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Absorption Spectra of the Europium Ion and Its Complexes 109
Fig. 19. Observed energy levels of singly ionized europium
= 1725 cm-1 and found good agreement between the calculated and observed values of the energy levels (Table 40).
Table 40. The observed and calculated energy levels for the second spectrum of europium (Eu II)
The calculation has confirmed the above statement on the groupings of energy levels derived from simple considerations.
The oscillator and 1 nv strength of the 4/7(8$7/2)6£>4/7(8$7/2) 60 transition in Eu II have been critically investigated [488,4X9]. The reported ionization potential is 11.21V.
The spectrum of doubly ionized europium (Eu2+, Eu III). — The complete data on the thim spectrum of europium is not yet available except for a short list of a few strong lined by Kmc . These lines occur at 39648.4, 39780.6, 40882.4, 40910.0, 41064.6, 42097.1 and 42543.6 cm *.
Crystal Spectra and the Crystal Field Effect
Until now we have been considering the free ions and may have been a little depressed by the lack of a fair amount of experimental data.
Spectroscopic Properties of Europium
However, such is not the case here, and, in fact, we will soon appreciate the amount of extensive studies already made on the absorption and fluorescence spectra of the rare earth ions, both in crystalline matrices and in solution.
Of the various crystal matrices, the cubic calcium fluoride (CaF2), the lanthanum trihalides (LaF3, LaCl3, LaBra) and ethylsulphate are popular host lattices. The spectra of rare earths with partly filled /-shells in doped crystals consist of very sharp lines, similar to those in atomic spectra, of closely spaced groups. Fig. 20 gives a summary of the crystal
3F3 —SD3 10.2
9 f-L ^ %7/2-*Gs
yy/Z -— -
_3h—=. *G7,Z 1£l
fin - 63/f7
5Ds - Sfr 2H9J2
— 3kJB.A—5/2 G*
2=3 <■—nt —
%2 Ce f1
_ ___—№\=z! _ L_ 1_
3% %/2 SIv 6^5/2 7F() 8S7/2 7fß RU
Pr Nd Pm
fZ f3 fit f5
%/3 7Fo 8S7/2 7fr6 eHl5/2 s^8 ¥Ij5/2 3H& ZF7/2
Sm Eu Gd Tb Dy Ho Er Tm Yb
f5 fB f7 f8 f9 flO f77 f12 f13
Fig. 20. Observed energy levels of the tripositive rare earth ions in doped
Absorption Spectra of the Europium Ion and Its Complexes 111
spectra of tripositive rare earth ions in a LaCl3 matrix. It will be seen that the 4/14-n configurations repeat the spectra of their conjugate 4fn with inverted multiplets. The sharp lines in the crystal spectra of the tripositive rare earths are due to the normally forbidden intra /—/ transitions, but may occur due to several factors among which asymmetry, vibronic interactions, electric quadrupole and magnetic dipole are important.
It is possible to distinguish the electric dipole and electric quadrupole transitions from the magnetic dipole ones using the following selection rules (for a detailed treatment of the selection rules see ref. 556—560).
For electric dipole:
AJ = O, ± 1 (not O -► O)
and if LS coupling holds:
AS = 0
AL = 0, ± 1 (not O -► O)
For electric quadrupole:
AJ < 2 (not 0 -*■ 0,1)
AL ^ 2 (not 0 -*■ 0,1)
For magnetic dipole:
AJ < 1 (not 0-^0)
AL < 1 (not 0 -► 0)
The only positively identified magnetic dipole transition at room temperature so far is the 7Fo -*■ transition in Eu8+ at ~ 5250 A. The abscence of centrosymmetry in crystal allows the changes in J and L upto seven units due to admixture of states (through sixth order terms) giving rise to weak electric dipole transition. The weak intensities of the intra f—f transition can be accounted for by the Laporte selection rule.
The onset of a 4/ 5d transition for the trivalent rare earth ions usually takes place in the vacuum ultraviolet. In the case of Ce3+, Eu3+ and Tb3+ aquo ions the / -► d transition occurs within the measurable range of most modem spectrophotometers (Fig. 20).
For the divalent rare earth ions the situation is slightly complicated, and they may be categorized into three classes according to the nature of their ground states.
(i) ions having the fn ground state
(ii) ions having ground states derived from e.g. Gd2+
(iii) ions for which the fn~H state lies very close to /»; e.g. Ce2+ and Tb2+
The f d transition in divalent rare earth ions occurs quite early and t.hi« broad intense / -*d band almost invariably blots out the weak f—f lines.
Spectroscopic Properties of Europium