4f—>5d Transitions

The divalent rare-earth ion Eu has the 4f electronic configuration at the ground states and the 4f 5d electronic configuration at the excited states. The broadband absorption and luminescence of Eu are due to 4f - 4 f 5d transitions. The emission of Eu is very strongly dependent on the host lattice. It can vary from the ultraviolet to the red region of the electromagnetic spectrum. Furthermore, the 4f-5d transition of Eu decays relatively fast, less than a few microseconds [33]. 

Let us now consider MMCT for the case in which the donating ion is a lanthanide ion with a partly filled 4/ shell M(/")M(d°)CT. The trivalent lanthanide ions with a low fourth ionization potential are Ce, Pr ", Tb ". Their optical absorption spectra show usually allowed 4f-5d transitions in the ultraviolet part of the spectrum [6, 35]. These are considered as MC transitions, although they will undoubtedly have a certain CT character due to the higher admixture of ligand orbitals into the d orbitals. In combination with M(d°) ions these M(/") ions show MMCT transitions. An early example has been given by Paul [36] for Ce(III)-Ti(IV) MMCT in borosilicate glasses. The absorption maximum was at about 30000 cm ... 

Divalent rare earth ions also have an outer electronic configuration of 4f"( including one more electron than for the equivalent trivalent rare earth). However, unlike that of (RE) + ions, the 4f " 5d excited configuration of divalent rare earth ions is not far from the 4f" fundamental configuration. As a result, 4f" 4f " 5d transitions can possibly occur in the optical range for divalent rare earth ions. They lead to intense (parity-allowed transitions) and broad absorption and emission bands. 

Jorgensen (7) has been the first who assigned the broad and strong absorption bands in the spectra of the trivalent lanthanides to either charge-transfer or 4f-> 5d transitions. Nugent and co-workers (2) have reported transitions of this kind for the actinides. In Table 1 we give some illustrative examples. 

As a general rule the c.t. bands shift to lower energies with increasing oxidation state, whereas Rydberg transitions (such as 4f- -5d transitions) shift to higher energies. It may, therefore, be expected that the lowest absorption bands of the tetravalent lanthanide ions will be due c. t. transitions and those of the divalent lanthanide ions to Af- -5d transitions. 

After this discussion of c.t. transitions on Ln i+ ions we now turn to the divalent lanthanides ions. Here the first allowed transitions in the spectra are 4f- -5d transitions as expected. They have been studied in detail. We will here mention some relevant results. 

The 4f-> 5d transitions of nearly all Ln + ions have been observed in CaF2 (9). Luminescence from these transitions has been studied in detail for Eu2+ 10), Sm + (77) and Yh + (12). In good approximation these spectra can be ascribed as transitions between the 4/ ground state and the d crystal-field components of the state. The influence of the surroundings on these transi-... 

We now turn to the common valency of the lanthanides, viz. three. Here we find that depending on the number of / electrons in the ground state the first allowed transition may be either a c.t. transition or a 4f > 5d transition. The stability of the half-filled and completely-filled shells serves as a starting point to predict which of the two transitions is to be expected for a special case the c. t. transitions are at relatively low energy in the case of Eu3+(4/6) and Yb3+(4 f ), the 4f- -5d transitions are at relatively low energy in the case of Ce +(4/i), Pr +(4/2), Tb +(4/ ). Loh 18) has measured the lowest 4f- 5d transitions of all trivalent lanthanides in CaF2 (see Table 3). 

Together with Sm another group of lines is often detected with the main line at 685 nm, which also has a very long decay time of several ms (Fig. 4. lOd). It is very close to the known resonance line of Sm. Under low power UV lamp excitation, the luminescence of Sm in fluorite is known only at low temperatures, starting from approximately 77 K, and is composed of narrow /-/ transition lines and a broad band of 4f-5d transitions (Tarashchan 1978 Krasilschikova et al. 1986). Evidently, under strong laser excitation, luminescence of Sm + may be seen even at room temperature, where 4f-5d luminescence is usually quenched because of radiationless transition. 

Other cases of approximately monatomic chromophores occur in 4f-+5d transitions now known in Sm11, Eu11, Tm11,28 Ybn, Cera, Prm, and Tbra.16 (The half-filled shell effect expressed by Eq. (3) is very conspicuous in this distribution of known species.) 5transitions are known in UIU, Np111, Puin, Paiy, U, Np, and Pu17. 5s-+5p transitions are known in complexes of Snn and Sbm and 6s — 6p in Tl1, Pbn, and Bira. The halide ions in solvents of not too high electron affinity and in crystals of alkali metal halides show absorption bands which to a certain approximation can be described as 3p - 4s(Cl), 4p — -5s(Br), and 5p - -6s(I). 

