Actinide optical spectroscopy

For 3 + lanthanide and actinide ions, almost all transitions within the f shell are electric dipole in nature. These transitions are formally parity (Laporte) forbidden. That such transitions are observable is attributed to non-centro-symmetric terms in the crystal-field Hamiltonian. Such terms have the effect of mixing higher-lying, opposite-pairty d and g states into the f shell. As Judd (1988) noted in a review of atomic theory and optical spectroscopy of rare earths No doubt that we shall eventually be able to calculate much of what we want with a high degree of accuracy. That day has not yet arrived. . 

This volume of the Handbook illustrates the rich variety of topics covered by rare earth science. Three chapters are devoted to the description of solid state compounds skutteru-dites (Chapter 211), rare earth-antimony systems (Chapter 212), and rare earth-manganese perovskites (Chapter 214). Two other reviews deal with solid state properties one contribution includes information on existing thermodynamic data of lanthanide trihalides (Chapter 213) while the other one describes optical properties of rare earth compounds under pressure (Chapter 217). Finally, two chapters focus on solution chemistry. The state of the art in unraveling solution structure of lanthanide-containing coordination compounds by paramagnetic nuclear magnetic resonance is outlined in Chapter 215. The potential of time-resolved, laser-induced emission spectroscopy for the analysis of lanthanide and actinide solutions is presented and critically discussed in Chapter 216. 

For the description of the linear and nonlinear optical properties of metallotetrapyrroles, TDDFT methods have proven [133-148] to be an excellent alternative to conventional highly correlated ab initio methods, such as SAC-CI, STEOM-CC, and CASPT2, for which these systems still represent a severe computational challenge, especially when transition metals, lanthanides or actinides are involved. The few highly correlated ab initio calculations dealing with the excited state properties of metallotetrapyrroles that have appeared to date only concern magnesium and zinc porphyrins and porphyrazines [149-151]. Application of TDDFT methods to the electronic spectroscopy of a variety of metallotetrapyrroles, including homoleptic and heteroleptic sandwiches, will be illustrated in this section. 

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

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. 

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... 

Actinides have particular spectroscopic properties which are characterized primarily by the / - / transitions within the partially filled 5f shell [42] and thus by a number of relatively weak but very sharp absorption bands. The optical spectra of actinides are characteristic for their oxidation states, and to a lesser degree dependent upon the chemical environment of the ion [43]. Thus spectroscopic investigation provides information on the oxidation state of an actinide element [42] and also serves to characterize the chemical states, such as hydrolysis products [44], various complexes [37, 45, 46] and colloids [29, 40]. Hence, laser-induced photoacoustic spectroscopy (LPAS) with its high sensitivity can be conveniently used for the speciation of aqueous actinides in very dilute concentrations [17-28]. 

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. 

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