Self-trapped hole

Hole centre, COT A self trapped hole forms a planar molecule, C03 (D3h symmetry) with 23 electrons (AB323 -type) gives... 

A theoretical analysis of the experimental kinetics for Vk centres in KC1-Tl, as well as for self-trapped holes in a-Al203 and Na-salt of DNA, is presented in [55]. The fitting of theory to the experimental curves is shown in Fig. 4.4. Partial agreement of theory and experiment observed in the particular case of Vk centres was attributed to the violation of the continuous approximation in the diffusion description. This point is discussed in detail below in Section 4.3. Note in conclusion that the fact of the observation of prolonged increase in recombination intensity itself demonstrated slow mobility of defects. In the case of pure irradiated crystals, it is a strong... 

Fig. 4.4. (a) The transient kinetics of tunnelling luminescence intensity increase due to hypothetical self-trapped holes in a-A Oj [53, 55], Dash-dotted line 1 - experimental, full line 2 - theoretical. Temperature increment 198.2 K —> 201.5 K (two curves above), and temperature decrease 204.1 K 201.5 K (two curves below), (b) Same for the Na-salt of DNA. Dash-dotted line 1 - experimental, full lines - theoretical for three-dimensional recombination (curve 2) and one-dimensional (curve 3). Temperature increase 141 —> 146 K. 

The electronic properties of RGS have been under investigation since seventies [3-7] and now the overall picture of creation and trapping of electronic excitations is basically complete. Because of strong interaction with phonons the excitons and holes in RGS are self-trapped, and a wide range of electronic excitations are created in samples free excitons (FE), atomic-like (A-STE) and molecular-like self-trapped excitons (M-STE), molecular-like self-trapped holes (STH) and electrons trapped at lattice imperfections. The coexistence of free and trapped excitations and, as a result, the presence of a wide range of luminescence bands in the emission spectra enable one to reveal the energy relaxation channels and to detect the elementary steps in lattice rearrangement. 

Two Cr ions (represented by large circles) move together, with a hole, to form a CI2 ion. This self-trapped hole , also called a V, center, is immobile. 

The STH has a high cross-section for recombination. Its short lifetime precludes its detection in pure AgCl by conventional EPR methods, except at temperatures close to 1.2 K [169]. In samples doped with deep electron traps, recombination is suppressed and an EPR spectrum from (AgCl6)4 can be observed up to about 50 K, at which point the hole becomes mobile and is annihilated. Although ENDOR data from the self-trapped hole center have... 

The EPR spectrum of the intrinsic STH was also recorded by ODMR in studies of nominally pure AgCl [171,172] and was later identified [173-175]. The self-trapped hole and shallowly trapped electron undergo donor-acceptor pair recombination which contributes to a blue-green (500 nm) luminescence from AgCl [69]. The species observed in the ODMR spectrum has g-factors, hyperfine, and superhyperfine matrices that are identical, within experimental error, to those observed earlier by EPR methods [68]. 

The STH resonance is enhanced in ODMR spectra by the addition of parts per million of known electron trapping dopants such as Ir3+ [176], Ni2 + [177], and Rh3+ [178]. This is probably because these dopants enhance distant pair formation at the expense of exciton formation [179,180]. Extrinsic self-trapped hole species have also been observed in the ODMR spectra of AgClj Br samples (see below) [111, 181,182],... 

The energy estimated from the analysis of the emission assuming pair recombination may suffer the same problem but it would be, again, an overestimate. Thus it is concluded that the relaxed, self-trapped hole lies, at most, 400 meV above the valence band. The data from the EPR studies of the decay of the self-trapped holes yielded a thermal trap depth equal to or greater than lOOmeV [164],... 

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]. 

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). 

H2 and H adsorption on simple metal oxides has been studied even less than that on transition metal oxides. H2 adsorption onto various defects on the MgO(lOO) surface has been treated theoretically using defect lattice techniques, including the relaxation of the lattice around the defects. Surface defects (F- and V-centers and self-trapped holes) were all found to activate dissociative chemisorption, resulting in the formation of OH radicals [127, 128]. This is in general agreement with the observed catalytic activity of MgO after the creation of F-centers by X-rays... 

Griscom, D. L., Self-trapped holes in amorphous silicon dioxide, Phys. Rev. B 40, 4224 (1989). 

Yamaguchi, M., Saito, K., and Ikushima, A. J. Fictive-temperature-dependence of photoinduced self-trapped holes in a-SiOj, Phys. Rev. B 68, 153204 (2003). 

Bennebroek MT, van Duijn-Arnold A, Schmidt J et al (2002) Self-trapped hole in silver chloride crystals. A pulsed EPR/ENDOR study at 95 GHz. Phys Rev B 66 054305... 

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