Centers with trapped electrons

The second important effect is that irradiation absorption generates active states of the photoadsorption centers with trapped electrons and holes. By definition (Serpone and Emeline, 2002) the photoadsorption center is a surface site which reaches an active state after photoexcitation and then it is able to form photoadsorbed species by chemical interaction with substrate (molecules, or atoms, or ions) at solid/fluid interface. In turn, the active state of a surface photoadsorption center is an electronically excited surface center, i.e. surface defect with trapped photogenerated charge carrier that interacts with atoms, molecules or ions at the solid/gas or solidfiquid interfaces with formation of chemisorbed species. ... 

In the field of metal-mediated and metal-catalyzed reactions, similar issues can be critical. Radical reduction by a HAT must be slow enough to allow any desired reaction to occur, but the HAT must also be fast enough to prevent any undesired trapping of the radicals by metal centers with unpaired electrons. 

Afterglow is due to the phenomenon that radiant recombination of electrons and holes is sometimes considerably delayed due to trapping of electrons or holes. Figure 3.28 gives a simple illustration. A semiconductor contains, next to the luminescent centers, also centers which trap electrons. Excitation with energy above E yields free electrons and holes. Let us assume that the holes are trapped by the luminescent center, whereas the electrons in the conduction band recombine with the holes yielding emission. 

Transition metal salts trap carbon-centered radicals by electron transfer or by ligand transfer. These reagents often show high specificity for reaction with specific radicals and the rates of trapping may be correlated with the nucleophilicity of the radical (Table 5.6). For example, PS radicals are much more reactive towards ferric chloride than acrylic propagating species."07... 

Since the original studies of F centers many other color centers have been characterized that may be associated with either trapped electrons or trapped holes. These are called electron excess centers when electrons are trapped and hole excess centers when holes are trapped. 

All varieties of color are mainly connected with two main absorption bands in the violet and yellow parts of the spectrum. The secondary bands are also present - in blue and green diapasons. The main absorption bands are connected with F and M-centers. The first one is anion vacancy, which traps electrons and the second is two neighboring anion vacancies with two trapped electrons. The short-wave band in fluorite is generated by mutual absorption of F and M-centers, while the long-wave band is connected with M-center absorption only. In the green varieties the REE (Sm ", Yb and Dy ) are also appreciable. Besides that, the centers O2, O3 and (Y, TR)02 sometimes have influence with resulting yellow and pink colors (Platonov 1979 Krasilschikova et al. 1986). 

Two further mechanisms are known to trap electronic charge in thin films intermolecular and resonance stabilization. In resonance stabilization, electron attachment to a molecular center produces an anion in a vibrationally excited state that is then de-excited by energy exchange with neighboring molecules. When the initial anion ground state lies below the band edge or lowest conduction level of the dielectric, then the additional electron may become permanently trapped at the molecular site. In this case, a permanent anion is formed (e.g., the case of O2 [220]). Intermolecular stabilization refers... 

In this connection it should be remembered that we can distinguish between recombination centers and traps. On the one hand, electron or hole traps can be considered as states from which trapped electrons or holes will with relatively high probability be reexcited thermally into the conduction or valence band. 

A description of the emission and capture processes at a trap will be useful before discussing the various experimental methods. Figure 1 depicts the capture and emission processes that can occur at a center with electron energy ET. The subscripts n and p denote electron and hole transitions, and the superscripts t and differentiate between thermally and optically stimulated processes. It is assumed here that only thermal capture processes are occurring. 

Fig. 14. Some Auger processes involving one-free carrier (boles as illustrated) The case of two trapped electrons on the same center is shown in (a), and the situation for trapping on nearby centers is shown in (b). The case of an exciton (isoelec-tronic) type center, with electron recombination to the trapped hole is shown in (c), and recombination with a free hole in (d) [note that in practice these two processes have to be considered in parallel (see, for example, Neumark, 1973)].

Regarding centers that trap two electrons, these have been analyzed by Neumark (1973) on the basis of He type wave functions and, more recently, by Jaros (1978) and by Riddoch and Jaros (1980). One conclusion from this work is that quantitative values of Auger cross sections depend strongly on the band structure, with the transition matrix element (MA) given by Neumark (1973) as... 

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