Absorption lanthanides

The other upconversion pathways are foremost relevant in doped inorganic materials. Multiionic mechanisms can be developed based on those fimdamental principles. The most frequent lanthanide ions for upconversion in inorganic materials are Pr +, Er +, and Tm +. Most of those inorganic materials use f- f transitions to achieve excited state absorptions. Lanthanide ions having both NIR and visible emission are thus needed. This limitation does not occur for two-photon absorption, which may use any lanthanide ion. For more details, the reader can refer to specific reviews. ... 

Absorption and Fluorescence Spectra. The absorption spectra of actinide and lanthanide ions in aqueous solution and in crystalline form contain narrow bands in the visible, near-ultraviolet, and near-infrared regions of the spectmm (13,14,17,24). Much evidence indicates that these bands arise from electronic transitions within the and bf shells in which the Af and hf configurations are preserved in the upper and lower states for a particular ion. 

In general, the absorption bands of the actinide ions are some ten times more intense than those of the lanthanide ions. Fluorescence, for example, is observed in the trichlorides of uranium, neptunium, americium, and curium, diluted with lanthanum chloride (15). 

The arc and spark spectra of the individual lanthanides are exceedingly complex. Thousands of emission lines are observed. For the trivalent rare-earth ions in soUds, the absorption spectra are much better understood. However, the crystal fields of the neighboring atoms remove the degeneracy of some states and several levels exist where only one did before. Many of these crystal field levels exist very close to a base level. As the soUd is heated, a number of the lower levels become occupied. Some physical properties of rare-earth metals are thus very sensitive to temperature (7). 

Absorption Spectra, of Aqueous Ions. The absorption spectra of Pu(III) [22541-70 ] Pu(IV) [22541 4-2] Pu(V) [22541-69-1] and Pu(VI) [22541-41-9] in mineral acids, ie, HCIO and HNO, have been measured (78—81). The Pu(VII) [39611-88-61] spectmm, which can be measured only in strong alkaU hydroxide solution, also has been reported (82). As for rare-earth ion spectra, the spectra of plutonium ions exhibit sharp lines, but have larger extinction coefficients than those of most lanthanide ions (see Lanthanides). The visible spectra in dilute acid solution are shown in Figure 4 and the spectmm of Pu(VII) in base is shown in Figure 5. The spectra of ions of plutonium have been interpreted in relation to all of the ions of the bf elements (83). 

Cerous salts in general are colorless because Ce " has no absorption bands in the visible. Trivalent cerium, however, is one of the few lanthanide ions in which parity-allowed transitions between 4f and Sd configurations can take place and as a result Ce(III) compounds absorb in the ultraviolet region just outside the visible. 

In general the lanthanides, including cerium, have a low toxicity rating (17), especially when they are present in material having low aqueous solubiUty. When orally adrninistered poor absorption from the gastrointestinal tract tends to result in the lanthanides generally having Httle effect. The anion is often an important deterrninant in toxicity. 

The role of cerium in these lighting phosphors is not as the emitting atom but rather as the sensitizer. The initial step in the lighting process is the efficient absorption of the 254 nm emission Ce ", with broad absorption bands in the uv, is very suitable. This absorbed energy is then transferred to the sublattice within the crystalline phosphor eventually the activator ion is fed and emission results. Cerium, as a sensitizer ion, is compatible in crystal lattices with other lanthanide ions, such as Eu and Tb, the usual activator atoms. 

Electronic absorption spectra are produced when electromagnetic radiation promotes the ions from their ground state to excited states. For the lanthanides the most common of such transitions involve excited states which are either components of the ground term or else belong to excited terms which arise from the same 4f" configuration as the ground term. In either case the transitions therefore involve only a redistribution of electrons within the 4f orbitals (i.e. f—>f transitions) and so are orbitally forbidden just like d—>d transitions. In the case of the latter the rule is partially relaxed by a mechanism which depends on the effect of the crystal field in distorting the symmetry of the metal ion. However, it has already been pointed out that crystal field effects are very much smaller in the case of ions and they... 

Absorption spectra and structure of lanthanide coordination compounds in solution. K. B. Yatsi-mirskii and N. K. Davidenko, Coord. Chem. Rev., 1979, 27, 223-273 (255). 

In what follows we briefly review some of the previous attempts to analyze the available spectra of plutonium (6). In addition, we estimate energy level parameters that identify at least the gross features characteristic of the spectra of plutonium in various valence states in the lower energy range where in most cases, several isolated absorption bands can be discerned. The method used was based on our interpretation of trivalent actinide and lanthanide spectra, and the generalized model referred to earlier in the discussion of free-ion spectra. 

Let us now consider MMCT for the case in which the donating ion is a lanthanide ion with a partly filled 4/ shell M(/")M(d°)CT. The trivalent lanthanide ions with a low fourth ionization potential are Ce, Pr ", Tb ". Their optical absorption spectra show usually allowed 4f-5d transitions in the ultraviolet part of the spectrum [6, 35]. These are considered as MC transitions, although they will undoubtedly have a certain CT character due to the higher admixture of ligand orbitals into the d orbitals. In combination with M(d°) ions these M(/") ions show MMCT transitions. An early example has been given by Paul [36] for Ce(III)-Ti(IV) MMCT in borosilicate glasses. The absorption maximum was at about 30000 cm ... 

Gruehn et al. [42] have reported on the isomorphy between modifications of LaTa309 and CeTajOg. Both contain trivalent lanthanide ions. The former is white, the latter is orange (P-modification), brass (O-modification) or yellow (M-modification). These colors are due to Ce(III)-Ta(V) MMCT absorptions, the Ce(III) and tantalate chromophores absorbing only in the ultraviolet. No doubt there are many more examples of this type in the literature. 

In view of this the divalent lanthanide ions are expected to show intense optical absorption in the whole visible region in compounds containing M(d°) ions. Since these compounds are not easy to prepare, our earlier warning is relevant. Only results on well-prepared and characterized compounds may be... 

Local Structure of the Eu2+ Impurity. From the experimental perspective, the doping of lanthanide ions into solid state materials can be probed by different instrumental technics such as nuclear magnetic resonance (NMR),44 extended X-ray absorption fine structure (EXAFS),45,46 or electron paramagnetic resonance (EPR),47 which instead of giving a direct clue of the local geometry offers only data that can be corroborated to it. From the theoretical point of view,... 

Patients with end-stage renal disease hyperphosphatemia ineffectively filter excess phosphate that enters the body in the normal diet.278 Elevated phosphate produces the bone disorder renal osteodystrophy. Skeletal deformity may occur, possibly associated with cardiovascular disease. Calcium deposits may further build up around the body and in blood vessels creating further health risks. The use of lanthanum carbonate is being promoted as an alternative to aluminum-based therapies.279,280 Systemic absorption, and cost have produced a clinical candidate, Fosrenol (AnorMED), an intriguing use of a lanthanide compound in therapy. 

The other notable feature of the optical spectroscopy of the lanthanide ions is that their absorption bands generally have very low extinction coefficients, because f-f transitions are... 

The sol-gel co-immobilization of a non-fluorescent blue indicator bromothymol blue (BTB) with an europium (Ill)-complex intense antenna mediated lanthanide dye represents a new scheme for the fluorescence analysis38. Luminescence spectra of europium (Ill)-complex shown in Figure 12 were found to be independent of pH changes in the range 1-10. Therefore, BTB, a non-fluorescent pH indicator with alkaline absorption maximum close to main europium emission band was added to the sol-gel mixture to shield reversibly the emission of the europium (Ill)-complex at different pH s without quenching of the antenna function. 

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