Photochemical process efficiency

The efficiency of a photochemical process [39] requiring high-intensity light sources, e.g., UV... 

Direct Photolysis. Direct photochemical reactions are due to absorption of electromagnetic energy by a pollutant. In this "primary" photochemical process, absorption of a photon promotes a molecule from its ground state to an electronically excited state. The excited molecule then either reacts to yield a photoproduct or decays (via fluorescence, phosphorescence, etc.) to its ground state. The efficiency of each of these energy conversion processes is called its "quantum yield" the law of conservation of energy requires that the primary quantum efficiencies sum to 1.0. Photochemical reactivity is thus composed of two factors the absorption spectrum, and the quantum efficiency for photochemical transformations. 

As such, the thermal process in equation (60) proceeds via the same reactive intermediates (arising from an adiabatic electron transfer) as that observed in the photochemical processes in equations (57) and (58). The proposed electron-transfer activation for the thermal retropinacol reaction is further confirmed by the efficient cleavage of benzpinacol with tris-phenanthroline iron(III), which is a prototypical outer-sphere one-electron oxidant195 (equation 61). 

The photochemical process built into 7 was encountered previously with regard to 2, Le, the capability of 02 in quenching excited states of sufficiently long lifetimes. In the case of 7, the process is so efficient that ambient levels of 02 completely kill off phosphorescence, even if the phosphor is enveloped by p-cyclodextrin. 

UV intensity measurements were made with an International Light 700A Research Radiometer. The measuring head was tightly covered with aluminum foil for zeroing, and then exposed to the lamp output under exactly the same conditions as the actual samples (i.e., same distance, angle, elevation, etc.). The results of these experiments were used to evaluate the quantum yield or efficiency of the photochemical process. Specifically, photolysis of AETSAPPE... 

The photo-thennal detoxification unit uses photo-thennal reactions conducted at temperatures higher than those used in conventional photochemical processes (200 to 500°C, rather than 20°C) but lower than combustion temperatures (typically greater than 1000°C). At these temperatures the developer claims that photochemical reactions are energetic enough to destroy wastes quickly and efficiently without producing complex and potentially hazardous by-products. 

While the aim of photochemical studies is generally to measure primary quantum yields, this is not always experimentally feasible. For example, NO reacts rapidly with N03 to form N02. Thus determination of 4il or (f>4b by measuring the NO and N02 formed can be complicated by this secondary reaction of NO with N03, and the measured yields of NO and N02 may not reflect the efficiency of the primary photochemical processes. 

In 1966, Chapman and co-workers proposed a nitro-nitrite photorearrangement as an efficient primary photochemical process for nitroarenes in which the nitro group is out of the plane of the aromatic rings. This is followed by dissociation into NO and a phenoxy-type radical ultimately quinones and other oxy products are formed (Chapman et al., 1966). 

Radical anions are produced in a number of ways from suitable reducing agents. Common methods of generation of radical anions using LFP involve photoinduced electron transfer (PET) by irradiation of donor-acceptor charge transfer complexes (equation 28) or by photoexcitation of a sensitizer substrate (S) in the presence of a suitable donor/acceptor partner (equations 29 and 30). Both techniques result in the formation of a cation radical/radical anion pair. Often the difficulty of overlapping absorption spectra of the cation radical and radical anion hinders detection of the radical anion by optical methods. Another complication in these methods is the efficient back electron transfer in the geminate cation radical/radical anion pair initially formed on ET, which often results in low yields of the free ions. In addition, direct irradiation of a substrate of interest often results in efficient photochemical processes from the excited state (S ) that compete with PET. 

As mentioned in the introduction, the ability to shape femtosecond laser pulses with unprecedented precision is the key to efficient control of photophysical and photochemical processes at the quantum level. In this section, we present the fundamentals of femtosecond pulse shaping and introduce specific pulse shapes that are used in the experiments and simulations presented in the following sections. We start with the electric field of a bandwidth-limited (BWL) femtosecond laser pulse written in terms of its positive frequency analytic signal... 

Before looking at the effect of the polymeric matrix on quantum yields and efficiencies of photochemical processes it is important to look first at variations which are due to the structure of the ketone chromophore itself which are observable regardless of whether the chromophore is in the solid, liquid, or gaseous state. The first of these is illustrated in Table II which illustrates the quantum yields for esters of dimethyl keto azelate (3). 

The dimethyl ester of this acid in solution shows a quantum efficiency photochemical products. On the other hand, when the same acid is copolymerized with a glycol to form a polymeric compound with molecular weight 10,000 the quantum yield drops by about two orders of magnitude, 0.012. The reason for this behavior appears to be that when the chromophore is in the backbone of a long polymer chain the mobility of the two fragments formed in the photochemical process is severely restricted and as a result the photochemical reactions are much reduced. If radicals are formed the chances are very good that they will recombine within the solvent cage before they can escape and form further products. Presumably the Norrish type II process also is restricted by a mechanism which will be discussed below. 

Two useful fluorescence parameters are the quantum yield and the lifetime. Quantum yield is a property relevant to most photophvsical and photochemical processes, and it is defined for fluorescence as in (1.101. More generally it is a measure of the efficiency with which absorbed radiation causes the molecule to undergo a specified change. So for a photochemical reaction it is the number of product molecules formed for each quantum of light absorbed ... 

Various metal nitrates, represented by silver nitrate, sensitize photopolymerization of AN, methaciylonitrile, a-chloroacrylonitrile, croto-nitrile and methyl methacrylate. The efficiency of photosensitization runs nearly parallel to the ease of reduction of the metal ion. Although there is little doubt that the monomer plays some role in the photochemical process, it is rather difficult to decide whether the primary act is direct oxidation of the monomer or electron transfer between metal ion and nitrate anion. 

In the case of a photochemical process the overall efficiency is given by the quantum yield5 which is defined as the number of molecules of photoproducts formed for each photon absorbed... 

The primary photochemical process that occurs when a photon is absorbed by the silver halide is the transfer of an electron to the conduction band. The quantum efficiency of this process is one. The efficiency of formation of a developable silver nucleus, however, depends on various chemical and physical secondary processes and may be much smaller. 

To assess the efficiency of the reactions listed in Table 7.2, Bolton et al. (1996) proposed a generally applicable standard for a given photochemical process. The proposed standard provides a direct link to the electrical efficiency. In this model, electrical energy per unit mass is calculated according to the quantum yield of the direct photolysis rate. Braun et al. (1997) calculated the quantum yield (O) according to Equation (7.13) ... 

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