Spectroscopy of Actinide Ions

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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Optical and Electron Paramagnetic Resonance Spectroscopy of Actinide Ions in Single Crystals... 

R. Stumpe, J. I. Kim, W. Schrepp, H. Walther, Speciation of Actinide Ions in Aqueous Solution by Laser-induced Pulsed Spectrophotoacoustic Spectroscopy, Appl. Phys. B 34, 203 (1984)... 

Conventional optical absorption spectrometry has detection limits of between 0.01 and 1 mM for the actinides. Highly sensitive spectroscopic methods have been developed, based on powerful laser light sources. Time resolved laser fluorescence spectroscopy (TRLFS), based on the combined measurement of relaxation time and fluorescence wavelength, is capable of speciating Cm(III) down to 10 mol/L but is restricted to fluorescent species like U(VI) and Cm(III). Spectroscopic methods based on the detection of nonradiative relaxation are the laser-induced photoacoustic spectroscopy (LPAS) and the laser-induced thermal leasing spectroscopy (LTLS). Like conventional absorption spectroscopic methods, these newly developed methods are capable of characterizing oxidation and complexation states of actinide ions but with higher sensitivity. 

Several spectroscopic techniques, namely, Ultraviolet-Visible Spectroscopy (UV-Vis), Infrared (IR), Nuclear Magnetic Resonance (NMR), etc., have been used for understanding the mechanism of solvent-extraction processes and identification of extracted species. Berthon et al. reviewed the use of NMR techniques in solvent-extraction studies for monoamides, malonamides, picolinamides, and TBP (116, 117). NMR spectroscopy was used as a tool to identify the structural parameters that control selectivity and efficiency of extraction of metal ions. 13C NMR relaxation-time data were used to determine the distances between the carbon atoms of the monoamide ligands and the actinides centers. The II, 2H, and 13C NMR spectra analysis of the solvent organic phases indicated malonamide dimer formation at low concentrations. However, at higher ligand concentrations, micelle formation was observed. NMR studies were also used to understand nitric acid extraction mechanisms. Before obtaining conformational information from 13C relaxation times, the stoichiometries of the... 

Sinkov, S.I. Choppin, G.R. Taylor, R.J. Spectrophotometry and luminescence spectroscopy of acetohydroxamate complexes of trivalent lanthanide and actinide ions, J. Solut. Chem., 36 (2007) 815-830. 

Time-resolved emission spectroscopy is gaining importance in the study of various chemical aspects of luminescent lanthanide and actinide ions in solution. Here, the author describes the theoretical background of this analytical technique and discusses potential applications. Changes in the solution composition and/or in the metal-ion inner coordination sphere induce modifications of the spectroscopic properties of the luminescent species. Both time-resolved spectra and luminescence decays convey useful information. Several models, which are commonly used to extract physico-chemical information from the spectroscopic data, are presented and critically compared. Applications of time-resolved emission spectroscopy are numerous and range from the characterization of the... 

Chloride complexation studies of the actinide ions An3+, An4+, An02, and AnO + (An = U, Np, Pu) were reported in several comprehensive reviews [293-295], More recent investigations on aqua and chloro complexes of U02+, NpOz+, Np4+, Pu3+, etc., by x-ray absorption fine structure spectroscopy (XAFS) were reported [296,297]. In particular, it was established for U(IV) and Th(IV) aqua ions and fluoride complexes that both M(IV) aqua ions are 10-coordinate with M — O bond distances for U(IV) and Th(IV) of 2.42 0.01 A and 2.45 0.01 A, respectively [297], Physical and chemical studies of uranium aqueous complexes are reported [298,299a], A series of articles is dedicated to specific sequestering agents for the actinides [299b-e],... 

Hydroxides. The hydrolysis of Np has been studied more than that of any other pentavalent actinide because it is the most stable oxidation state for Np and it is an actinide ion of significant concern for environmental migration. Pentavalent uranium disproportionates in aqueous solution at pH values where hydrolysis would occur. Hydrolysis products for Pa, Pu, and Am are very similar to, but much less stable than those of Np, so only Np hydroxides will be described in detail. Neptunyl hydrolyzes at about pH 9, to form the stepwise products, Np02(0H) and Np02(0H)2 ", which have been identified by optical absorbance and Raman spectroscopy. " In addition to the hydroxide these complexes likely have two or three inner-sphere waters in the equatorial plane and pentagonal bipyramidal coordination geometry. 

Spectral methods (spark source mass spectrometry SSMS, secondary ion mass spectrometry SIMS, inductively coupled argon plasma for emission spectroscopy ICAP-ES) which avoid separation steps are increasingly applied for multi-element analysis. Hot extraction is used for 0, N, H determinations. Oxygen is also determined by activation analysis, nitrogen after adaptation of classical methods (micro-Kjeldahl). Combination and comparison of different, independent methods are desirable, but hampered by the often limited availability of samples of actinides. 

In view of the ease and success of lasing lanthanide ions, only some compelling reason such as the requirement of a specific wavelength would warrant development of some of the actinide lasing schemes discussed. Perhaps additional spectroscopy will reveal advantages of using actinide ions in other valence states and hosts for efficient laser action. 

Achieving laser action is a result of a favorable combination of many spectroscopic properties of an ion in a given host. The ability to predict and demonstrate stimulated emission is therefore a powerful confirmation of our understanding of the spectroscopy of lanthanide and actinide ions and a motivation for further study of these ions. 

Relativistic Ab Initio Model Potential embedded cluster calculations on the structure and spectroscopy of local defects created by actinide impurity ions in solid hosts are the focus of attention here. They are molecular like calculations which include host embedding effects and electron correlation effects, but also scalar and spin-orbit coupling relativistic effects, all of them compulsory for a detailed understanding of the large manifolds of states of the 5f" the 5f" 6d configurations. The results are aimed at showing the potentiality of Relativistic Quantum Chemistry as a tool for prediction and interpretation in the field of solids doped with heavy element impurities. 

In this Section, we will show the results of sfss CGWB-AIMP embedded cluster calculations on the structure and spectroscopy of several actinide ions in chloride hosts. Altogether, they are aimed at showing the potentialities of present days wavefunction-based ab initio methods of Quantum Chemistry in materials made of heavy element ions in solids, with several open-shell electrons and a very large number of excited states of interest. 

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