4f transition

Rare earth ions (band emission) Ce + 5d-4f Transition is parity allowed and spin selection is not appropriate =y fully allowed (10-100 ns)... 

In contrast to the lanthanide 4f transition series, for which the normal oxidation state is +3 in aqueous solution and in solid compounds, the actinide elements up to, and including, americium exhibit oxidation states from +3 to +7 (Table 1), although the common oxidation state of americium and the following elements is +3, as in the lanthanides, apart from nobelium (Z = 102), for which the +2 state appears to be very stable with respect to oxidation in aqueous solution, presumably because of a high ionization potential for the 5/14 No2+ ion. Discussions of the thermodynamic factors responsible for the stability of the tripositive actinide ions with respect to oxidation or reduction are available.1,2... 

In order to analyze the 4f 4f transitions of Pr3"1" in YLF, we first consider only MOs with dominant Pr 4f character among the MOs obtained in section 4.2.4 for the PrFg- cluster. Therefore, the number of possible electron configurations (number of the Slater determinants) is 91 as shown in table 1. 

The correlation crystal-field extension to the Hamiltonian becomes more important when many high-energy states are observed. For the 4P 4f transitions, many new energy lev-... 

Ishizumi and Kanemitsu (2005) have studied PL properties of Eu3+ doped ZnO nanorods fabricated by a microemulsion method. The PL of bound exciton recombination and ZnO defects was observed near 370 and 650 nm under 325-nm light excitation, but no emission of Eu3+ occurred. On the other hand, the sharp PL peaks due to the intra-4f transitions of Eu3+ ions appeared under nonresonant excitation below the band-gap energy of ZnO (454 and 457.9 nm) in addition to direct excitation to 5D2 (465.8 nm). Therefore the authors concluded that the energy transfer occurs from the ZnO nanorods to Eu3+ ions through ZnO-defect states. This energy transfer mechanism seems very different from the previous one and more spectroscopic evidence is required to confirm it. 

When Co2+ ions occur in tetrahedral coordination, the ground-state becomes 4A2 4F), and spin-allowed transitions are to the 4T2(4F), 4TX(4F) and 47 (4P) states (fig. 3.12). Note that the 4A2 —> 4T2(4F) transition provides a direct measure of A,. When Co2+ ions are present in distorted tetrahedral sites, as in the cobaltian staurolite lusakite ( 4.4.3), each of the excited triplet crystal field states is resolved into several levels, so that numerous polarization-dependent absorption bands occur in the visible to near infra-red spectra (cf. fig. 4.7). 

The first known work on the intensities of the intra 4f—4f transitions is that of Van Vleck [53]. This was followed by the work of Jurbner and coworkers [54,55]. In 1962 Judd [56] and Ofelt [57] independently proposed the theory of forced electric-dipole transition which enabled the compilation of oscillator strengths for lanthanide aquo ions. 

In the absorption spectra of Ln3+ ions, the 4f-4f transitions are located in the near infrared, visible and near ultraviolet regions. Only in the case of Eu3+ and Sm3+ is it possible to observe the transition from the first and second excited levels of the ground multiplet, having low energy values and populated at room temperature. 

Sometimes broad emission bands are observed as in the case of Eu2+ and Ce3+. In these cases the emission is due to 5d-4f transitions. The d-f emission spectra are broad because of the (i) participation of the d electrons, (ii) d-f transitions are allowed and have a time scale of microseconds. Typical f-f and d-f emission spectra are shown in Figs 12.19 and 12.20, respectively. 

Crystal field theory, intensities of 4f-4f transitions, Judd-Ofelt theory of electric-dipole transitions, covalency model of hypersensitivity, dynamic coupling mechanism, solution spectra, spectral data for complexes, solvent effects, fluorescence and photochemistry of lanthanide complexes are dealt with in spectroscopy of lanthanide complexes. 

Fig. 7.10. Linear and log spectra in the vicinity of 4D in Mo32+ are shown in the top frame. Also shown is the synthetic collisional-radiative synthetic spectrum, with Ne-like lines shown in solid, Na-like transitions depicted by the dotted lines, the Mg-like transition shown as the dashed line and F-like transitions shown by dash-dot lines. The spectrum of the Kr26+ 4C, 4D and 4F transitions, with Na-and Mg-like 2p -4d satellites is shown in the bottom frame. Also shown is the synthetic spectrum, with Ne-like lines shown in solid, Na-like transitions depicted by the dotted lines and the Mg-like 2p -4d transition shown as the dashed line...

Halides can form colour centres with absorption bands in the visible range. Doped with europium whose decay time of the allowed 5d —< 4f transition is of the order of 1 fis, they are the most appropriate materials. Because of the presence of fluorine, only BaCIF Eu and BaBrF Eu have a high enough chemical stability for screens fabrication. The highest efficiencies of photostimulated luminescence have been obtained with the fluorobromide (Amax = 390 nm). 

In contrast to the situation with the 3d transition metals in particular, the 4f-4f transitions in the electronic spectra of lanthanide complexes rarely serve any diagnostic purpose. It may be noted, however, that the spectra of the octahedral [LnXe] " ions (X = Cl, Br) have particularly small extinction coefficients, an order of magnitude lower than the corresponding aqua ions, due to the high symmetry of the environment. 

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