Local field intensity factor

In nonpolar solvents, Eu chelates studied in the present work exhibited luminescence kinetics closely following the single exponential decay law over the intensity range of about three orders of magnitude. In agreement with the previous observations [5, 6], an increase in the refractive index of the nonpolar medium leads to a systematic decrease in the excited-state lifetime of the Eu ion. This effect is accounted for by the well-known refractive-index dependence of the radiative rate y where and are radiative rates in a dielectric medium and in vacuo, respectively, n is the refractive index of the medium, and/[ ) is the local-field correetion factor [4, 5]. 

Huge local enhancement of the near-field intensity by factors of... 

A final point worth mentioning is the effect of local fields on the optical nonlinearities of strongly QC nanostructures. These arise from embedding QD s in a medium of different dielectric constant (2). One requires to know how the field intensity inside the particle varies at saturation in excitonic absorption. This has been approached theoretically by defining a local field factor f such that Em = f Eout (2). The factor f depends on the shape of the QD and the dielectric constant of the QD e = + E2 relative to that of the surrounding medium. Here... 

Similarly to IR, classical theories have also been proposed in the literature for Raman intensities in solution [29,32-38], The starting point is again the definition of the local field Eso1 acting on the molecule. In all cases the local field factor is defined as / = S-i/S-c, with 5 sc being the scattering intensity. 

Irwin (40) gave an alternative formulation to fracture by considering the distribution or field of stresses around a crack in an elastic material. He proposed that such a distribution could be expressed as a function of a parameter K, known as the stress intensity factor, and he established that the fracture would occur when K exceeds a critical value characteristic of each material. Figure 14.33 shows a sharp crack of length 2a in an infinite lamina subjected to a tensile stress ct. The equations defining the local stresses an, a22 < 12 are (42)... 

Second, Irwin found that the stress field surrounding a crack could be defined uniquely by a stress-field parameter termed the stress-intensity factor, K. He postulated that fracture occurs when the value of K exceeds some critical value. K, often referred to as the material fracture toughness. Thus K relates the magnitude of the stress-intensity local to the crack in terms of the applied loadings and the geometry of the structure in which the crack is located. A crack in a solid may be stressed in three different modes as depicted in Fig. 2.18. Mode I opening, and hence the Mode I value for the stress intensity factor Ki, is the most critical situation in bonded joints. 

In two different investigations of single pentacene molecules in -terphenyl [10, 36] it was found that calculated values for 1 were all much smaller than the experimental values. In these calculations, however, the triplet state was treated as a single level. By using the correct expression (Eq. 8) and approximate corrections for the local field, the saturation intensities of [36] were recalculated by the authors of this chapter and much better agreement between experiment and theory than in [36] was found. The experimental values of Is did still show quite a large scatter which did not follow the variations in the ISC parameters 23 and which were found to be different up to a factor of three from molecule to molecule [36] (see also 1.2.4.3). Therefore, this distribution of saturation intensities may arise due to differences in local fields and variations in the orientation of the molecular transition dipole moments with respect to the electric field of the exciting laser. 

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