Self-trapped hole state

The lifetime of the impurity center produced by electron trapping is obviously important to the photographic process. It is affected by the identity of the central metal ion, its valence state, the composition of the ligand shell and the composition of the host lattice. The first example of a ligand effect was reported for self-trapping of photoholes in bromide-doped AgCl (see above) [181]. The lifetimes and trap depths of the self-trapped hole states [AgClj jBr ]4- increased as x increased [181]. 

In Part IV, the A center is identified as a trapped hole state, but its detailed nature is also not clear. Although a three-center bond model has been proposed (Morigaki et al, 1980a), we think that the A center corresponds to the self-trapped hole state proposed by Tsang and Street (1979). 

The stability of the self-trapped hole center in AgCl is enhanced by its association with bromide ion impurities and its recombination cross-section is reduced. As a consequence, the lifetimes of electron states are increased. For example, EPR signals from both shallowly trapped electrons and STHs can be detected during bandgap exposures at temperatures well above 50 K, a situation that does not occur in the pure material [177]. 

It has been shown theoretically that an extra electron or hole added to a one-dimensional (ID) system will always self-trap to become a large polaron [31]. In a simple ID system the spatial extent of the polaron depends only on the intersite transfer integral and the electron-lattice coupling. In a 3D system an excess charge carrier either self-traps to form a severely locahzed small polaron or is not localized at all [31]. In the literature, as in the previous sections, it is frequently assumed for convenience that the wavefunction of an excess carrier in DNA is confined to one side of the duplex. This is, of course, not the case, although it is likely, for example, that the wavefunction of a hole is much larger on G than on the complementary C. In any case, an isolated DNA molecule is truly ID and theory predicts that an excess electron or hole should be in a polaron state. 

The ejection of atoms or molecules from the surface of solid in response to primary electronic excitation is referred to as electronically stimulated desorption (ESD) or desorption induced by electronic transitions (DIET). Localization of electronic excitations at the surface of RGS induces DIET of atoms both in excited and in ground states, excimers and ions. Most authors (see e.g. Refs. [8,11,23,30] and references therein) discuss their results on DIET from RGS in terms of three different desorption mechanisms namely (i) M-STE-induced desorption of ground-state atoms (ii) "cavity-ejection" (CE) mechanism of desorption of excited atoms and excimers induced by exciton self-trapping at surface and (iii) "dissociative recombination" (DR) mechanism of desorption of excimers induced by dissociative recombination of trapped holes with electrons. 

Direct verification of DR-mechanism of DIET was provided [21] by combining the state-selective photoexcitation of the sample and the controlled thermally induced release of electrons from electron traps (Fig.9a). In RGS, after electron-hole pair creation at selective excitation by photons with energies E>Eg, the hole may survive and be self-trapped if the electron is captured by any kind of traps [32], In solid Ar at T>2 K the main part of electron traps is not active [12], the electron-hole recombination occurs before self-trapping the holes, and, therefore, the concentration of W-band emitting centers decreases (Fig.9a). On the contrary, the heating... 

What is the nature of the charge-separated states Are those self-trapped states [24], are there multiple states stabilizing holes or electrons in different configurations ... 

Consider as a specific example KCI, a very simple compound indeed. In Chapter 2, its lowest optical absorption band was mentioned to be due to the 3p -> 3p 4s transition on the Cl ion. The excited state can be considered as a hole on the Cl ion (in the 3p shell) and an electron in the direct neighbourhood of the Cl ion, since the outer 4s orbital spreads over the K inns. Now we consider what happens after the absorption process. The hole prefers to bind two Cl" ions forming a Vk centre this centre consists of a C " pseudomolecule on the site of two O" ions in the lattice. The electron circles around the Vk centre. In this way a self-trapped excitnn is formed. An exciton is a state consisting of an electron and a hole bound together. By the relaxation process (Cl" Vk.c) the exciton has lowered its energy and is now trapped in the lattice. 

Fig. 3.7. Schematic representation of relaxed excited states in an alkali halide. a ground slate h-. self-trapped cxciton consisting of a centre and an electon c F.H pair centre. The electron is represented by its orbit (drawn line) marked by the letter e, the CIJ pseudomolecule (i.e. the trapped hole) by CI-CI. See also text...

This example shows clearly that the emission process is very different from the (simple) absorption process. For all details the reader is referred to the literature [SJ. Finally we draw attention to the fact that the life time of the relaxed self-trapped exciton in the alkali halides is longer 10 s) than expected for an allowed transition (I0 - I0 "s). This is ascribed to the fact that the emitting state contains an amount of spin triplet character. Such a triplet state arises when the spins of the electron and the hole are oriented parallel. The emission transition becomes (partly) forbidden by the spin selection rule (see Chapter 2). 

An additional complication arises from the fact that the probability of an electron (or hole) being self-trapped due to the electron - phonon interaction increases strongly as the electronic wave function shrinks in size to the order of atomic dimensions (Emin, 1982). A consequence of this is that electrons in disorder-induced localized states are believed to be more susceptible to small polaron formation and self-trapping than are ordinary extended-state electrons (Emin, 1984 Cohen et al, 1983). Thus, not only does the disordered structure of amorphous semiconductors introduce new physical phenomena, namely, the mobility edge, but also the effect of known phenomena, such as the electron - phonon interaction, can be qualitatively different. 

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