The expertise and specialized equipment required in the handling of the other actinides, coupled with their very limited supply,served to discourage photochemists from fundamental investigations of these elements. As a result, no report of actinide photochemistry (save that of uranium) is to be found in the open literature prior to 1969. 

The first attention given to actinide photochemistry was for the purpose of identifying any photochemical activity which might alter the efficiency of the extraction or exchange processes. Subsequently, the identification of photochemically active species of uranium and plutonium gave some indication that the photoreactions could be turned to a useful end and, perhaps, offer a cleaner way to separate actinides from each other and from the other elements accompanying them in nuclear fuel elements. 

To summarize the promise of actinide photochemistry briefly, it has been found that all three major actinides have a useful variety of photochemical reactions which could be arranged in a sequence to achieve a separations process that requires fewer reagents, and, understandably, a reduced volume of waste solutions. Most of the impact of photochemistry on reprocessing is admittedly speculative since only the chemical feasibility has been demonstrated. 

The solution photochemistry of the actinides begins with uranium none has been reported for actinium, thorium, and protactinium. Spectra have been obtained for most of the actinide ions through curium in solution (5). Most studies in actinide photochemistry have been done on uranyl compounds, largely to elucidate the nature of the excited electronic states of the uranyl ion and the details of the mechanisms of its photochemical reactions (5a). Some studies have also been done on the photochemistry of neptunium (6) and plutonium (7). Although not all of these studies are directed specifically toward separations, the chemistry they describe may be applicable. 

Sostero, S. Traverso, 0. Bartocci, C. DiBernardo, P. Magon, L. Carassiti, V. "Photochemistry of Actinide Complexes. III. The Photoproduction Mechanism of Uranium(V) Oxochloro Complexes," Inorg. Chim. Acta., 1976, 19, 229. 

Topics, which have formed the subjects of reviews this year, include the luminescence kinetics of metal complexes in solution, photochemical rearrangements of co-ordination compounds, photochromic complexes of heavy metals with diphenylthiocarbazone derivatives, the photochemistry of actinides, actinide separation processes, and light-induced electron-transfer reactions in solution and organized assemblies. A discussion has also appeared on assigning excited states in inorganic photochemistry. ... 

Knowledge of actinide photochemistry is limited mainly to the complexes of uranium, especially those containing the uranyl ion, The lowest excited state of this ion is... 

Since the bulk of actinide photochemistry has so far been concerned... 

The photochemistry of lanthanide and actinide ions is mainly limited to photoredox reactions. Photosubstitution has not been studied because the metal ions are labile, and therefore ligand substitution reactions are vapid even under thermal conditions. 

The majority of the photochemical studies with actinide ions have been carried out with the uranyl (UC ion. This ion is yellow in color both in the solid and solution states. The early photochemistry of this ion has been reviewed. " Excitation of this ion results in an LMCT absorption that involves a transition from an essentially nonbonding 7r-orbital on oxygen into an empty 5/orbital on uranium. This LMCT assignment is that given to the weak visible bands in the absorption spectrum at 500 nm and 360 nm. The absorption spectrum also shows a series of bands of increasing intensity to higher energy. The positions of the absorption bands of are very sensitive to both temperature and the chemical environment... 

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