Screening local field effects

G2, to G3, and to G4, the effective enhancement was 10%, 36%, and 35% larger than the value estimated by the simple addition of monomeric values. The enhancement included the local field effect due to the screening electric field generated by neighboring molecules. Assuming the chromophore-solvent effect on the second-order susceptibility is independent of the number of chro-mophore units in the dendrimers, p enhancement can be attributed to the inter-molecular dipole-dipole interaction of the chromophore units. Hence, such an intermolecular coupling for the p enhancement should be more effective with the dendrimers composed of the NLO chromophore, whose dipole moment and the charge transfer are unidirectional parallel to the molecular axis. 

The interactions between the molecule and the environment can lead to distortions in the electrical properties due to the susceptibility of the molecules and the properties of the host matrix. The refractive index of the matrix acts as a screening factor, modifying the optical spectra and interaction between charges or dipoles embedded within it. Local field effects change the interaction with an electromagnetic field and should be considered along with orientation factors in the dipolar interaction. 

However, in the ease of eolloidal semiconduetor nanoeiystals experiments show an essential depression of the decay compared to the valne determined by Eq. (2). To explain this depression, Wehrenberg, Wang, and Guyot-Sioimest" noted that the above expression, derived for point-like emitters, does not take into account the local field effect, or screening of the radiation field inside a nanociystal of finite size. If a spherical nanociystal of permittivity snc is embedded into a dielectric host of permittivity Shost and snc > e host, then the internal electric field (inside the nanociystal) int is weaker in comparison with the external field E xt in the host, Ei t = S E xt, where the screening factor S is found to be ... 

The Exact Nature of the Protein as a Phonon Bath and Dielectric Environment. At this point there exists no satisfactory quantitative description of dielectric screening and local field effects in energy transfer, except in the limit of large donor-acceptor separations (the l/n factor). This appears to be an important point to resolve, since the effect on the rate can amount to a factor of 4. A particular challenge will be to calculate or measure medium effects in a protein as opposed to a dielectric continuum. 

The one-particle approximation that has been described above does not take into account the local polarization due to the charge transfer from one atom to the other, the so-called local field effects. There are at least two ways to take into account the charge polarization into a tight-binding formulation. The first approach comes from solid-state physics. It is based on the calculation of the screening matrix, which represents... 

Unlike the case of metals and semi-conductors, surface and bulk screening effects in insulators have been little studied. In this section, we will review the general properties of the dielectric constant - its small wave vector and low-frequency limits - and we will put a particular emphasis on local field effects. 

In screening processes, local field effects are the manifestation of the spatial inhomogeneities of the material and, in particular, its discrete atomic structure. The response to a charge localized in h depends upon both f and h and not only upon f — tq. 

The local field factor F, intervening in Eq. (3) takes account of the already-mentioned screening effects and is given by... 

A closer inspection of Fig. 8.1.3 reveals that the screened potential is not represented by perfectly straight lines the bumps, barely visible in Ge but clearly apparent in GaAs, are not computational noise and they reflect a well defined physical effect microscopic variation of local fields. By treating in the... 

Effect of off-diagonal dynamic disorder (off-DDD). The interaction of the electron with the fluctuations of the polarization and local vibrations near the other center leads to new terms VeP - V P, Vev - Vev and VeAp - VAPd, VA - VAd in the perturbation operators V°d and Vfd [see Eqs. (14)]. A part of these interactions corresponding to the equilibrium values of the polarization P0l and Po/ results in the renormalization of the electron interactions with ions A and B, due to their partial screening by the dielectric medium. However, at arbitrary values of the polarization P, there is another part of these interactions which is due to the fluctuating electric fields. This part of the interaction depends on the nuclear coordinates and may exceed the renormalized interactions of the electron with the donor and the acceptor. The interaction of the electron with these fluctuations plays an important role in processes involving solvated, trapped, and weakly bound electrons. 

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