Crystal light absorption

To determine of Ce(IV) in acid soluble single crystals, a simple and sensitive method is proposed. The method is based on the reaction of tropeoline 00 oxidation by cerium(IV) in sulfuric acid solution with subsequent measurement of the light absorption decrease of the solution. The influence of the reagent concentration on the analysis precision is studied. The procedure for Ce(IV) determination in ammonium dihydrophosphate doped by cerium is elaborated. The minimal determined concentration of cerium equal to 0.04 p.g/ml is lower than that of analogous methods by a factor of several dozens. The relative standard deviation does not exceed 0.1. 

Light absorption of a dye, crystal violet, adsorbed from solution on the surface of film or fiber, before and after grafting, and measured by absorption of visible light at about 600 nm. 

Figure 4. Kinetics of surface grafting of polyethylene (PE) and polypropylene (PP) films with acrylamide (AM) by the vapor phase method measured as light absorption at 600 nm after dipping in an aqueous solution of crystal violet (CV).

Figure 16. Surface grafting of polypropylene film strips after 10 sec. irradiation measured as light absorption after dipping in aqueous crystal violet solution. The presoaking solutions contain 0.2 M benzophenone (all) and 1.3 M acrylamide (1), 0.8 M (2),...

Figure 10.2 Orientation of an aminoazobenzene dye for maximum light absorption in a liquid crystal display...

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. 

Silicon is the most popular material for photovoltaic (PV) power. Another material is gallium arsenide (GaAs), which is a compound semiconductor. GaAs has a crystal structure similar to that of silicon, but it consists of alternating gallium and arsenic atoms. It is well suited for PV applications since it has a high light absorption coefficient and only a thin layer of material is required, which reduces the cost. 

Hirota used doped crystals to observe weak Ti-<- So absorption spectra by phosphorescence excitation spectroscopy. Triplet excitons of the host are formed by direct light absorption. The guest molecules, chosen to have lower triplet energy, act as traps and emit guest phosphorescence. 

In this case the initial act of light absorption leads to the formation of an exciton, rather than a free electron or hole. Such an exciton, as it wanders through the crystal, may meet a lattice defect and annihilate on it, the energy of the exciton being utilized to ionize the defect, i.e., to transfer an electron or hole localized on the defect to the free state (the mechanism of Lashkarev-Juze-Ryvkin, see 90, 91). If such a defect is a foreign particle chemisorbed on the crystal surface, the result will be a change in the character of the bond between this particle and the surface. Thus, the interaction of the lattice excitons with the chemisorbed particles may cause a change in the relative content of the different forms of chemisorption and... 

Light absorption by a semiconductor results in the creation of electron-hole pairs, by means of either direct or indirect momentum transitions depending on the crystal structure. Momentum is conserved in direct transitions, while a change in momentum is... 

Absorption measurements, however, on such fatty acid layers mixed with dye molecules which are deposited on glass slides can be used for estimating light absorption in the layer of dye molecules at the crystal surface. A monomolecular layer of dye molecules with an absorption coefficient of 105 M-1 cm-1 as for... 

For instance, the value of AE = 1.7 eV measured for single crystals of phthalo-cyanine, leads us to conclude that the 18-ji-electron system is responsible for both light absorption and conduction. 

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