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Europium - Sinha S.P.

Sinha S.P. Europium - Springer-Verlag, 1967. - 88 p.
Download (direct link): europium1967.djvu
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Experimental evidence for charge transfer phenomena between di and trivalent rare earth ions in a CaF2 matrix has been presented by Welber [620]. Reversible phototransfer of electrons in the CaF2: Sm3+: Eu2+ system was achieved [620] at 4°K, whereas the system CaF2:Tm3+:Eu2+ failed in this respect.
Chang [621] studied the fluorescence characteristies Eu3+ doped Y2O3 crystals and claimed to have observed transitions originating from the 5Z>2 level of Eu3+ in addition to the prominent 5D0 -*-7F2 transition (r = 870 pLsec) at 6115 A (16350 cm-1). Incorporation of other rare earth ions as codopents in Y2O3: Eu3+ crystals produced [622] pronounced effects on the fluorescence intensity and decay time of the 5Do -*■ 7F2 transition.
The fluorescence spectra of several mixed oxide systems have been extensively studied by Bril and Wanmaker [623], who reported the following decay times (in milliseconds).
(МгОз: Eu, 8 msec; (МгОз — B2O3 : Eu, 2.0 msec; а20зВ20з: Eu, 1.6 msec; EuPC>4, 0.6 msec.
The fluorescence spectra of Sm3+, Eu3+, Tb3+ and Dy3+ in a MgO matrix has also received attention [624]. In thorium oxide matrices, samples with low Eus+ concentration (Tho.ee Euo.14 О1.93) show [579] strong fluorescence whereas concentration quenching is observed in the sample Tho.5 Euo.s 01.75-
Several studies on absorption and fluorescence properties of Eu3+ in silicate [625] and borate [625] glasses have appeared.
A strong blue fluorescence at 4200 A is observed [627] for Eu2+ doped CaF2 or NaCl. The luminescence decay of Eu2+ doped CaF2 is reported
9 Sinha, Ешорішп
130 Absorption Spectra of the Europium Ion and Its Complexes
by Tolstoi [628] to be about 0.63 [xsec in the temperature range —183° to 60° C.
c) Fluorescence spectra of complexes in the solid state and in solution. — The fluorescence properties of rare earth complexes with oiganic ligands are very attractive. The luminescence of a particular metal complex may result from one of the following effects [629].
a) Emission resulting from the ligand part of the molecule, due to perturbation by the metal ion.
b) Emission occuring from the metal ion perturbed by the ligand.
c) Emission due to intramolecular energy transfer (IMET) from the excited ligand to the metal ion resulting in a radiative transition within the metal ion levels.
In most of the europium complexes the fluorescence is due to the last effect. The intramolecular energy transfer within europium and terbium complexes was first observed by Weissman [630] in 1942, and subsequently investigated by many other workers.
The following steps sum up the basic mechanism of the IMET process as involved in rare earth complexes.
Step I (Sfo -* SiiK): Absorption of the ultraviolet radiation by the organic moiety of the complex excites the singlet ($lig) state to the first excited singlet state (>S£g). The absorbed energy can dissipate either by a radiative transition, $£g -> $Iig, giving rise to molecular fluorescence, or by radiationless energy migration from S^g to the triplet (T) state.
Step II (S£g T): This radiationless intersystem crossing of energy is the most important factor for the IMET mechanism and is markedly influenced by the nature of the ligand. Often ligand substitution produces changes in the fluorescence yield. Deactivation of the triplet state energy may occur by a T -* S]lB transition giving molecular phosphorescence and competing favourably with the intramolecular energy transfer process. The triplet state is actually the donor (energy transferring) level.
Step III (T~*4:f): By transferring energy to the 4f acceptor level (resonance level) via a radiationless process the organic moiety (ligand) goes back to the ground state while indirectly exciting the coordinated rare earth ion. The efficiency of this process is mainly dependent on the nature of the T — 4/ overlap (talking very naively of course) or on the energetic locations of the T and 4/ states. Thus again the nature of the ligand will be significant in determining the efficiency of the energy transfer. The closer the donor triplet is to the 4/ acceptor level the better is the energy transfer.
Step IV (f -* f): At this final stage a radiative transition, if occur-
The Luminescence Spectra of the Europium Ion
131
ing, within the rare earth ion states gives rise to characteristic line emission.
At any step during the process, thermal dissipation of energy through vibrational coupling with the solvent or the matrix may bring deactivation of the next step and no / -► / fluorescence is then expected. The temperature dependence of the fluorescence yield and the lifetime of several (3-diketonates of Eu3+ have been studied by Bhaumik [631].
We have seen earlier that Eu3+ possess two resonance levels, 5Z>o and ®Di, from which fluorescence transitions to the J manifold of 7F takes place. The ®Do -► 1Fq transition is strictly forbidden for regular octahedral symmetry but is observed in some complexes due to the lack of centro-symmetry. The intensity of the fluorescence transition is not directly dependent only on the amount of T 4f energy transfer, but mainly on the transition probabilities from a particular resonance level to the various J manifolds. However, the transition probabilities are sensitive functions depending on the ligand. A schematic representation of the intramolecular energy transfer in a Eu3+ complex is given in Fig. 23.
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