Quantum yield, definition product

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. 

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. 

Consider a Stern-Volmer-type analysis of a system such as in Figure 16.8, but with one additional process, the conversion of A to photochemical product B with rate constant Show that a plot of relative quantum yield for product formation vs. [Q] is linear and can give a value for the lifetime of A if we assume a value for C,- The definition of relative quantum yield is the quantum yield in the absence of quencher divided by the quantum yield in the presence of quencher. 

Among the several examples of photoelectron producing complexes, Fe(CN)4-has been studied in greatest detail.170-177) The quantum yield for photoelectron production is strongly wavelength dependent, Table 28,177 and shows definitively... 

Here an important definition for the rate constant of free carrier production, k, is given. The latter differs from the ionization rate constant by a multiplier equal to the charge separation quantum yield tpm, obtained in the Markovian approximation. This difference indicates that the number of photogenerated ions that avoid geminate recombination and become free is less than their total amount, cpm < 1. 

Since the only excited products of the charge recombination are triplets, their quantum yield is given by the definition identical to that of Eq. (7.55) ... 

For the experimental determination of the 0, it is necessary to quantify the light output of the direct chemiluminescent process. The experimental definition of the direct chemiluminescence quantum yield is given in Eq. 36, that is, the initial rate of photon production (/q ) per initial rate of dioxetane decomposition k )[D]o). Alternatively, the total or integrated light intensity per total dioxetane decomposed can be used. The /t )[Z)]o term is readily assessed by following the kinetics of the chemiluminescence decay, which is usually first order. Thus, from a semilogarithmic plot of the emission intensity vs. time, the dioxetane decomposition rate constant kjj is obtained and the initial dioxetane concentration [Z)]o is known,especially if the dioxetanes have been isolated and purified. In those cases in which the dioxetanes are too labile for isolation and purification, [/)]o is determined by quantitative spectroscopic measurements or iodometric titration. 

To arrive at a photochemical mechanism which is consistent with all of the observed photochemical as well as thermal results, the various possible energy levels or excited states of the primary photochemical products should be considered in detail. Such a mechanism has been proposed (5), which can account for the high quantum yields observed in the far ultraviolet. However, this is somewhat speculative, because the experimental difficulties encountered in the photochemical studies appear to be even more insurmountable than those encountered in the thermal decomposition. More quantitative data are needed on the photochemical system before its mechanism can be definitely established. 

The efficiency of any photophysical or photochemical process is a function of both the properties of the reaction environment and the character of the excited state species. The fundamental quantity which is used to describe the efficiency of any photo process is the quantum yield (0) it is useful in both quantifying the process and in elucidating the reaction mechanism. Quantum yield has the general definition of the number of events occurring divided by the number of photons absorbed. Therefore, for a chemical process 0 is defined as the number of moles of reactant consumed or product formed divided by the number of einsteins (an einstein is equal to 6.02 X 10 photons) absorbed. Since the absorption of light by a molecule is a one-quantum process, then the sum of the quantum yields for all primary processes occurring must be one. Where secondary reactions are involved, however, the overall quantum yield can exceed unity and for chain reactions reach values in the thousands. When values of 0 are known or can be measured for a specific photochemical reaction the rate can be determined from ... 

Quantum yield (quantum yield is shown in (7). In some cases, the input number of electrons is used as the numerator in (7). It is noteworthy that stable reduction products of CO2 require multi-electron transfer, as described above. 

For example, the quantum yield (QY or d>) is a fundamental parameter in heterogeneous photocatalysis, whose definition should be carefully considered. A standard definition has been given in terms of a particular reaction, with a defined reactant or product, at a given wavelength A. [78] ... 

According to our definitions, the well studied valence isomerization of quadricyclane to norbomadiene in the presence of appropriate transition metal complexes is also regarded as a photoinduced catalytic reaction. This reaction was recently discussed by Kutal[35], [36] as photo-generated catalysis. The observed quantum yield exceeds unity (( )s = 1.6) because a chain reaction is involved. Photochemically formed [Ru(bpy)3] , e.g., initiates the chain reaction by oxidation of quadricyclane to the corresponding cation radical which acts as chain carrier. The quadricyclane cation radical undergoes an isomerization to the more stable norbomadiene cation radical. The oxidation of quadricyclane by the latter one leads to the formation of norbomadiene as the product upon regeneration of the chain carrier. 

This transformation fits the definition of a photocatalytic reaction, whereby a thermally active catalyst is photogenerated. This situation differs from that observed with a photoassisted reaction, which is a photolytic reaction that ceases when the light source is removed. In photocatalytic systems the reaction is catalytic in the number of incident photons, and therefore the quantum yield 0 > 1. In photoassisted reactions where continuous irradiation is required to obtain the product, the quantum yield (j> <... 

Wigner s three steps are (WSl) The determination of potential energy surfaces, which gives, in the words of Wigner, the behaviour of all molecules present in the system during the reaction, how they will move, and which products they will yield when colliding with definite velocities, etc. (p. 29). The solution of this problem requires the calculation of a potential energy surface, which is a quantum chemistry problem that was solved, somewhat unsatisfactorily, by Bom and Oppenheimer(1927). 

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