Extinction coefficient anisotropic

Infrared ellipsometry is typically performed in the mid-infrared range of 400 to 5000 cm , but also in the near- and far-infrared. The resonances of molecular vibrations or phonons in the solid state generate typical features in the tanT and A spectra in the form of relative minima or maxima and dispersion-like structures. For the isotropic bulk calculation of optical constants - refractive index n and extinction coefficient k - is straightforward. For all other applications (thin films and anisotropic materials) iteration procedures are used. In ellipsometry only angles are measured. The results are also absolute values, obtained without the use of a standard. 

Attention must now be paid to the exponential factor, exp( 2nir (n iij)/A), in Equation 6.5, where (n it) is known as the complex refractive index of a substance. It can be seen that the effect of this factor upon the electromagnetic wave increases with the distance Irl that the light travels in that medium. In the general case of an anisotropic medium, n and are referred to as a specific set of axes, usually chosen to coincide with the optical axes of the medium. For example, the axes of maximum and minimum transmittance are selected for anisotropic absorption. The extinction f for an anisotropic medium is related to the extinction coefficient through Equation 6.9. 

One approach to a solution of this problem was put forward by Hansen (14), who derived general equations which express the overall experimental film absorbance in terms of the external reflectance of the substrate. These relations contain within them expressions for the individual anisotropic extinction coefficients in each geometric orientation. Solution of these general equations for the anisotropic extinction coefficients allows for an unambiguous description of the dipole orientation distribution when combined with a defined orientation model. 

Next, we consider the absorbance due to a dichroic adlayer adsorbed onto the waveguide surface with the optical constants as indicated in Fig. 4. The optical properties of the dichroic layer are described by the different extinction coefficients k, ky, and k in each Cartesian direction. The reflectance of the waveguide-adlayer-cover system follows the analysis found in Macleod [9] with the anisotropic coefficients taken from Horowitz and Mendes [10]. By assuming a thin and weakly absorbing adlayer, the following expressions are obtained for the absorbance as measured through a guided mode at each polarization [8] ... 

Absorption spectra of polysilanes anisotropic absorption for helical conformation, 549 bathochromic shifts, 549 effect of conformation on extinction coefficient, 547, 548/ low-energy feature for al -trans conformations, 547, 548/, 549 Acetylenes... 

The lower part of Figure 3.1 shows a simplified model of the excited states. Only two excited states are represented, but each represents a set of actual levels. The lifetimes of all these levels are assumed to be very short in comparison of those of the two excited states, and form the cross section for absorption of one photon by the trans and the cis isomers, respectively. The cross sections are proportional to the isomers extinction coefficients, y is the thermal relaxation rate it is equal to the reciprocal of the lifetime of the cis isomer (x ). tc and ct are the quantum yields (QYs) of photoisomerization they represent the efficiency of the trans->cis and cis—>trans photochemical conversion per absorbed photon, respectively. They can be calculated for isotropic media by Rau s method, which was adapted from Fisher see Appendix A) for anisotropic media, they can be calculated by a method described in this chapter. Two mechanisms may occur during the photoisomerization of azobenzene derivatives—one from the high-energy 7C-7t transition, which leads to rotation around the azo group, i.e., - M=N-double bond, and the other from the low-energy transition, which... 

To evaluate photoisomerization and photo-orientation parameters, and should be known, was calculated from the absorption spectrum of the polymer solution before irradiation, assuming the same extinction coefficient in the film and in solution bq was determined by the Fisher s method, modified by Rau, which holds not only for isotropic but also for anisotropic samples when the isotropic absorbance is considered (vide infra). For this determination, the isotropic absorbance change was recorded versus the irradiating light intensity, and the sample absorbance change was extracted for an irradiation flux extrapolated to infinity for three drbierent combinations of irradiation and analysis wavelengths 488-488, 532-488, and 532-532 nm, irradiation and analysis, respectively. These experiments... 

Whenever extinction is refined, it is important to check in the. 1st file whether the extinction coefficient refines to sensible values with a reasonably small standard uncertainty. This is the case here, so we will keep EXTI in. Unfortunately, removing the E AD P constraint causes the carbon atom C(6) to be NPD again. This is not unusual for global pseudo-symmetry, as the abovementioned correlation among symmetry-related but not equivalent atoms can lead to problems with anisotropic refinement. Therefore, we decide to live with this constraint and finalize the refinement by adjusting the weighting scheme. The final model in P2 ln is in the file s-05a.res. 

When an electric field is applied to a chemical system which exhibits both electrical and optical anisotropy, both the c, and the Cj terms in the fundamental Eq. (4.21) may be field dependent. Note that the usual extinction coefficients of optically anisotropic molecules reflect random average values ij of all chromophore orientations of the system when measured with polarized light. 

The interplay of these criteria is well illustrated by the two main classes of compound that have historically dominated the development of anisotropic dyes. Azo dyes tend to offer better order parameters, larger extinction coefficients, and better solubility in liquid crystals, whereas the anthraquinone materials tend to have better photochemical stability. The position now is that following a number of years of intensive development stable anisotropic dyes in a variety of colors are available commercially. Colors may be mixed to give black, high-order-parameter mixtures. Note that such blacks are metameric and in principle will change hue with changes in the illuminant, although in practice very acceptable mixtures can be obtained. 

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