Energy phonon assisted

The phonon-assisted polaron-like hopping model is unique because it is built upon an understanding of the dynamical nature of DNA in solution. The fundamental assumption of this model is that the introduction of a base radical cation into DNA will be accompanied by a consequent structural change that lowers the energy for the system. 

Fig. 7.18 The radiative recombination time r as a function of the blue shift of the photon energy AE from the bulk silicon band edge zero-phonon transitions (dots) TO phonon-assisted transitions (line). This scatter plot shows the radiative time for each member of an ensemble uniformly distributed around a cubic geometry. The top scale indicates the equivalent cube size. Redrawn from [Hy2],...

F ure 5.19 The energy-level schemes of donor D and acceptor A centers for (a) resonant energy transfer and (b) phonon-assisted energy transfer. 

The processes involved are stepwise energy transfer, cooperative sensitization of luminescence and cooperative luminescence. As an example of stepwise energy transfer a system containing Er and Yb may be considered. The latter ion absorbs at 970 nm (10,300 cm ) and in phonon assisted an Er ion... 

Figure 6. Schematic representation of phonon-assisted energy exchange. Note that energy gaps EA and EB do not match, and the mismatch is AE. The exchange takes place via the liberation of phonons equal in energy to AE.

Van Uitert and Iida (55) suggested the applicability of the phonon-assisted-transfer mechanism to rare earth-rare earth energy exchange. They were able to correlate the emission intensity of the 5D0 level of trivalent europium or the5 D4 level of trivalent terbium with the closest, but definitely lower-lying, level observed for a second rare-earth ion. 

Johnson et al. (55) have reported a phonon-assisted energy exchange from trivalent erbium to trivalent thulium or to trivalent holmium. In this case, these authors were able to rule out resonance exchange completely Of some importance is that these systems are useful for laser oscillators, and the energy exchange results in a substantial decrease in threshold. 

If there are several AP minima of close energy, then at low temperatures one should take into account two-phonon-assisted transitions between these minima. In Ref. [15] (see also Ref. [14]) it was found that the rate of these transitions depends on temperature as 7 3. However, as it was already mentioned above, in Ref. [9] it was found that the contribution of the two-phonon-assisted transitions between different Jahn-Teller minima of the AP to the ZPL width at low temperatures is described by the T5 law. Note that an increase of the Jahn-Teller interaction leads to a decrease of the rate of these transitions. Therefore, in the strong Jahn-Teller interaction limit this broadening mechanism becomes unimportant. 

Neodymium systems have the potential for quantum cutting because Nd3+ has a high lying 4f" state, 2G(2)9/2, at about 47 000 cm-1, which has a 7000 cm-1 gap above the next lower level, 2F(2)7/2 (Camall et al., 1988). This energy gap is sufficient to prevent non-radiative relaxation between the two states, and emission from the 2G(2)y/2 state can be expected. Exciting the 2G(2)9/2 state directly is impractical, due to the very low transition probability from the ground state. However, if efficient absorption into the 5d band occurs, then the 2G(2)9/2 state may be populated via non-radiative phonon-assisted relaxation, resulting in 2G(2)9/2 emission. 

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... 

Energy transfer, particularly, phonon-assisted energy transfer processes must be considered in evaluating lanthanide luminescence decays, because they contribute in many cases to a major part of the observed lifetime. Based on the theoretical models described in section 3, we have conducted Monte Carlo simulations of energy transfer and its effect on luminescence decay for lanthanide ions in nanociystals and compared the calculated results with experi-... 

Direct pumping of poison centers as well as energy transfer from co-activators, and energy transfer from activators all represent an energy loss in the system. In addition, activators and co-activators also have non-radiative decay routes. Because non-radiative decay is usually phonon-assisted, non-radiative decay is exacerbated by increasing temperature and manifests itself by a characteristic temperature at which luminescence is quenched. The crystallographic relations that provide optimum sensitizer -activator energy transfer are outlined by Blasse (9)... 

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