Rare Earth Ions Line Emission

After these general considerations some special cases will be dealt with. 

This ion has a half-filled 4/shell which gives a veiy stable S7/2 ground state. The excited levels are at energies higher than 32000 cm. As a consequence the emission of Gd is In the ultraviolet spectral region. The 87/2 level (orbitally nondegenerate) cannot be split by the crystal field. This limits the low-temperature emission spectrum to one line, viz. from the lowest ciystal field level of the P /2 level to S7/2. However, usually the real spectrum consists of more than one line for several reasons. 

The Do- Fj emission is very suitable to survey the transition probabilities of the sharp spectral features of the rare earths. If a rare-earth ion occupies in the crystal lattice a site with inversion symmetry, optical transitions between levels of the 4/  

If there is no inversion symmetry at the site of the rare-earth ion, the uneven cry.stal field components can mix opposite-parity states into the 4/ -configurational levels (Sect. 2.3.3). The electric-dipole transitions are now no longer strictly forbidden and appear as (weak) lines in the spectra, the so-called forced electric-dipole transitions. Some transitions, viz. those with AJ = 0, 2, are hypersensitive to this effect. Even for small deviations from inversion symmetry, they appear dominantly in the spectrum. 

In NaGd02 Eu the Dd- I 2 emission transition dominates, but other lines are also present. The Eu case is so illustrative, because the theory of forced electric-dipole transitions [8] yields a selection rule in case the initial level has J - 0. Transitions to levels with uneven J are forbidden. Further J = O- J = 0is forbidden, because the total orbital momentum does not change. This restricts the spectrum to D()- Fi, present as magnetic-dipolc emission, but overruled by the forced electric-dipole emission, 

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There are a few green Tb -based phosphors, suitable for application in fluorescent lamps. Despite intensive research, no substitute for Y203 Eu with the same spectral properties has been found, leaving it the only red primary with line emission at about 611 nm. The width and position of the emission bands originating from optical transitions within the f-electronic shell are almost independent of the chemical environment. The relative intensity of the separate bands, however, depends on the crystal lattice. The transitions on many rare-earth ions are spin- and parity-forbidden and therefore rather slow (in the ms range). However, for a number of rare-earth ions, broad emission bands are also known, due to d f emission, e.g. 

Once in an excited state, the system will relax towards the equilibrium state (of the excited level) by dissipating heat. From this state or nearby levels the system returns to the ground state, thereby emitting radiation. The emission too, therefore, consists of a broad band. Line emission is found in the case of small Ar as for example in the case of rare earth ions. The emission generally lies at a lower energy than the absorption. This displacement of emission with respect to absorption is known as the Stokes shift. 

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). 

Emission of rare earth ions is due to the optical transition involving f levels (Tb3+ 4f8 Gd3+ 4f7 Eu3+ 4f7). The f electrons are well shielded from the chemical environment and f-f emission spectra consist of sharp lines. These optical transitions are generally slow within a time scale of microseconds to milliseconds because the f-f transitions are partially forbidden along with spin forbiddenness of many transitions. 

The unique properties of rare earth ions are (i) the spectral positions of the emission lines are independent of the host lattice, (ii) some of the ions, Tb3+, Eu3+ emit at spectral positions, enabling high lumen efficacies along with a very good quality of white light. 

Figure 4 shows three emission spectra that are representative of the three cases. Characteristic examples of case (a) are the trivalent rare earth ions. The value of S is so small for these ions that the spectra consist in good approximation of the zero-vibrational transitions only. Figure 4a gives as an example the emission spectrum of the Gd " ion in LaBaOe. It consists of one strong electronic line at about 310 nm,... 

This is due to the fact that the emission of the tantalate group in YTa04 is at such hi energies (viz. 30000 cm" )> the spectral overlap is no longer with the forbidden narrow absorption lines of the rare earth ions, but with allowed, broad bands. The critical interaction distance, Rq, has been estimated to be 10 A in the case of tantalate to terbium transfer. 

Rare-earth ion emission is not necessarily sharp line emission, as we will see now. 

At first sight, cncigy transfer between identical rare earth ions seems to be a process with a low rate, because their interaction will be weak in view of the well-shielded character of the 4/electrons. However, although the radiative rates are small, the spectral overlap can be large. This originates from the fact that AR 0, so that the absorption and emission lines will coincide. Further the transfer rate will easily surpa.ss the radiative rate, since the latter is low. In fact energy migration has been... 

