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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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Sinha [380] whilst examining the terpyridyl complexes of the rare earths found only a very slight increase of the s value for the 5L7 band (e 3) compared to that in the aquo ion (e = 2.36). The 5L? transition in the Eu(Terp)Cl3 complex occurs at 25390 cm-1 as a small sharp peak whilst that for the Eu(Terp)2d3 complex is split into two components occuring at 25410 and 25340 cm-1 respectively. The broad electron transfer bands are superposed on the bIn transition and occur at 24460 and 24350 cm-1 for Eu(Terp)Cl3 and Eu(Terp)2Cl3 complexes respectively.
Crystal and Solution Spectra of the Eu2+ Ion
The spectrum of the Eu2+ (4/7) ion both in aqueous solution [593] and in the solid state [594—598] has been studied by a number of people. In addition to the usual very strong, broad fn transitions, weak,
sharp lines are also observed in the ultraviolet region. These weak lines may possibly be due to the transitions from 8jS7/2 to the 6P, 67 and 6Z) terms of the p configuration. The Faraday rotation measurements [549] on Eu2+ in the cubic fluoride lattice showed that the upper level contained some Pj character.
Two strong absorption bands between 45000 and 25000 cm-1 due to the fn -*-d transition are observed in crystals. The absorption maxima of Eu2+ in different host crystals are compared in Table 47 with those of the aquo ion. The lower wavenumber band (designated as band B) almost
Table 47. Wavenumber8 of the absorption maxima of the Eu2+ ion in different environments
Band Aquo ion CaF2 SrF2 BaF2 KC1 KBr
A 40320 45050 43480 42700 40130 40210
B 31200 27150 27870 28470 29150 29000
Av 9120 17900 15610 14130 10980 11210
invariably shows a “staircase” structure. One might imagine that these fn d bands would be strongly influenced by the change in crystal field which is apparent from Table 47. Considering them individually band A shows a red shift and band B a blue shift in changing the host lattice from CaF2 to SrF2 to BaF2.
126
Spectroscopic Properties of Europium
The splittings of the ground state, 8$7/2, of the Eu2+ ion 11 a cubic crystalline field of alkaline ea rth fluorides [600] and alkaline earth oxides [601] have been investigated. This state remains unaffected by a first order crystal field, and in fact the splitting can only arise from higher order perturbations (due to spin-spin, spin-orbit and others) involving excited states. Title [600] has shown that the splitting of the ground state of Eu2+ varies linearly with crystal field in changing host lattice from CaF2 to SrF2 to BaF2. The nearest neighbour distances arc CaF2 = 2.361 A; SrF2 = 2.54 A and BaF2 = 2.663 A. Similar behav dut has also been observed for Eu2+ doped alkaline earth oxides [601].
Paramagnetic reasonance studies on Eu2 f doped CdS [602], CdSe
[603] and CdTe [603] show that the zero field splitting of the ground state is indeed very small (—0.5 cm-1). The separation between the ®jP7/2 and ®/S»7/2 states n CdSe and CdTe is of the order of ~32000 cm-1 (4eV) [603].
Part II
The Luminescence Spectra of the Europium Ion in Different Environments
Of the rare earths, trivalent Sm, Eu, Tb and Dy usually show strong v :si )le fluorescence either in their * ;rystals or in solution. Although these ions constitute the middle of the rare earth series, the fluorescence is by no means restricted to them, and, in fact, fluorescence has now been observed for the whole rare earth series in doped anhydrous LaCk crystals. Fluorescence spectra are especially valuable for obtaining information about the different levels of the lowest multiplet of the ion, as at low temperature discernible fine structure is often observed.
There has been a great stimulation of interest on the fluorescence properties of the Eu3+ ion because of its interesting use in Laser studies (see next chapter). The bright red fluorescence lines of Eu3+ at 17250 and 19020 cm-1 were earlier assigned by Sayre and Freed [564] to the transitions from the 5Z)o and 5Di excited levels to the 7Fq ground level. However, fluorescence transitions from the 5Do and 5Di levels to all the multiplets of 1F have been observed. Although the relative positions of these lines vary only slightly with the changes of environment (i.e. host lattice or ligand), drastic changes in fluorescence efficiency and lifetime have been observed due to environmental effects. As the f—f transitions are Laporte forbidden ones, greater asymmetry in the environment will undoubtedly enhance the fluorescence intensity. There are several factors which contribute to the mechanism of fluorescence emission, and one
The Luminescence Spectra of the Europium Ion
127
would imagine them to vary with environmental changes. For convenience we shall discus*, the fluorescence properties of europi urn under the following three headings a) in solution, b) in inorganic compounds and c) in complexes.
a) Fluorescence spectra in solution. — Gallagher [604] has recently studied in detail the absorption and fluorescence spectra of the Eu3+ ion in aqueous solution with various anions, such as, perchlorate, chloride, L trate, EDTA, DTPA. The fluorescence spectra of perchlorate solutions were found to be similar to those of the chlorides. However, the ®Do 7F2 transition is more intense in the case of the nitrate solutions, and strong enhancement of this fluorescent transition i noticeable on increasing the nitrate ’on concentration. Incidentally, nitrate ion forms stronger complex than chloride or perchlorate, and there arc infrared evidences [297a, 367J for covalently bonded nitrate groups in the solid state. An increase of about ten fold in ■ntensity of the transition is obtained
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