Impurity Absorption

The nature of light absorption in a crystal is of no significance for theory. What is important here is that this absorption be photoelectrically active, i.e., results in a change of the concentration of free carriers in a crystal. This process may take the form either of the so-called intrinsic absorption accompanied by the transition of an electron from the valency to the conduction band, or of the so-called impurity absorption caused by an electronic transition between the energy band and the impurity local level. 

Dielectric constant of mixed crystalline solutions and polarization of impurity absorption bands... 

Thus, in the frequency range of impurity absorption uq u>, the only resonance term in eqn (5.43) provides a contribution with the form... 

Equation (5.44) makes it possible to analyze the dependence of the intensity of impurity absorption on the polarization of the incident light. Rashba has treated this problem within the framework of the theory of small-radius excitons (2), (12) for the isotopic substitutional impurities. 

The nature of the dependences n (u>) and nn(w) are known to be dependent on the signs of the coefficients fi and n2, respectively (1). Therefore, the experimental results for the ratio (5.53) can, in principle, be used to identify the spatial dispersion effects of the crystal matrix. However, the decay of the excited states of both the impurity and the matrix makes this difficult. If the levels of these states are wide enough then it becomes practically impossible to sneak up on the frequency fli(O) and to distinguish the impurity absorption from the matrix absorption. 

An interesting modification of the sharp transitions Fo to Do of Eu in Eu3GasOi2 has been reported by Van der Ziel (1973) and analyzed in more detail by Van der Ziel and Van Uitert (1973). The transition-line modification is assumed to be caused by an overlap with the broad-band impurity absorption of Pt their analysis convincingly supports this assumption. Such interactions could have significant impact on possible laser applications using rare-earth ions and selected impurities. The existence of inequivalent sites of Er in Gd3Ga50i2 and other garnets and their identification has been reported by Ashurov et al. (1976) possible applications of these new centers are discussed. 

These absorptions can be divided into three categories impurity absorptions due to gases or bound hydrogen isotopes, the infrared cutoff or multiphonon edge, and the fundamental structural vibrations. 

Transmission. The transmittance of glasses is limited by electronic excitiations and light scattering in the UV, by vibronic excitations in the IR, and by reflections and impurity absorptions within the transmission window (in the visible part of the spectmm) Fig. 3.4-27. The UV absorption edge is temperature dependent. An example is shown in Fig. 3.4-28. 

A severe problem is presented by the purification of liquid crystalline solvents. Especially for applications in optical spectroscopy extremely (spectroscopically) pure solvents are often desirable. To the author s knowledge spectroscopically pure liquid crystals have not yet been prepared. The cholesteryl derivatives contain impurities which absorb between 330 nm and 270 nm and which emit a broad structureless emission band centered at 400 nm. Most probably these impurities are steroids with more than one double bond. In the author s laboratory the purest cholesteryl derivatives have been obtained by recrystallization first from etanol and subsequently from dioxane. By this procedure the impurity absorption could be reduced by a factor of 10. 

Absorption losses caused by electronic transitions and molecular vibrations are examples of intrinsic mechanisms. For sihca, the electronic transitions are in the ultraviolet (UV) wavelengths, whereas the molecnlar vibrations are in the infiared (IR). The tails of these transitions bracket the usefiil range of low loss between the visible and the near-infrared wavelengths. Impurity absorption losses are examples of extrinsic mechanisms. The main impurities in fibers made by vapor deposition techniques are the transition metal ions and OH ions. Impurities such as iron cause absorption in the UV and visible wavelengths. Their concentrations need to be lednced to the few parts per bilhon range to reduce their contributions to... 

The solid line (Fig. 4) is the spectrum of a large standard composite of representative plant product. Although it is not apparent in this spectrum, many plant batches show as unknown impurity absorption at 11.3 n. It is, therefore, impossible to use the 11.07 band of 2,5-dichlorophenol for the analysis. The 12.58 n absorption is coincident with a 2,3,6-trichlorophenol band. However, with proper correction for the 2,3,6 interference, accurate values for the 2,5-dichlorophenol were obtained. Calculation with reference to the pure 2,4,5-trichlorophenol spectrum gives... 

The concentration polarization is a result of decreasing surface concentration of reactant. The restriction can be a result of gas-phase transport limits, impurity absorption at the catalyst surface, liquid blockage in low-temperature fuel cells, or other reasons. The thermodynamic (Nernst) and kinetic concentration polarization at an electrode can be written as... 

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