The Rare Earth Ions

How Does a Luminesceni Material Absorblts Excitation Energy  

How is the parity selection rule relaxed Vibrations have only a very weak influence. For interesting consequences of this influence the reader is referred to Ref. [13]. Of more importance are the uneven components of the crystal-field which are present when the rare earth ion occupies a crystallographic site without inversion symmetry. These uneven components mix a small amount of opposite-parity wave functions (like 5d) into the 4/wavefunctions. In this way the intraconfigurational 4/° transitions obtain at least some intensity. Spectroscopists say it in the following way the (forbidden) 4/-4/transition steals some intensity from the (allowed) 4/-5rftransition. The literature contains many treatments of these rare earth spectra, some in a simple way, others in considerable detail [1,16,17,18,19]. 

If the absorption spectra of the rare earth ions are measured at high enough eneigy, allowed transitions are also observed as will be discussed now. 

4 The Rare Earth Ions 4f-5d and Charge-D-ansfer lyansitions) 

The allowed optical transitions of the rare earth ions mentioned above are intercon-figurational and eonsist of two different types, viz. 

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The calculation of the magnetic moments of the rare-earth ions by... 

Here L, S, and J are the quantum numbers corresponding to the total orbital angular momentum of the electrons, the total spin angular momentum, and the resultant of these two. Hund predicted values of L, S, and J for the normal states of the rare-earth ions from spectroscopic rules, and calculated -values for them which are in generally excellent agreement with the experimental data for both aqueous solutions and solid salts.39 In case that the interaction between L and S is small, so that the multiplet separation corresponding to various values of J is small compared with kT, Van Vleck s formula38... 

It is then shown that (excepting the rare-earth ions) the magnetic moment of a non-linear molecule or complex ion is determined by the number of unpaired electrons, being equal to ms = 2 /S(S + 1), in which 5 is half that number. This makes it possible to determine from magnetic data which eigenfunctions are involved in bond formation, and so to decide between electron-pair bonds and ionic or ion-dipole bonds for various complexes. It is found that the transition-group elements almost without exception form electron-pair bonds with CN, ionic bonds with F, and ion-dipole bonds with H2O with other groups the bond type varies. 

Although the role of rare earth ions on the surface of TiC>2 or close to them is important from the point of electron exchange, still more important is the number of f-electrons present in the valence shell of a particular rare earth. As in case of transition metal doped semiconductor catalysts, which produce n-type WO3 semiconductor [133] or p-type NiO semiconductor [134] catalysts and affect the overall kinetics of the reaction, the rare earth ions with just less than half filled (f5 6) shell produce p-type semiconductor catalysts and with slightly more than half filled electronic configuration (f8 10) would act as n-type of semiconductor catalyst. Since the half filled (f7) state is most stable, ions with f5 6 electrons would accept electrons from the surface of TiC>2 and get reduced and rare earth ions with f8-9 electrons would tend to lose electrons to go to stabler electronic configuration of f7. The tendency of rare earths with f1 3 electrons would be to lose electrons and thus behave as n-type of semiconductor catalyst to attain completely vacant f°- shell state [135]. The valence electrons of rare earths are rather embedded deep into their inner shells (n-2), hence not available easily for chemical reactions, but the cavitational energy of ultrasound activates them to participate in the chemical reactions, therefore some of the unknown oxidation states (as Dy+4) may also be seen [136,137]. 

The photoluminescence of these nanoparticles has very different causes, depending on the type of nanomaterial semiconductor QDs luminescence by recombination of excitons, rare-earth doped nanoparticles photoluminescence by atom orbital (AO) transitions within the rare-earth ions acting as luminescent centers, and metallic nanoparticles emit light by various mechanisms. Consequently, the optical properties of luminescent nanoparticles can be very different, depending on the material they consist of. 

The CTI band has nothing to do with the varying f-electron configuration of the rare earth ions. If this would be the case, the... 

If the rare-earth ion is immersed in a crystal field, the perfect symmetry of the free ion is destroyed, leaving parity in some cases not quite a good quantum number. Under this circumstance, electric-dipole transitions become quite possible. It was Van Vleck (25), in his classic 1937 paper The Puzzle of Rare Earth Spectra, who first pointed out that the weak electric dipole emission was due to this mixing of states of opposite parity by the crystal field. 

The authors made no attempt at quantitative emission-intensity measurements. They did make the qualitative observation, however, that the rare-earth-ion emissions from the D20 solvated crystals were much more intense than the emissions from the H20 solvated ones. 

Nardi and Yatsiv (141) studied the temperature dependence and the decay times of europium emissions in europium dibenzoylmethide. In this compound the ultraviolet radiation absorbed by the organic component is transferred to the rare-earth ion and fluorescence is emitted from two levels, namely, the SD0 and the 5DI. The compound was prepared by treating a solution of EuC13 in ethanol with a solution of dibenzoylmethane in ethanol. The compound was precipitated by the addition of piperidine. 

Rare earth ions, especially Eu+3, Sm+3, Gd+3, Tb+3, and. Dy+3 with electronic configuration (/s through / ), emit characteristic line spectra from their 4/energy states. The energies of the emitting levels are fairly low so that the rare earth ions can be vety useful species as acceptors to detect triplet states in solution by energy transfer. Such quenching processes... 

There are, however, many cases of interest in which we may want to determine the splitting of a state that is well characterized by its total angular momentum, J. This will in fact be the only thing of importance in the very heavy elements, for example, the rare earth ions, where states of particular L cannot be used since the various free-ion states of different J are already separated by much greater energies than the crystal field splitting energies. 

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