Quantum yield definitions

The quantum yield (J x(X) is equal to the amount nx of photochemical or photophysical events x that occurred, divided by the amount rip of photons at the irradiation wavelength X that were absorbed by the reactant, nx/np (Equation 2.24).k Both nx and np are measured in moles or einstein (1 einstein= 1 mol of photons) and the dimension of PX is unity. 

The quantum yield of product formation 4 B is distinct from the chemical yield of product B. The chemical yield may approach 100% if B is the only photoproduct formed, yet J B may be low when photophysical processes dominate over product formation. Conversely, the quantum yield of reactant disappearance, P A, may be high, even though the chemical yield of product B is low, when the reaction produces mainly products other than B. 

The quantitative analysis of the reaction progress is a task that chemists are well trained to do, but the determination of the amount p of photons absorbed is not. Both are needed 

---

Quantum Yield Definitions. Having in mind that the population of the excited state is N (c) or N (0), correspondingly, we can define the absolute values of the quantum yields for these cases as follows ... 

However, (photo)chemists do not usually think in quantities such as the number of molecules consumed or formed by a reaction (cf Tab. 3-7), but in concentrations given in mol Therefore, the quantum yield definition must be converted into a concentration dependency, as is demonstrated by Eqs. 3-11 to 3-13. 

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. 

Quantum parameters are important and useful efficiency estimators in photocatalysis (Cabrera et aL, 1994). These parameters are based on a number ratio , either of photoconverted molecules over absorbed photons or photoconverted molecules over photons entering the reactor, as described in Table 6.1 Using this idea as the basis there are several possible quantum yield definitions ... 

Equation (6-22) shows that energy efficiency evaluations using PTEF require not only a maximum quantum yield definition at initial conditions, based on the energy absorbed by the catalyst, but also rjoH, the fraction of the photon energy used in forming OH groups. The product of these two parameters provides an assessment of the energy efficiency of a photocatalytic reactor system. 

Danapbos, 338 Danaphryne, 339 Decanal, 32, 35, 39, 41 Decapoda (decapods), 47, 48 Definitions, xv luciferin, xix-xxi photoprotein, xxi, xxii quantum yield, xvi, 361 Dehydrocoelenterazine, 173-176, 206-214... 

Quantula (Dyakia), 180, 334 Quantum yield, xvi, 361, 362 aequorin, 104, 106, 110 aldehydes in bacterial bioluminescence, 36, 41 Chaetopterus photoprotein, 224 coelenterazine, 85, 143, 149 Cypridina luciferin, 69-71 definition, xvi, 361 Diplocardia bioluminescence, 242 firefly luciferin, 12 fluorescent compound F, 73 Latia luciferin, 190 pholasin, 197 PMs, 286... 

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. 

For phenomena involving electrons crossing the phase boundary (photocurrents, electron photoemission), the quantum yield j of the reaction is a criterion frequently employed. It is defined as the ratio between the number of electrons, N, that have crossed and the number of photons, that had reached the reaction zone (or, in another definition, the number of photons actually absorbed by the substrate) J=N /N. ... 

We now focus our attention on the presence of the unperturbed donor quantum yield, Qd, in the definition of R60 [Eq. (12.1)]. We have pointed out previously [1, 2] that xd appears both in the numerator and denominator of kt and, therefore, cancels out. In fact, xo is absent from the more fundamental expression representing the essence of the Forster relationship, namely the ratio of the rate of energy transfer, kt, to the radiative rate constant, kf [Eq. (12.3)]. Thus, this quantity can be expressed in the form of a simplified Forster constant we denote as rc. We propose that ro is better suited to FRET measurements based on acceptor ( donor) properties in that it avoids the arbitrary introduction into the definition of Ra of a quantity (i />) that can vary from one position to another in an unknown and indeterminate manner (for example due to changes in refractive index, [3]), and thereby bypasses the requirement for an estimation of E [Eq. (12.1)]. 

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).

The preceding treatment assumed, for simplicity, that the quantum yield of the isolated dipole (i.e., at z= oo) was 100%. Here we assign it a more general value of qQ. The following definitions are useful ... 

---