Differential quantum yield

Differential quantum yield. It is considered as a dynamic characteristics of the system and is defined as the ratio of the reaction rate to the photon flux (calculated horn the radiant flux, the intensities of the rays of the UV lamp, the transmittance of the optical window of the reactor, and the absorbance of TiC>2). A quantum yield of ca. 0.1 was found in the region where r varies linearly with . 

Also, care must be taken when discussing organic photochemical reactions to differentiate between the term yield as an organic chemist usually employs it and quantum yield as understood by photochemists. In the former sense the yield refers to the molecules of product per... 

Solving the corresponding differential equations for steady-state conditions leads to the following expressions for the fluorescence quantum yields A and B of A and B bands ... 

Direct labeling of a biomolecule involves the introduction of a covalently linked fluorophore in the nucleic acid sequence or in the amino acid sequence of a protein or antibody. Fluorescein, rhodamine derivatives, the Alexa, and BODIPY dyes (Molecular Probes [92]) as well as the cyanine dyes (Amersham Biosciences [134]) are widely used labels. These probe families show different absorption and emission wavelengths and span the whole visible spectrum (e.g., Alexa Fluor dyes show UV excitation at 350 nm to far red excitation at 633 nm). Furthermore, for differential expression analysis, probe families with similar chemical structures but different spectroscopic properties are desirable, for example the cyanine dyes Cy3 and Cy5 (excitation at 548 and 646 nm, respectively). The design of fluorescent labels is still an active area of research, and various new dyes have been reported that differ in terms of decay times, wavelength, conjugatibility, and quantum yields before and after conjugation [135]. New ruthenium markers have been reported as well [136]. 

We often differentiate between the primary quantum yield, which focuses on only the first event (here the quantum yield cannot be >1), and secondary quantum yield, which focuses on the total number of molecules formed via secondary reactions (here the quantum yield can be high). The common emission quantum yield measurement involves the comparison of a very dilute solution of the studied sample with a solution of approximately equal optical density of a compound of known quantum yield (standard reference). The quantum yield of an unknown sample is related to that of a standard by equation 16.5... 

First, we note that the number of photons absorbed rather than the number of incident photons has to be taken into account. Second, integrations over extended time periods most likely bear substantial errors because the intensity of the source may fluctuate or drift. As a consequence of this, the only exact measure for the efficiency of a photochemical reaction is the true differential quantum yield, which needs to be determined for each step of the reaction. Similar to thermal reactions, photochemical reactions may be complex. Accordingly, the only correct measure is the so-called partial (true differential photochemical) quantum yield, which is defined for each linearly independent step of the reaction. 

One of the most important features of a photoreaction is the value of the quantum yield ( )i of compound i, which is the quantifying answer to the question How effective In principle, the quantum yield is the ratio of the number of reacting molecules to the number of quanta absorbed. In praxis there are several definitions of the quantum yield true (only light absorbed by the reactant is considered) and apparent (there are other absorbers present), differential (at the moment ) and integral (mean). In the previous rate equation, ( )e and (j) are the true differential yields. The monoexponential kinetics of Equation, 1.2 or 1.4 allow one to determine the yields in systems where the starting solution is already a mixture of E- and Z-forms (which can happen easily if the E-form is not prepared under strict exclusion of light). It turns out, however, that the yalues of the Z —> E quantum yield are especially sensitive to small errors in the E values. 

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