Nonradiative resonance mechanism

One of the outstanding properties of these substances is the extreme tunability of the electronic states under high pressure. In many cases a red shift on the order of 2000 cm-1/GPa has been observed (Yersin and Riedl, 1995). In addition, an effective nonradiative energy transfer from the cyano donor complexes to the f elements has been observed. In the case of Eu[Au(CN)2]3-3H20 this process even totally quenches the otherwise very intense and broad emission from the [Au(CN)2] layers. However, because of the very strong red shift of the donor electronic states, it is possible to shift the donor states over different levels of the f element. Especially, resonant and nonresonant energy transfer conditions can be achieved to study the transfer mechanism. 

Fig.13a-c. Schematic representation of the sequence of events in a simple three-level photon avalanche excitation mechanism a nonresonant GSA to generate one ion in level 1 b resonant ESA to generate one ion in level 2 c nonradiative cross relaxation to generate two ions in level 1... 

Let the basis set still be the BO states starting points. Sim we wish to focus upon all the diverse molecular phenomena which are classify as involving radiationless processes, it is necessary to center attention upon th molecule. This focus is best obtained by considering the effective Hamiltoniar Hett, for the molecule which accounts for all relaxation mechanisms other tha the intramolecular nonradiative decay. (The use of effective Hamiltonians is popular in considering the relaxation processes associated with studies of magnetic resonance 37L) For the present case, the effective Hamiltonian is 16>17)... 

It is useful in discussion of weak coupling between nanostructures to remember the nonradiative mechanism of Forster resonant energy transfer from an excited molecule (a donor) to some other molecule (an acceptor) which can be in the ground or in an excited state. The probability of such a transfer is determined by the Coulomb nonretarded (instantaneous) dipole-dipole interaction between molecules and is proportional to Rp/R6 where Rp is the Forster radius and R is the distance between molecules. For organic materials the Forster radius is usually about several nanometers and strongly depends on the overlapping... 

The theory of resonant nonradiative ED-ED ET was formulated by Forster, and extended by Dexter to include other interaction mechanisms. From the Fermi Golden Rule, the ET rate, WDA, between an excited donor (D ) and an acceptor (A) in nondegenerate states is proportional to the square of the interaction matrix element, /, [358] ... 

Other resonant (as well as nonresonant) quenching mechanisms are however, possible, for this system. For example, considering the fact that both nonradiative transitions are EQ allowed, the site-site coupling strength is given by... 

Nonradiative relaxation from states can be effected via fluorescence resonance energy transfer. The rate of energy transfer in donor-acceptor pairs is directly related to their distance, which can of course be affected by analyte complexation. Excitation energy transfer leads to quenching of the donor, but also to enhancement of the acceptor fluorescence therefore, the ratio of fluorescence intensities at donor and acceptor emission wavelengths provides information on complexation via changes in donor-acceptor distance. The chapters by Valeur, Bouas-Laurent, and Krafft describe uses of the energy transfer mechanism. 

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