Isotropic extinction

An isotropic extinction parameter, of type I and Lorentzian distribution (in the formalism of Becker and Coppens [16]), was also refined. The motions of the non-H atoms were described by anisotropic parameters, while those of the H atoms by isotropic B s. All these displacement parameters were included among the refinable quantities of the model, for a total of 1161 variables in a single least-squares matrix. 

Isotropic samples will have no effect on the polarized light no matter how the crystal is oriented, since all crystal axes are completely equivalent. This effect is known as complete or isotropic extinction (Fig. 3a). Noncrystalline, amorphous samples will exhibit the same effect. 

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. 

Fig. 3 Schematic representations of extinction character (a) isotropic, (b) parallel, (c) symmetrical, and (d) oblique extinction.

Parallel or symmetrical extinction. Can be isotropic if viewed down the c axis, but then a six-sided silhouette should be observed. 

Extinction parameters isotropic or anisotropic Occupancy parameters ... 

The extinction I is proportional to the concentration C of light absorbers in the medium and the directional nature of absorption in an anisotropic medium is denoted by the vector g, known as the molar extinction. Because Equation 6.8 was derived for a wave propagating in the vacuum, the contribution of the extinction to the light intensity is lost in Equation 6.7. For an optical wave propagating in an isotropic medium, Equation 6.7 can be rewritten in the form of Equation 6.10. 

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