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12 Handbook on the Physics and Chemistry of Rare Earths place as magnetic dipole since in this case the parity exclusion rule does not apply. Spin-orbit coupling weakens selection rules on DL and DS. As a conse- quence of these facts, many trivalent rare earth ions exhibit luminescence with intensities high enough for practical uses. The decay times of the luminescence due to 4f–4f transitions are mostly in the range ms–ms, while, in the case of luminescence due to a spin-allowed transition between levels having equal spin multiplicity, they are much shorter, $105 s. The spectra of the 4f–4f transi- tions are line shaped and the energies of the levels involved in the transitions are well defined and mostly independent of the nature of the host. The energy level diagram for trivalent rare earth is commonly referred to as Dieke diagram (Peijzel et al., 2005). This diagram is useful because the energies of the J multi- plets vary by only a small amount in different host crystals. The diagram allows rapid identification of the energy levels in new hosts and has been a crucial tool in the design of materials suitable for phosphors or lasers. 3.1.2.2 4f–5d Transitions This is concerned with the 4fn15d1 configurations. 4fn15d1 levels may be understood as formed by the electron in the 5d orbital interacting with the 4fn1 core. The 5d orbitals interact strongly with the surroundings (crystal field) so that 4fn15d configurations of rare earth ions in solids are very different from those of the free ions. The 4fn!4fn15d1 absorption consists of parity-allowed intense and broad, bands corresponding to the components of 5d electronic levels split by the crystal field. For most of the trivalent rare earth ions, inter- configurational transitions 4fn!4fn15d1 correspond to wavenumbers exceed- ing 50,000 cm1, and are thus not accessible to UV excitation. In the case of Ce3+ and Eu2+, however, they have smaller energies and are usually accessible from near-UV to visible radiations. The spectra of the 4f–5d transitions are broad band shaped and can be tuned by modifying the coordination environ- ment. The decay times of the luminescence arising from 4f to 5d transitions are short, usually in the range of 40–70 ns for Ce3+ and $1 ms for Eu2+. 3.1.3 Energy Transfer The process by which the excitation energy of an ion migrates to another ion is called energy transfer. It is very important to understand this effect in order to develop efficient luminescent materials. There are several types of energy transfer processes (Goldberg, 1966). Here, it is convenient to introduce terms which are frequently used, such as donor/acceptor. A donor (D), sometimes called sensitizer, is a species giving out energy, while an acceptor (A), some- times called activator, is a species accepting the energy of the donor. (1) Resonant energy transfer The energy of an excited ion migrates to another one of the same species that is in its ground state. This mechanism of this type of transfer is divided into three categories.PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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