Models lanthanide spectra

Magnetic and spectroscopic properties of free atoms depend on the interplay of the interactions Hi and H2, since they determine the magnetic moment and the energy spectrum of the atom. Models of this interplay (coupling models) are assumed for lanthanide and d-transition elements. We shall examine in a simple way possible couplings, and point out the difficult case of actinide atoms. 

Crystal field theory is one of several chemical bonding models and one that is applicable solely to the transition metal and lanthanide elements. The theory, which utilizes thermodynamic data obtained from absorption bands in the visible and near-infrared regions of the electromagnetic spectrum, has met with widespread applications and successful interpretations of diverse physical and chemical properties of elements of the first transition series. These elements comprise scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel and copper. The position of the first transition series in the periodic table is shown in fig. 1.1. Transition elements constitute almost forty weight per cent, or eighteen atom per cent, of the Earth (Appendix 1) and occur in most minerals in the Crust, Mantle and Core. As a result, there are many aspects of transition metal geochemistry that are amenable to interpretation by crystal field theory. 

The superposition model has also been applied to experimental crystal field parameters obtained for lanthanides [31] substituted into host lattices of oxides, zircons, anhydrous trihalides, oxysulphides, alkaline earth fluorides and some other cubic crystals. The intrinsic parameters obtained from the analysis are given in Table 8.23. The solution spectrum of Er3+ aquo ion is given in Fig. 8.29. 

The shifts and line broadenings induced by various lanthanide shift reagents in the proton spectrum of pinacoline have been described. A method of analysis which combined a two-site model for co-ordination of the lanthanide to the carbonyl group with several methods for averaging over internal rotations of the methyl and t-butyl... 

In principle, all possible excitations contribute to the photoelectron spectrum and the proper quantum mechanical amplitude must be calculated. For the lanthanides, the atomic limit corresponds to the assumption that the photoelectron spectrum is dominated by those processes, where the photon hits a particular ion and causes an excitation on that ion without disturbing the remainder of the crystal. In the standard model, the lanthanide ion would initially be in its bivalent / configuration with the Hund s rule ground state multiplet (Table 1 in Section 2.2), and would be transferred into some multiplet within configuration... 

Since only information on the energy spectrum and wave functions of the ground multiplets is necessary for interpretation and prediction of magnetic properties of lanthanide compounds, and bearing in mind an essential role of electron-phonon interaction, we shall confine ourselves in this case to semiphenomenological models of the crystal field which allow one to represent parameters Bpq as definite functions of structural parameters of the crystal lattice. All the models developed until recently are... 

The one-electron theory, discussed previously, explains satisfactorily the features observed in the spectra of simple and 3d transition metals. The theory is valid for systems having a continuous DOS above Ep. Discrepancies between theory and experiment were observed for lanthanides. The breakdown of the one-electron model occurs because the excited core electron and/or projectile electron may occupy 4f orbitals, which are quite localized about the excited ion. Wendin (1974) has made an attempt to explain the spectral features on the basis of a two densities of states model one for the scattered projectile electron and the other for the excited ion with an electron-hole pair. This model is able to explain some of the spectral features. More theoretical work, taking into account the core-level widths, core-hole lifetime broadening, many-body and other effects contributing to the spectrum, is needed to provide a more plausible explanation for the APS spectra. 

There are several recent experimental studies on the CeO diatomic molecule. Schall et al. (1986) have studied CeO using the sub-doppler Zeeman spectroscopy. Again, the ligand-field model is so successful in explaining the observed spectra due to the ionic nature of the diatomic lanthanide oxide. Linton et al. (1979, 1981, I983a,b) as well as Linton and Dulick (1981) have studied the electronic spectrum of CeO using absorption, emission as well as laser spectroscopic method. There are many 0-0 bands for... 

Spectral bands of an aquated lanthanide ion arising from vibronic contributions were reported first by Haas and Stein (1971) in their study of the emission spectrum of aquated Gd. These bands are termed vibronic because they arise from a simultaneous change in the electronic state of the metal ion and the vibrational state of a coordinated ligand. Stavola et al. (1981) noted additional examples of such bands and presented a theoretical model based on the importance of electronic factors for calculating the intensities of lanthanide-ion vibronic transitions. Their theoretical model also predicts selection rules for such transitions. The intensities of observed bands assigned by these workers as being vibronic typically were at least 50 times weaker than the parent purely electronic band. Faulkner and Richardson (1979) have... 

Again, it has been X-ray diffraction data on crystalline solid salts that has provided models for the species present in solution. Nd + is a lanthanide(III) ion whose 4f-4f absorption spectrum has spectral features sensitive to changes in coordination environment ( hypersensitivity ), and this was applied to studying the Nd (aq) ion (1968). In the hypersensitive region of the spectrum around 8000 A, the spectra of solid [Nd(H20)9] (Br03)3 and of dilute aqueous solutions of Nd + are very similar the spectrum of Nd + in concentrated HCl solution is quite different, resembling the... 

The L absorption spectra in lanthanide vapors may be interpreted through this type of approximation. Figure 8 models an atomic Lm absorption spectrum through a superposition of a lifetime broadened distribution of atomic oscillator strengths and an arctan shaped continous absorption. Due to the relatively large core hole... 

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