Rare Earth Energy Levels

Rare-earth chelates, energy transfer, 203 Rare-earth, energy levels of, 25 Recombination luminescence, 160 Redox potential excited state, 111 of PS I and PS II, 282 Redox reactions, 218... 

Figure 8 Splitting of rare earth energy levels owing to the interelectronic, spin-orbit, and crystal field interactions...

III. Rare Earth Energy Levels and Electronic States... 

Lanthanide luminescence apphcations have already reached industrial levels of consumption. Additionally, the strongly specific nature of the rare-earths energy emissions has also led to extensive work in several areas such as photostimulable phosphors, lasers (qv), dosimetry, and fluorescent immunoassay (qv) (33). 

Van Uitert and Iida (55) suggested the applicability of the phonon-assisted-transfer mechanism to rare earth-rare earth energy exchange. They were able to correlate the emission intensity of the 5D0 level of trivalent europium or the5 D4 level of trivalent terbium with the closest, but definitely lower-lying, level observed for a second rare-earth ion. 

For 2,7-dmnapy complexes of the same rare earth, the recorded intensities of the nitrate complexes are always greater than those in which acac is the anion. Conductivity measurements on nitromethane solutions of M(acac)3(2,7-dmnapy) give A values of 6-9 which are typical of nonelectrolytes (II, 12). IR spectra of the acac complexes have vCO absorptions at approximately 1600 and 915 cm" and i/M-0 bands at 405 and 313 cm" These absorptions and the lack of a 1700-cm band indicate that both oxygens of each of the acac units are coordinated to the metal (II, 30). The more intense fluorescence of the nitrate complex may result from the presence of a second 2,7-dmnapy ligand which would increase the coordination number of the rare earth from 8 to 10. The triplet state of acac is reported at 25,300 cm (31). The triplet state of napy at 22,210 cm" is closer to the rare earth resonance levels and may contribute to a more efficient energy transfer which in turn would enhance fluorescence intensity. 

Fig. 7. Energy levels of the most commonly used rare-earth activators.

Martin, W. Zalubas, R. Hagan L. Atomic Energy Levels, the Rare Earths National... 

Dieke, G.H. and Crosswhite, H. (1968) Crosswhite Spectra and Energy Levels of Rare Earth Ions in Crystals, Interscience Publishers, New York. 

Zalubas, W.C.M.R. and Hagan, L. (1978) Atomic Energy Levels - The Rare-Earth Elements, US Government Printing Office, Washington, DC. 

Weak crystalline field //cf //so, Hq. In this case, the energy levels of the free ion A are only slightly perturbed (shifted and split) by the crystalline field. The free ion wavefunctions are then used as basis functions to apply perturbation theory, //cf being the perturbation Hamiltonian over the / states (where S and L are the spin and orbital angular momenta and. 1 = L + S). This approach is generally applied to describe the energy levels of trivalent rare earth ions, since for these ions the 4f valence electrons are screened by the outer 5s 5p electrons. These electrons partially shield the crystalline field created by the B ions (see Section 6.2). 

In principle, all of the elements of the periodic table can be used to iucorporate foreign ions in crystals. Actually, only a number of elements have been used for optically active centres in crystals in other words, only a number of elements can be incorporated in ionic form and give rise to energy levels within the gap separated by optical energies. The most relevant centers for technological applications (although not the unique ones) are based on ions formed from the transition metal and rare earth series of the periodic table, so we will focus our attention on these centers. 

Figure 6.1 An energy-level diagram for trivalent lanthanide rare earth ions in lanthanum chloride (after Dieke, 1968).

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