Quantum yield fluorescence, definition

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Definition and Uses of Standards. In the context of this paper, the term "standard" denotes a well-characterized material for which a physical parameter or concentration of chemical constituent has been determined with a known precision and accuracy. These standards can be used to check or determine (a) instrumental parameters such as wavelength accuracy, detection-system spectral responsivity, and stability (b) the instrument response to specific fluorescent species and (c) the accuracy of measurements made by specific Instruments or measurement procedures (assess whether the analytical measurement process is in statistical control and whether it exhibits bias). Once the luminescence instrumentation has been calibrated, it can be used to measure the luminescence characteristics of chemical systems, including corrected excitation and emission spectra, quantum yields, decay times, emission anisotropies, energy transfer, and, with appropriate standards, the concentrations of chemical constituents in complex S2unples. 

Fig. 1.6. Strategy for the choice of a fluorescent probe. Av, , and t are the Stokes shift, quantum yield and lifetime, respectively (see definitions in Chapter 3).

Hence the quantum yield of fluorescence 4> n the absence of any external quenching is, from definition, (5.28)... 

In our discussion above, it was pointed out that a molecule in the excited state can return to lower energy levels by collisional transfer or by light emission. Since these two processes are competitive, the fluorescence intensity of a fluorescing system depends on the relative importance of each process. The fluorescence intensity is often defined in terms of quantum yield, represented by (X This describes the efficiency or probability of the fluorescence process. By definition, XL is the ratio of the number of photons emitted to the number of photons absorbed (Equation 5.6). 

The relative fluorescence quantum yield defined in Eq. (3.8) is the ratio of the stationary singlet excitation concentration in the presence of quenchers to the same concentration in their absence. By substituting into this definition N from Eq. (3.654a), we confirm that the fluorescence quantum yield obeys the Stem-Volmer law (3.363) with the same constant as in Eq. (3.364), but with the contact [Pg.339]

By definition of the differential quantum yield (Equation 3.15), the photon flux emitted as fluorescence, m,pem, is equal to the fraction of the incident photon flux <7m,p° that is absorbed by the fluorescent compound times the quantum yield of fluorescence [Pg.118]

Extension of our technique described here is of course limited by various factors. The laser intensity cannot be increased greatly because of the damage of lenses and mirrors of the microscope. Therefore, the concentration of a fluorophore to be bonded as a probe must be rather high, especially when its fluorescence quantum yield is low. Thus, it is difficult to say definitely the smallest size of a particle which is amenable to the decay measurement, but in a fortunate case, a particle as small as 1 um can be examined (8). When a particle is larger than a few tens ym, it is also possible to examine spatially heterogeneous decays within the particle (see Fig.2). 

The quantitative assessment of photochemical activity is facilitated by introducing the quantum yield. In the practice of photochemistry a variety of quantum yield definitions are in use depending on the type of application. There are quantum yields for fluorescence, primary processes, and final products, among others. In atmospheric reaction models, the primary and secondary reactions usually are written down separately, so that the primary quantum yields become the most important parameters, and only these will be considered here. Referring to Table 2-4, it is evident that an individual quantum yield must be assigned to each of the primary reactions shown. The quantum yield for the formation of the product Pf in the ith primary process is the rate at which this process occurs in a given volume element divided by the rate of photon absorption within the same volume element. 

Writing equations for these processes allows definition of the fluorescence quantum yield, 0, and the fluorescence decay time, t,. In terms of sums of the various first order rate constants (nb. other possible deactivation pathways also exist). 

Since the transition probabilities for spontaneous emission and stimulated absorption are proportionah and since ka(X) is related to the absorption coefficient y(A), the absorption spectrum can be used to establish kf. The fluorescence quantum yield then determines k. The measured lifetime in the particular experiment is, using the definition (6.7),... 

Although reaction of the acid chlorides with hydrogen peroxide and base did not lead to the isolation of definite cyclic peroxides, chemiluminescence was observed. On reacting 9,10-diphenyl anthracene-2,3-dicarboxylicacid dichloride (30) with hydrogen peroxide/tert. amine (e. g. ethyl-dicyclohexylamine-, dicyclo-hexylamine - or urea/H202) in dimethyl phthalate light is emitted. The emission spectrum matched the fluorescence of 9,10-diphenylanthracene-2,3-dicarboxy-late (Xmax. ca. 460nm). The quantum yield is low, with ca. 10 einsteins/mol. 

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