Optical quenching

In X-ray fluorescence, self-absorption due to optical quenching of the sample is always observed. For solid samples, radiation can be observed only at a depth of a few micrometres. The intensity P of radiation, after travelling a distance dx in a material which a lineic absorption coefficient p (cm ), will decrease by dP for a penetration angle of 90°. The integrated form of the expression dP = —pP dx is comparable to that described in colorimetry ... 

Absorption by the matrix can cause either an underestimation of the results due to optical quenching, or an overestimation of the results when some of the X-ray fluorescence causes secondary excitation of other elements. For example, the presence of iron in aluminium causes an increase in the fluorescence of the latter because fluorescence emanating from iron excites aluminium. 

Recent measurements utilizing the crossed-beam technique have been performed as follows.37 A metastable helium beam is formed by electron-impact excitation of a thermal helium beam effusing from a multichannel array. The optical quenching method12 described earlier is applied to obtain results for He(2 5 ) and He(23S) separately. The target gas beam is... 

Carlos and co-workers [51,52] have also used these devices to detect spin-dependent effects in the photocurrent and an optical quenching of the forward bias EDMR signal. The SQWs have thus been used to study a number of interesting effects and as test samples toward obtaining a better understanding of the EDMR processes that will be transferable to other materials systems. 

When the material whose absorption spectrum overlaps, more or less, the emission spectrum of the solute is contained in a prepared sample, the fluorescence generating from the solute is partly absorbed to cause the optical quenching. 

The strength of the optical quenching depends on the sort and density of colours. In general, red and yellow cause strong quenching, but blue causes weak. Even though a quencher is colourless, the optical quenching occurs if the quencher has an absorption spectrum which partly overlaps an emission spectrum of solute. 

Measurements will be repeatable based on the presumption that the sample is inherently uniform in analyte concentration. It is also important to think about the possibility of re-absorption of the emitted fluorescence (the optical quenching... 

In addition to the above there is another kind of quenching which has been called optical quenching. In this the sample components do not interfere. Dirty scintillation vials may interfere with light emitted by the sample and prevent its passage to the detector. 

When drawdown is high, the film may be uniaxially oriented and the properties of the final film isotropic. In the manufacture of strapping tape this effect is accentuated. If the cast or quenched film is to be used to feed an orientation tine, additional attention must be given to the amorphous—crystalline nature of the film ia the draw processes so that maximum strength can be achieved and uniform gauge and optical quality maintained. Slot casting is used for the orientation of these resins, polyesters, polyamides, and a variety of others. 

Sheet can be produced by melt extmsion, but in this case a three-roll stack of quenching roUs is generally used (Fig. 2). More than three roUs may be used where necessary. The roUs may be mounted vertically or horizontally. The web is extmded through a slot die in a thickness close to the desired final thickness. The die is in very close proximity to the first chill roU or chill-roll nip. The web may be cast horizontally directly onto the upper chill roU of the stack as shown (Fig. 2), or it may be extmded into the first nip directly. The roUs quench the sheet and provide the surface polish desired. In some applications, matte or embossed roUs maybe used to impart special surface characteristics for certain functions. Where the utmost in optical (glazing) quality is desired the trend has been to mount the roU stack horizontally. The hot melt is then extmded vertically down into the first nip. This avoids problems associated with sag of a horizontal hot melt no matter how short the distance between die and quench. 

The dye is excited by light suppHed through the optical fiber (see Fiber optics), and its fluorescence monitored, also via the optical fiber. Because molecular oxygen, O2, quenches the fluorescence of the dyes employed, the iatensity of the fluorescence is related to the concentration of O2 at the surface of the optical fiber. Any glucose present ia the test solution reduces the local O2 concentration because of the immobilized enzyme resulting ia an iacrease ia fluorescence iatensity. This biosensor has a detection limit for glucose of approximately 100 ]lM , response times are on the order of a miaute. 

Fig. 22 Optical trains of the commercially available scanners. (A) absorption, (B) fluorescence quenching and true fluorescence.

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. 

GTP is a safe operation. A runaway polymerization can be quickly quenched with a protonic solvent. Since the group transfer polymerization goes to completion, no unwanted toxic monomer remains the silicone group on the living end after hydroxylation is removed as inactive siloxane. The living polymer in GTP is costlier than traditional polymerization techniques because of the stringent reaction conditions and requirements for pure and dry monomers and solvents. It can be used in fabrication of silicon chips, coating of optical fibers, etc. 

In order to address the possible influence of positional disorder, we have chosen to analyze the way basic operations such as translations and rotations affect the properties calculated for highly symmetric configurations. This approach could provide guidelines to prevent the loss of significant optical coupling between the ground state and the lowest excited state, and hence the quenching of luminescence in the solid state. 

The use of enantiomerically pure (R)-5-menthyloxy-2(5.//)-furanone results in lactone enolates, after the initial Michael addition, which can be quenched diastereoselectively trans with respect to the /J-substituent. With aldehydes as electrophiles adducts with four new stereogenic centers arc formed with full stereocontrol and the products are enantiomerically pure. Various optically active lactones, and after hydrolysis, amino acids and hydroxy acids can be synthesized in this way317. 

Optical recording systems, 6, 126 Optical resolution amine metal complexes, 2,25 Oral contraceptives iron deficiency, 6, 764 Orbital angular momentum quenching, 1,262 /-Orbital systems... 

Durst and coworkers were the first to report the condensation of chiral a-sulphinyl carbanions with carbonyl compounds477. They found that metallation of ( + )-(S)-benzyl methyl sulphoxide 397 followed by quenching with acetone gives a mixture of dia-stereoisomeric /i-hydroxy sulphoxides 398 in a 15 1 ratio (equation 233). The synthesis of optically active oxiranes was based on this reaction (equation 234). In this context, it is interesting to point out that condensation of benzyl phenyl sulphoxide with benzaldehyde gave a mixture of four / -sulphinyl alcohols (40% overall yield), the ratio of which after immediate work-up was 41 19 8 32478. 

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