The spectroscopic method is based on pressure-induced changes in absorption or/and emission spectra. The idea is to relate the pressme-induced shift of the fluorescence lines of the specific material to the value of the pressure. The material selected for the luminescence pressure sensor should be characterized by strong intensity of the emission line(s), which should be stable at a broad range of pressures and temperatures and the energy of which is possibly related linearly to pressure. It is also important that the emission of the sensor does not overlap the emission of the sample. Considering the above-mentioned requirements, the Raman fluorescence and photoluminescence of transition-metal and rare-earth ions were used. Raman modes of nitrogen [49], which is the pressure-transmitting medium, and Raman frequencies of diamond chips [51] have been used. Recently, a pressure-induced shift of the Raman line 1332 cm of the face of the DAC culet was proposed to estimate pressure < 1,000 kbar [50, 52]. 

In this part of the chapter we have looked at the energy diagram and associated optical transitions of a number of rare-earth ions. These diagrams and transitions are nowadays well known. The influence of the crystal lattice on the situation and intensity of absorption and emission bands or lines can also be well understood. In the next part we consider the efficiency of the luminescence. 

If we consider the optical spectrum of a rare-earth ion, either in absorption or in emission, we are immediately struck by a large number of sharp lines. Their position seems to be independent of the surroundings. Their intensity ratios vary strongly, indicating certain selection rules, which are reflected by the branching ratio defined as the ratio of a specific radiative transition from a given level divided by the sum of all the radiative transitions from this level. The branching ratio is defined as )3y = (the ratio between the transition probability A... 

The previously discussed radiative transition probabilities and line strengths are connected to the peak-stimulated emission cross sections of the rare-earth ion in a glass by... 

The emission spectra of the europium(III) ion in Cs2NaEuCl6 and in Cs2Na(EUxYi-x)Cl6 (x = 0.01 and 0.1) have recently been reported by Serra and Thompson (1976). In this compound the rare earth ion is in a site of perfect octahedral symmetry and as a result of the inversion symmetry only magnetic dipole transitions are allowed. The usually more intense electric dipole transitions are replaced by weak vibronic transitions. In the doped compound all possible magnetic dipole,transitions between the levels (J = 0, 1, 2, 3) and the Fj levels (J = 0,1,2,3,4) have been observed. Weak additional lines on the magnetic dipole transitions and the weak vibronic lines have been correlated with the various vibrational modes of the compound. The emission spectra complement the absorption spectra determined earlier by Schwartz (1975) and illustrate how the two types of spectra can be used to obtain a more complete picture of the energy levels. 

Most of the solid-state lasers employ as active material crystals or glasses doped with rare-earth or actinide ions, because these ions exhibit a large number of relatively sharp fluorescent lines, covering the whole visible and near-infrared spectrum 380) search for new laser materials and investigations of the characteristics of laser emission at different temperatures of the active material and with various pump sources have improved knowledge about the solid state spectra and radiationless transitions in laser media 38i). 

As stated in an earlier paragraph, the sharp emission and absorption lines observed in the trivalent rare earths correspond to/->/transitions, that is, between free ion states of the same parity. Since the electric-dipole operator has odd parity,/->/matrix elements of it are identically zero in the free ion. On the other hand, however, because the magnetic-dipole operator has even parity, its matrix elements may connect states of the same parity. It is also easily shown that electric quadrupole, and other higher multipole transitions are possible. 

Intramolecular transfers between two ehromphores separated by insulating groups can lead to absorption by one chromophore and emission from the other. Complexes of rare earths specially of Eu+3, Sm+3. Gd+3 and Dy+3 emit line spectrum characteristic of the central metal ion when absorption takes place in the ligand moeity. 

Phosphors doped with rare-earth elements show two types of CTL spectrum, namely emission from the excited species and recombination radiation, simultaneously. Figure 21 shows the CTL spectrum from the TL-phosphor BaS04 Eu in air containing ethanol vapor. The emission band with fine spectrum components at 420 nm is attributed to the excited HCHO. The line spectrum components peaking at 580 and 615 nm are attributed to the electronic transitions within Eu3+ ions. 

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