Mechanism of photogeneration

In this chapter we have attempted to summarize and evaluate scientific information available in the relatively young field of microwave photoelectrochemistry. This discipline combines photoelectrochemical techniques with potential-dependent microwave conductivity measurements and succeeds in better characterizing the behavior ofphotoinduced charge carrier reactions in photoelectrochemical mechanisms. By combining photoelectrochemical measurements with microwave conductivity measurements, it is possible to obtain direct access to the measurement of interfacial rate constants. This is new for photoelectrochemistry and promises better insight into the mechanisms of photogenerated charge carriers in semiconductor electrodes. 

The second important mechanism of photogeneration is photostimulation of long-lived stable cation-radicals of the donor PI fragments, representing the hole (major carriers) captured by deep centers (PSC). Accumrrlation of the cation-radieals in the dark and photo processes leads to the dependenee of photovoltaic characteristics on the number of charge discharge cycles of the sample. 

Pai DM, Enck R (1975) Onsager mechanism of photogeneration in amorphous selenium. Phys Rev B 11 5163... 

The calculated (/>ct and d values are given in Table 10.9 also. The increase of (/ with increasing TNF coverage indicates that the TNF aids not only the exciplex formation but also the e-h pair formation. The mechanism of photogeneration for X-H2PC in the presence of an electron donor or acceptor is therefore extrinsic, namely excitation of X-H2PC results in an... 

Pfleger et al. [369] have studied photoconduction in undoped poly(phenylacetylene) which they prepared by coordination polymerization of phenylacetylene using the metathesis catalyst WOCU/Pl Sn. The polymer thus obtained was predominantly in the c/5-transoidal form, as demonstrated by IR spectra, and had a molecular weight of (A/ ) 91 000. The photoconduction threshold has been detected at 410 nm, although absorption of the film extended up to 550 nm. It is suggested that the mechanism of photogeneration is intrinsic by its nature. The formation of initial charge carrier pairs occurs by an exciton autoionization process [38]. From the temperature dependence, as well as Irom the field dependence of the quantum yield, the pair separation distance was established to be ca 2.2 nm. 

Choi W, MR Hoffman (1997) Novel photocatalytic mechanisms for CHClj, CHBrj, and CCljCO degradation and fate of photogenerated trihalomethyl radicals on TiOj. Environ Sci Technol 31 89-95. 

Magnetic field effects on the reaction kinetics or yields of photochemical reactions in the condensed phase have been studied [20-23]. They have proved powerful for verifying the mechanism of photochemical reactions including triplet states. Previously, we obtained photogenerated triplet biradicals of donor-acceptor linked compounds, and found that the lifetimes of the biradicals were remarkably extended in the presence of magnetic fields up to 1T [24]. It has been reported that Cgo and its derivatives form optically transparent microscopic clusters in mixed solvents [25,26]. The clustering behavior of fullerene (C o) is mainly associated with the strong three-dimensional hydrophobic interactions between the C o units. Photoinduced... 

Klimov, V. I. 2006. Mechanisms for photogeneration and recombination of mul-tiexcitons in semiconductor nanocrystals Implications for lasing and solar energy conversion./. Phys. Chem. B 110 16827-16845. 

The overall process performance, as measured by photon efficiency (number of incident photon per molecule reacted, like the incident photon to current conversion efficiency, or IPCE, for PV cells), depends on the chain from the light absorption to acceptor/donor reduction/oxidation, and results from the relative kinetic of the recombination processes and interfacial electron transfer [23, 28]. Essentially, control over the rate of carrier crossing the interface, relative to the rates at which carriers recombine, is fundamental in obtaining the control over the efficiency of a photocatalyst. To suppress bulk- and surface-mediated recombination processes an efficient separation mechanism of the photogenerated carrier should be active. 

The photogenerated electrons and holes can recombine in bulk or on the semiconductor surface releasing energy in the form of heat or a photon. The electrons and holes that migrate to the semiconductor surface without recombination can, respectively, reduce and oxidize water (or the reactant) and are the basic mechanism of photocatalytic hydrogen production, see Fig. 6.3. 

Two main models are usually discussed for the mechanism of the spectral sensitization. The excitation of the sensitizer by absorbed light and electron transfer from the excited sensitizer to the semiconductor is the first model. The alternative mechanism consists of the transfer of the excitation energy from the sensitizer to the semiconductor. This energy is used for photogeneration of the charge carriers in the sensitized photoconductor. In the first case the excited singlet level of the sensitizers has to be located above the conduction band of the semiconductor for realization of the electron transfer. For hole transfer the basic sensitizer level has to be located lower than the valence band of the sensitized photoconductor. The energy transfer mechanism does not need a special mutual location of the semiconductor and sensitizer levels. 

The reaction of photogenerated singlet oxygen with 2,5-dimethylthiophene results in ring opening (70TL791, 70TL795) the c/s-sulfine (149) and the rrans-diketone (150) are the main products. The mechanism of this reaction has been speculated upon. Since (149) itself is photoinert, the two products must arise by different pathways. The key intermediate is (148), formed by cycloaddition of the thiophene with 02 this can lead to the two observed products as shown in Scheme 28. An alternative pathway for the sulfine would be by attack of 02 on sulfur to yield (151), followed by the transformations shown. 

The addition of Fe ions to the solution also increased the rate of photo-catalytic degradation. The TOC reduction was 80% in the presence of Fe ions and 10% in the absence of Fe ions. pH is the key parameter in determining the photocatalytic degradation of anilines as well as other compounds previously studied. The high photocatalytic degradation near the pHpzc has also been observed for other compounds studied with Ti and Zn. The major mechanism is the attack of photogenerated OH radicals on the aniline molecule under alkaline conditions. The addition of Fe ions to the solution can enhance the formation of OH radicals and lead to higher photocatalytic rates. 

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