Impurity quenched

The optimum precipitate is obtained by a more elaborate heal treatment the alloy is solution heat-treated (heated to dissolve the impurity), quenched (cooled fast to room temperature, usually by dropping it into oil or water) and finally tempered or aged for a controlled time and at a controlled temperature (to cause the precipitate to form). 

In addition, the pure solid (N(C2Hs)4)2MnBr4 is recommended as standard. Its phosphorescence yield is high, , = 0.8, the material has only medium absorption despite its high concentration, and there is no long-distance impurity quenching because the luminescence centers are electronically isolated from one another by the large counter ions. 

At — 20 °C. the lifetime in both solvents was approximately the same, as was the lifetime at 77°K. (which was assumed to be the natural lifetime ro). In view of the high viscosity of glycerol at — 20°C., it is reasonable to assume that impurity quenching is negligible, and the same must therefore have been true for ethanol at this temperature. The ratio of ro to r at — 20 °C. is equal to (kp + kh)/kp. The values of the rates of intersystem crossing from triplet to ground state (fa) at —20° were thus derived (see Table I). They are apparently independent of viscosity, a... 

Bimolecular reactions such as quenching, either by molecules of the same kind, self-quenching, or by added substances, impurity quenching, inhibit emission because frequency of bimolecular collisions in gases as well as in solution, k 1010. v can compete with fluorescence emission. Solvent quenching may involve other physical parameters as well such as solute-solvent interactions. Since the solvent acts as the medium in which the solute molecules are bathed, solvent quenching may be classified under unimolecular processes and a clear distinction between it and internal conversion St - S0 is difficult. 

The temperature sensitivity of phosphorescence mainly arises from fast impurity quenching processes. At low temperatures and rigid glassy medium, emission is a rule rather than exception. 

Solvent quenching Self-quenching Impurity quenching Electronic energy... 

The first equation, (3.136a), describes the impurity quenching of A and may be considered separately from another one intended for the description of energy accumulation and dissipation at reaction partners, Bs. 

This is the biexcitonic analog of what we obtained for impurity quenching in Eq. (3.288). It determines the initial distribution of ions that can be obtained from Eq. (3.772), setting LR = WR = 0 ... 

The benzene triplet has not been observed to phosphoresce in either the gas or liquid phase, as noted previously but in view of the lifetime measured In glassy media at low temperature this is not surprising, since very small amounts of quenching impurities would reduce emission to undetectable levels. Some detailed investigations have shown that there is an intramolecular process which Induces radiationless decay of the benzene triplet state, other than impurity quenching. 

Reaction quantum yield determinations using conventional chemical actinometers in isotropic solution are problematic in studies employing liquid crystalline solvents, because liquid crystals (particularly smectic phases) tend to scatter and/or reflect light, and impurity quenching... 

One solution to these problems is to irradiate the actinometer as a mixture in the liquid crystal of interest (33). This procedure tends to eliminate artifacts due to light scattering and impurity quenching, but can introduce new ones if the actinometer is solubilized differently than the substrate is in the liquid crystal or more generally, if the nature of its reactivity differs from that of the substrate. Thus, the actinometer should be such that its structure (length and breadth are most important) and reactivity are as close as possible to those of the substrate of interest. 

Pearlstein RM. Impurity quenching of molecular excitons. I. Kinetic comparison of FSrster—Dexter and slowly quenched Frenkel excitons in linear chains. J. Chem. Phys. 1972 56 2431-2442. 

This facilitates the relative importance of radiationless decay by internal conversion or by quenching through collision with traces of impurities. Consequently, phosphorescence is rarely observable in fluid media. An important exception is in the case of ketones which have lowest energy - (mr ) triplet excited states (4). Here photon emission occurs at rates of 10 to 10 sec , fast enough to compete with solvent or impurity quenching if care is taken to deoxygenate the samples and purify the solvents. For molecules such as acetone, acetophenone, benzo-phenone, biacetyl and benzil, phosphorescence is readily observed in fluid solution at ordinary temperatures with (1/e) lifetimes of 50-500 ys. Heavy atoms promote phosphorescence rates. Dibromoacetonaph-thone (5), with a lowest (TnT ) triplet state is a useful phosphorescence probe of micellar systems. There is a whole literature on heavy-atom induced room-temperature phosphorescence applications in analytical chemistry (6),... 

Two factors besides phosphorescence lead to deactivation of The classical radiationless coupling of the excitation energy to vibrational modes of the matrix, which Jones and SiegeP investigated, and a contribution due to impurity quenching. One can write the non-radiative rate constant knr as a sum of a vibrational and a quenching component... 

The relaxation temperature is frequency dependent. In the nmr experiment the maximum corresponds to a methyl group rotational frequ cy of 10 Hz. In the phosphorescence quenching experiment, the triplet lifetimes are on the order of seconds. Impurity diffusion is coupled in an yet unknown way to the methyl group rotation. From a kinetic point of view, -35 is the temperature where the rate of impurity quenching exceeds that of other processes in limiting the phosphorescence decay time. 

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