Electron paramagnetic chemical analysis

Deep state experiments measure carrier capture or emission rates, processes that are not sensitive to the microscopic structure (such as chemical composition, symmetry, or spin) of the defect. Therefore, the various techniques for analysis of deep states can at best only show a correlation with a particular impurity when used in conjunction with doping experiments. A definitive, unambiguous assignment is impossible without the aid of other experiments, such as high-resolution absorption or luminescence spectroscopy, or electron paramagnetic resonance (EPR). Unfortunately, these techniques are usually inapplicable to most deep levels. However, when absorption or luminescence lines are detectable and sharp, the symmetry of a defect can be deduced from Zeeman or stress experiments (see, for example, Ozeki et al. 1979b). In certain cases the energy of a transition is sensitive to the isotopic mass of an impurity, and use of isotopically enriched dopants can yield a positive chemical identification of a level. 

It is doubtful that a distinction can be made between the various charge distributions proposed by purely chemical means, although this might be possible in principle. We thus rely on physical methods such as optical studies, x-ray analysis, polarography, magnetic susceptibility, and electron paramagnetic resonance (EPR) studies for a determination of the stmcture of these complex cations. This is not to say that the chemistry of these compounds is no longer of... 

The second volume of this new treatise is focused on the physicochemical properties and photochromic behavior of the best known systems. We have included chapters on the most appropriate physicochemical methods by which photochromic substances can be studied (spectrokinetic studies on photostationary states, Raman spectroscopy, electron paramagnetic resonance, chemical computations and molecular modeling, and X-ray diffraction analysis). In addition, special topics such as interactions between photochromic compounds and polymer matrices, photodegradation mechanisms, and potential biological applications have been treated. A final chapter on thermochromic materials is included to emphasize the chemical similarities between photochromic and thermochromic materials. In general, the literature cited within the chapters covers publications through 1995. However, in several cases, publications from as late as 1997 are included. 

Polypyrrole was one of the earliest conducting polymers produced. In 1968 DalT-Olio et al. [2] oxidized pyrrole in aqueous sulfuric acid to obtain a powdery precipitate on a platinum electrode. Pyrrole black, as this material became known, had a conductivity of 8 S cm". Chemical analysis determined the presence of 75% poly pyrrole and 25% sulfate anions. An intense electron paramagnetic resonance signal indicated that the polymer possessed a large number of free spins and gave a g value of 2.0026 (comparable to that of a free electron, g = 2.0023). 

An analysis of the accumulated material makes it possible to formulate the basic distii uishing features of this type of aging, determined by the specifics of the elementary processes. The chemical reactions initiated in rubbers by irradiation and occurring with the participation of free radicals in most cases do not lead to the development of chain processes. The presence of free radicals in irradiated polymers has been demonstrated directly by means of the method of electron paramagnetic resonance. 

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