Frenkel excitations

Frenkel exciton The fourth absorption peak is polarized parallel to the long axis. In thin films it lies at 5.7-6.0 eV (Lane et al. 1997). This state is a highly localized intraphenyl (Frenkel) excitation which lies at 6.16 eV in biphenyl. Its A -dependence is plotted in Fig. 11.9. 

Bittner ER (2007) Frenkel exciton model of ultrafast excited state dynamics in AT DNA double helices. J Photochem Photobiol A 190 328-334... 

The potassium ions that are produced occupy cation lattice sites, but no anions are produced so electrons occupy anion sites. In this situation, the electron behaves as a particle restricted to motion in a three-dimensional box that can absorb energy as it is promoted to an excited state. It is interesting to note that the position of the maximum in the absorption band is below 4000A (400nm, 3.1 eV) for LiCl but it is at approximately 6000 A (600 nm, 2eV) for CsCl. One way to explain this observation is by noting that for a particle in a three-dimensional box the difference between energy levels increases as the size of the box becomes smaller, which is the situation in LiCl. Schottky, Frenkel, and F-center defects are not the only types of point defects known, but they are the most common and important types. 

Excited-state relaxation, in photochemical technology, 19 109-111 Excitons. See also Frenkel exciton in double heterostructure OLEDs, 22 217... 

In tightly bound (Frenkel) excitons, the observed peaks do not respond to the hy-drogenic equation (4.39), because the excitation is localized in the close proximity of a single atom. Thus, the exciton radius is comparable to the interatomic spacing and, consequently, we cannot consider a continuous medium with a relative dielectric constant as we did in the case of Mott-Wannier excitons. 

It should be noted that, besides free electrons and holes, the role of the free valencies in the crystal may be played by the so-called Frenkel excitons. The latter are, roughly speaking, excited atoms or ions of the lattice which can transfer their state of excitation to similar neighboring atoms or ions. As an example, we may take again the CU2O lattice in which a Frenkel exciton (in the same rough model) is represented by an excited Cu+ ion with the following electronic structure ... 

The earlier formulation due to Frenkel (1931) is appropriate to molecular crystals where the overlap between the orbitals even of excited states is weak. If i(q) is the wave function for a molecule in its ground state and excited state, then a Slater determinant Wa formed from the product... 

The molecular character of the absorption, photoconductivity and luminescence spectra points out that Frenkel type excition formation takes place at the... 

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