In the case of photolysis of acidic aqueous solutions of Eu2+ ion at 366 nm, the excited state results from 4f —> 5d transitions localized on the metal centre. These excited states have also considerable MLCT character because of strong mixing of metal 5d orbitals with ligand orbitals. In the case of the reaction of Eu3+ with H2 which occurs on photolysis at 254 nm, the photo reaction is due to the formation of an LMCT excited state. This process has been successfully used in the photochemical separation of Eu from other members of the lanthanides because Eu2+ is the only member of the lanthanide series which is at suitably low energy that an LMCT state is accessible [98]. Yb3+ and Sm3+ ions behave in a similar fashion to Eu3+ as far as their photochemical behaviour is concerned. Aqueous solutions of Sm3+ or Yb3+ containing 2-propanol on photolysis at 185 nm give hydrogen and acetone as products probably by a mechanism similar to Eu3+ ion. 

In solutions, the energy of the 4f—> 5d transitions is lowered by about 15000 cm-1 as compared to gaseous ions and the corresponding parity allowed and relatively intense bands are observed in the ultraviolet region in Ce3+, Pr3+ and Tb3+ ions. As shown in Fig. 8.14, the electronic absorption spectra of the majority of the trivalent lanthanides in the UV-vis spectral range involves only f-f transitions. 

The Ce3+ ion has characteristic absorption in the near UV region due to 4f-5d transitions. In solution broad intense bands with half-width of 1000-2000 cm-1 and extinction coefficient of the order of 102—103 1 mol cm-1 are observed due to 4f-5d transitions. 

Rare earth phosphors based lamps have efficiency of 100 lm/W and CRI of >80. BaMgAl 10O17 doped with Eu2+ (BAM) is used as blue emitting phosphor in fluorescent lamps. Photons are absorbed by Eu2+ ions and causes 4f-5d transition. Emission from Eu2+ occurs. 

In the last few years, only a few studies have been devoted to the optical properties of Sm3+ ions in fluoride glasses [138,139], The main result concerns the excitation of the 4G5/2 level by a three-fold up-conversion process including a direct two-photon absorption mechanism. Izumitani et al. have succeeded in stabilizing divalent samarium in fluoroaluminate and fluorohafnate glasses [140], Absorption spectra reveal a strong 4f - 5d transition whose maximum is located around 320 nm. Emission of Sm2+ ions in fluoroaluminate glass occurs in the red, between 680 nm and 810 nm, from the 5D0 excited state to the 7Fj (J = 0, 1, 2, 3, 4) levels. 

Not all the lanthanide ions give rise to f-f transitions, including obviously the f and f species, La + and Lu +. Likewise there are no f-f transitions for the f (Ce +) and f(Yb +) ions, as with only a single L-value there is no upper 4f state. Transitions between Fs/2 and Fv/2 are seen in the case of Ce + as a rather broad band in the infrared region around 2000 cm Ce + and Yb +do, however, give rise to broad 4f" 4f" 5d transitions (as indeed do many lanthanides). Even an ion like Eu +, which has several absorptions in the visible region of the spectrum, has only weak absorptions, so many of its compounds appear colourless the only three tripositive ions whose compounds are invariably coloured are Pr +, Nd +, and Er +. 

Kazuyoshi Ogasawara, Shinta Watanabe, Hiroaki Toyoshima and Mikhail G. Brik, First-principles calculations o/4f 4f 5d transition spectra 1... 

The energy of 4f 5d transitions is much lower for divalent ions (Figure 10). For the most stable ions, Eu + (4f ) and Yb + (4f " ), the corresponding absorption bands in oxide or fluoride hosts most often he in the UV range. 

Alkaline-earth fluorides have been the principal hosts for divalent lanthanide lasers. These are relatively soft, optically isotropic materials. Lanthanides enter the alkaline earth sites substitutionally without charge compensation. Because these sites have inversion symmetry, only magnetic-dipole or vibronic transitions are allowed between 4f states. These are weak and the resulting radiative lifetimes are long. In comparison, the radiative lifetimes of 5d->4f transitions, which are parity allowed, are-short. The 4f->5d transitions are broad and thus provide good absorption bands for optical pumping. 

Brik, First-principles calculations of 41" 4f" 5d transition spectra 1... 

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