Quench lamp

The principle of an experimental setup used for PglES with helium metastables is shown in Fig. 11. The metastables are produced in a hot cathode discharge, running at approximately 10-2 torr and 0.1 A in a differentially pumped chamber. The quenching lamp allows removal of... 

The branching ratio is 1 1000 in favor of the ground state. The beam is scattered from a second supersonic beam, and the electronically excited atoms are detected. As excitation transfer can occur during the collision (e.g., He + Ne— He + Ne ), a second quench lamp is sometimes installed in front of the detector, to enable study of the energy-transfer process separately. 

Sj metastables are unaffected. Data collection sweeps are performed with the quench lamp alternatively on and off and subtraction of the two sets of data allows the time-of-flight distributions for the 2 Sq metastable alone to be determined. 

However, the optical train illustrated in Figure 22B allows the determination of fluorescence quenching. The interfering effect described above now becomes the major effect and determines the result obtained. For this purpose the deuterium lamp is replaced by a mercury vapor lamp, whose short-wavelength emission line (2 = 254 nm) excites the luminescence indicator in the layer. Since the radiation intensity is now much greater than was the case for the deuterium lamp, the fluorescence emitted by the indicator is also much more intense and is, thus, readily measured. 

Color reproduction of the chromatograms can be achieved by color photography — the best, but also the most expensive method of documenting thin-layer chromatograms. It can be used not only to produce true-color reproductions of colored zones but also — with the aid of a Reprostar (Fig. 64) or a UVIS analysis lamp (Fig. 6) — of fluorescent or fluorescence-quenched zones. When photograph-... 

The required exposure times are difficult to estimate. They are best found by trial and error. Documentation of fluorescence quenching at A = 254 nm usually only requires one trial. The exposure time found to be adequate here is normally suitable for all following exposures of fluorescence quenching if the exposure conditions are maintained constant (camera type, film type, distance of objective and lamp, aperture etc.). The exposure time required for fluorescent chromatograms is primarily dependent on the intensity of the fluorescence and, therefore, has to be optimized for each chromatogram. It is best to operate with a range of exposure times, e.g. aperture 8 with exposures of 15,30,60,120 and 240 seconds. Experience has shown that one exposure is always optimal. 

Thin-layer plates were made with silica gel-calcium sulfate and each contained a mixture of zinc silicate and zinc cadmium sulfide as phosphors. Separated components are generally visible under ultraviolet light by fluorescence quenching. This was true, in part, for the pyrethrins, except that some of the separated components possessed a natural fluorescence under the ultraviolet lamps. 

The single-electron transfer from one excited component to the other component acceptor, as the critical step prior to cycloaddition of photo-induced Diels Alder reactions, has been demonstrated [43] for the reaction of anthracene with maleic anhydride and various maleimides carried out in chloroform under irradiation by a medium-pressure mercury lamp (500 W). The (singlet) excited anthracene ( AN ), generated by the actinic light, is quenched by dienophile... 

Our results demonstrated clearly that the lifetime data are more sensitive to subtleties of the micromechanistic photophysics. In this case we were able to establish inadequacies of the two-component model that were not detected by intensity quenching measurements alone. It is also clear that resolution of the detailed mechanism in these complex polymer systems will require even better lifetime data than we are able to obtain with a conventional flash lamp-based time-correlated photon counting system. 

Together with Sm another group of lines is often detected with the main line at 685 nm, which also has a very long decay time of several ms (Fig. 4. lOd). It is very close to the known resonance line of Sm. Under low power UV lamp excitation, the luminescence of Sm in fluorite is known only at low temperatures, starting from approximately 77 K, and is composed of narrow /-/ transition lines and a broad band of 4f-5d transitions (Tarashchan 1978 Krasilschikova et al. 1986). Evidently, under strong laser excitation, luminescence of Sm + may be seen even at room temperature, where 4f-5d luminescence is usually quenched because of radiationless transition. 

The limitations on the total pressure in the FP-RF cell are far less severe than those for FFDS. The lower end of the pressure range that can be used is determined by the need to minimize diffusion of the reactants out of the viewing zone. The upper end is determined primarily by the need to minimize both the absorption of the flash lamp radiation by the carrier gas and the quenching of the excited species being monitored by RF. In practice, pressures of 5 Torr up to several atmospheres are used. The kinetic analysis is again typically pseudo-first-order with the stable reactant molecule B in great excess over the reactive species as outlined earlier. Table 5.5 gives some typical sources of reactive species used in FP-RF systems. 

A gated deuterium lamp which has a full width at half-maximum (FWHM) ofabout2nsanddecay time of 1 ns has been used. The decay curves are deconvolved by numerical convolution technique with the assumption that the delta-pulse response is a single exponential function. A programme is used that varies the lifetime until the sum oi the squares or tne deviations between the observed and the calculated decay curves is a minimum (Fig. 11.5). If t0 = unquenched fluorescence lifetime and t = lifetime of quenched... 

Due to its excellent pigmentation properties, especially its light stability and universal insolubility, carbon black has been used as a black pigment since early times. It was produced for this purpose by burning oils, fats, or resinous materials. The flame was either quenched on a cool surface (impingement black) or cooled in special chimneys (lamp black), where the carbon black was deposited. 

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