Energy transfer, radiative/nonradiative

Fig. 10. Energy level and upconversion scheme for the Er3+ and Yb3+ codoped system. Full, dashed and curved arrows indicate radiative transition, energy transfer and nonradiative relaxation processes, respectively (redraw after...

Energy Transfer. In addition to either emitting a photon or decaying nonradiatively to the ground state, an excited sensitizer ion may also transfer energy to another center either radiatively or nonradiatively, as illustrated in Figure 4. 

Nonradiative energy transfer is induced by an interaction between the state of the system, in which the sensitizer is in the excited state and the activator in the ground state, and the state in which the activator is in the excited and the sensitizer in the ground state. In the presence of radiative decay, nonradiative decay, and energy transfer the emission of radiation from a single sensitizer ion decays exponentially with time, /. 

Noncontact Interactions (Nonradiative and Radiative Energy Transfer).197... 

As seen from (1) and (2), intermolecular processes may reduce essentially the lifetime and the fluorescence quantum yield. Hence, controlling the changes of these characteristics, we can monitor their occurrence and determine some characteristics of intermolecular reactions. Such processes can involve other particles, when they interact directly with the fluorophore (bimolecular reactions) or participate (as energy acceptors) in deactivation of S) state, owing to nonradiative or radiative energy transfer. Table 1 gives the main known intermolecular reactions and interactions, which can be divided into four groups ... 

The next group of bimolecular interactions (3) shown in Table 1, includes noncontact interactions, in which fluorescence quenching occurs due to radiative and nonradiative excitation energy transfer [1, 2, 13, 25, 26]. Energy transfer from an excited molecule (donor) to another molecule (acceptor), which is chemically different and is not in contact with the donor, may be presented according to the scheme ... 

In Chapter 5, we discuss in a simple way static (crystalline field) and dynamic (coordinate configuration model) effects on the optically active centers and how they affect their spectra (the peak position, and the shape and intensity of optical bands). We also introduce nonradiative depopulation mechanisms (multiphonon emission and energy transfer) in order to understand the ability of a particular center to emit light in other words, the competition between the mechanisms of radiative de-excitation and nonradiative de-excitation. 

At present it is universally acknowledged that TTA as triplet-triplet energy transfer is caused by exchange interaction of electrons in bimolecular complexes which takes place during molecular diffusion encounters in solution (in gas phase -molecular collisions are examined in crystals - triplet exciton diffusion is the responsible annihilation process (8-10)). No doubt, interaction of molecular partners in a diffusion complex may lead to the change of probabilities of fluorescent state radiative and nonradiative deactivation. Nevertheless, it is normally considered that as a result of TTA the energy of two triplet partners is accumulated in one molecule which emits the ADF (11). Interaction with the second deactivated partner is not taken into account, i.e. it is assumed that the ADF is of monomer nature and its spectrum coincides with the PF spectrum. Apparently the latter may be true when the ADF takes place from Si state the lifetime of which ( Tst 10-8 - 10-9 s) is much longer than the lifetime of diffusion encounter complex ( 10-10 - lO-H s in liquid solutions). As a matter of fact we have not observed considerable ADF and PF spectral difference when Sj metal lo-... 

The nonradiative energy transfer must be differentiated from radiative transfer which involves the trivial process of emission by the donor and subsequent absorption of the emitted photon by the acceptor ... 

For energy transfer to occur, the energy level of the excited state of D has to be higher than that for A and the time scale of the energy transfer process must be faster than the lifetime of D. Two possible types of energy transfers are known—namely, radiative and nonradiative (radiationless) energy transfer. 

Figure 24. Schematic representation of the proposed radiative and nonradiative processes occurring in nanocrystalline Mn2+ CdS. The straight lines represent radiative processes and the curved lines represent nonradiative processes. (1) Absorption to generate excitonic excited state. (2) Energy transfer to defect. (3) Energy transfer to Mn2+ via defect. (4) Radiative decay of defect. (5) Radiative decay of Mn2+. (6) Direct energy transfer to Mn2+. [Adapted from (122).]...

Fig. 16. Energy transfer processes in Eu chelates (from Hayes and Drickamer (1982)). Wiggly arrows represent nonradiative processes. Solid and dashed arrows represent radiative processes, with a lesser probability for the dashed arrows a, b, c, d see text.

The first term is due to spontaneous radiative relaxation and nonradiative phonon relaxation as described in eq. (13), where / , is the probability of ion i in the excited state. The second term is due to energy transfer induced by ion-ion interaction, where W es and W A are rates of resonant and phonon-assistant energy transfer, which depend on distance between donor and acceptor RtJ. For resonant energy transfer... 

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