Electron excess center

Since the original studies of F centers many other color centers have been characterized that may be associated with either trapped electrons or trapped holes. These are called electron excess centers when electrons are trapped and hole excess centers when holes are trapped. 

The F center is an electron excess center and arises because the crystal contains a small excess of metal. Similar metal excess F centers exist in compounds other than... 

The Electron Excess Center. In their earlier paper Schulte-Frohlinde and Eiben (57) had assigned the line A to the O ion and the other line to the stabilized electron subsequently they have reversed this assignment, and are therefore in agreement with other authors. However, the line with g = 2.0006 has been interpreted in different ways, although all interpretations relate it to the radiation-produced electron. Thus Schulte-Frohlinde and Eiben (57, 58) consider the species responsible for this line to be a stabilized free electron, while Ershov et al. (16) and Henriksen (23) identify it with a solvated electron or a po-laron in the same sense as these two terms are used in the radiation chemistry of water and aqueous solutions. According to the above authors, this species is not found in pure ice because of Reaction 30, whereas in alkaline systems such a reaction should not occur. (Henriksen does not offer any explanation about the specific role of alkali hydroxide in stabilizing the solvated electron. ) Both of these hypotheses can be shown to be incorrect. Thus, if Reaction 30 occurred to any extent in pure ice, one should be able to detect H atoms in neutral ice with a yield of at least as high as the maximum yield of the solvated electrons, viz.. 

The decay of the anions (or the electron excess center) appears to proceed through several known steps. In carboxylic acids, protonation of the anion is a highly favored first step above 4.2 K. At 77 K or higher the protonated anion decays to form the acetyl radical by loss of water. This is followed by proton abstraction of a type CH2R. Such a scheme is illustrated in equation (4)... 

The reaction is induced by nucleophilic addition of the hydroxide anion to one of the two carbonyl groups. Then the respective substituent R migrates with the bonding electrons to the adjacent carbon atom (a 1,2-shift). Electron excess at that center is avoided by release of a pair of r-electrons from the carbonyl group to the oxygen ... 

Like other 7r-excessive heterocycles9 (e.g., azoles), the main reactions of azapentalenes are electrophilic substitutions at electron-rich centers (nitrogen or carbon atoms) in the molecule. 

The presence in azoles of both pyrrole-like and pyridine-like heteroatoms leads to a highly perturbed n-electron distribution. As a result, these molecules often display along with ir-excessive centers, atoms with a rather high rr-deficiency, sometimes even higher than in typical azines. We first consider non-fused azoles (Table 1). 

One of the commonest forms of hole-excess center imparts color to the minerals smoky quartz and amethyst. These minerals are forms of silica, containing aluminium as an impurity. The AP+ substitutes for 81 +, and to preserve charge neutrality equal amounts of H+ are incorporated into the crystal. The smoky purple color arises in the electron deficient [A104] group. It is formed when an electron is liberated from a neutral [A104] group by ionising radiation is trapped on one of the H+ ions present. Other hole centers have been described in a variety of crystals. 

As candidates for the reversible hydrogen-adsorption centers, F, 5 line, oxid-red, and thermolum qualify as to stability. F is eliminated by its polarity (electron excess) and the last two are too poorly characterized to be useful to discuss. 5-Line has the proper polarity and could be the reversible site, although some physically unidentified center could also qualify. The adsorption (on 5-line) could hardly be a reconstitution of the original hydroxyl group (plus a hydrogen atom) since that would not be easily reversible. 

The measurement results of electrical resistance of irradiated films (Figure 24.19) speak in favor of the offered mechanism. At irradiation doses of up to 10 cm" both materials manifest an increase in resistance provoked by the accumulation of helium at boundaries, which results in enlargement of electron scattering centers. Further, chromium nitride experiences a catastrophic growth of resistance, which results in collapse. The vanadium nitride forms a stable system of opened channels, through which the excess helium escapes. The value of electrical resistance remains unchanged. 

An excellent apparatus for coloring crystals is the heat pipe. A detailed description of the heat pipe would be out of place in this article, but an excellent review of the method is given by Mollenauer (see Bibliography). Briefly, the heat pipe maintains a zone of pure alkali metal vapor at a precisely controlled pressure. An uncolored crystal is lowered into the alkali vapor for 30-60 min, during which time excess alkali ions diffuse into the crystal until an equilibrium is established. To maintain charge neutrality, negative ion vacancies with electrons (F-centers) must diffuse into the crystal in equal concentration. The ultimate density of F-centers in the crystal is precisely controlled by adjusting the vapor pressure. 

GouiaryBS, Adrian FJ (1960) Wave functions for electron-excess color centers in alkali halide crystals. Solid State Physics 10 127-247... 

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