Electronic spectra of the lanthanides

Because the / orbitals are so well shielded from the surroundings of the ions, the various states arising from the / configurations are split by external fields only to the extent of 100 cm 1. Thus when electronic transitions, called/—/transitions, occur from one / state of an / configuration to another J state of this configuration the absorption bands are extremely sharp. They are similar to those for free atoms and are quite unlike the broad bands observed for the d—d transitions. Virtually all the absorption bands found in the visible and near-uv spectra of the lanthanide +3 ions have this linelike character. The intensities of the/—/bands show measurable sensitivity to the nature of the coordination sphere but the relationship is complex and not quantitatively understood. 

We will discuss applications of the APS technique to simple as well as multicomponent systems. The results of the applications cited are compared with those from other techniques wherever available. The derivation of the DOS directly from the APS spectrum of 3d transition metals has been dealt with by various authors (Dose et al. 1981). The one-electron theory explains satisfactorily the 3d transition metal spectra, but fails when applied to the lanthanide metals. The electronic structures of the lanthanide metals and their intermetallics as obtained from APS spectra are also discussed. 

Why do the electronic absorption spectra of the lanthanide ions show sharp bands differing from the broad bands in the spectra of the 3d transition elements ... 

Fig. 11. The 4d->4f core electron energy loss spectra of the lanthanides. Spectra are shown for two primary energies E. The theoretical relative intensities of electric dipole transitions of Sugar (1972a) are shown where available (Strasser et al. 1985a).

The actual absorption bands that occur in the spectra of the lanthanides and actinides and which are associated with electronic transitions have been divided into three types. 

The X-ray spectra of the rare earths are similar to those of all other elements. The spectra consist of only a few lines and absorption edges that result from electronic transitions among the inner-most shells of the atonl. Increases in atomic number cause a simple shift of the X-ray spectrum to higher energies or to lower wavelengths. Consequently, the X-ray spectrum is unique and characteristic for each element. With exciting radiation obtainable from commercially available 50- or 60-kilovolt X-ray tubes, the L series spectra of the lanthanides are readily excited. To excite the K spectra with adequate intensity, 100-kilovolt power supplies are required. Lines from the K series are used for the determination of Sc and Y. For lanthanide analyses, nearly all analysts recommend lines in the L series. 

The atomic spectra of the actinides are very complex and it is difficult to identify levels in terms of quantum numbers and configurations (6). The chemical behaviour of the elements is dictated by the configurations of the electrons around the nucleus and in the case of the actinides it is the competition between the 5/ 1 7 s2 and the 5 /n 1 6 d 7 s2 levels that dictates these chemical properties. A comparison of the /-energy levels of the lanthanides and the actinides shows that less energy is required for the promotion of the 5 / -> 6 d levels than for the 4/ -> 5 d levels in the lanthanides. As a result of this lower energy requirement by the actinides they have the tendency to display higher valences since the bonding electrons are more readily available. It is only at the commencement of the second half of the actinides that there is commencement of properties which echo those of the lanthanides. 

The electronic absorption spectra of the trivalent lanthanides are characterized by a number of sharp bands with low intensities (e < 10). These bands are assigned to transitions between the levels of the 4/ configuration. The theories of these transitions have been discussed by various authors (290-297). 

It was not until quite recently that the presence of octahedral lanthanide(III) complexes with CN = 6 has been demonstrated. In 1966 Ryan and Jergensen were able to prepare the anionichexahalide complexes of the lanthanides, [MXe] " in non-aqueous (nitriles) solvents [33, 34). Complexes of the type [(C6H5)3PH]3 [MXe] and [C5H5NH]3 [MXe], where M =Pr, Nd, Sm, Eu, Dy, Er, Yb and X- = Cl, Br were obtained as solids. The electronic (/- -/) transitions in these and in hexaiodide, [Mle] -, complexes are extremely weak, except for the magnetic dipole allowed transitions [35). Both electronic and electron transfer spectra (4f- 5d) indicated the presence of octahedral MX species. 

The heteroatoms listed in Table 10 form complexes of stoichiometry [XWi0O36]8-. The structure, based on determinations of the CeIV and UIV anions, can be regarded as the attachment of two quadridentate ligands, derived from W6Oi9 through the loss of one W06 octahedron, to the heteroatom (Figure 18). The site symmetry of the latter is approximately D4d. The XW10 anions are of only moderate stability in aqueous solution (pH5.5-8.5). Electronic, vibrational and 170 NMR spectra have been recorded. The emission spectra and luminescence properties of several of the lanthanide anions have been discussed.84... 

Comparison of electronic absorption spectra of the Ln-TTHA complexes in the solid state and in solution has shown that the monomeric species with Ln3+ coordination numbers 10 and 9 also occur in solution for the light and heavy lanthanides, respectively [39,41,43]. In addition, these studies suggest the presence of another species with one uncoordinated N-atom for the Nd3+ and Eu3+ systems. Absorption spectra [39,41,43],luminescence [45] and H Nuclear Magnetic Resonance Dispersion (NMRD) studies [46] have shown that oligomeric species also occur in solution, particularly below pH 5. 

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