Hole self-trapping

The smaller value of i found experimentally suggests that the coupling constant g is larger than we assumed. In our model of DNA we considered hole self-trapping due to interaction with longitudinal vibrations only. As discussed in Sect. 2.1, other degrees of freedom may also contribute to the coupling constant. 

L. Kantorovich, A. Stashans, E. A. Kotomin, and P. W. M. Jacobs, Int. J. Quantum Chem., 52, 1177 (1994). Quantum-Chemical Simulations of Hole Self-Trapping in Semi-Ionic Crystals. 

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 energy E will necessarily have this minimum, but its value at this point can be positive or negative only in the latter case will a stable self-trapped particle (i.e. a small polaron) form. This is most likely to occur for large effective mass, and thus for holes in a narrow valence band or for carriers in d-bands. If the polaron is unstable then there is practically no change in the effective mass of an electron or hole in equilibrium in the conduction or valence band. 

It will be seen that a barrier exists resisting self-trapping this has been observed as a time delay by Laredo et al (1981,1983) for holes in AgCl, indicating a barrier height of 1.8 meV. 

The triangular planar (D3h symmetry) CO/ molecular ion with 24 electrons (AB324-type) in CaC03 is easily ionized by radiation to electron and hole centres self-trapped in the lattice or an oxygen vacancy type C02 molecular ion at the anon site. Molecular orbital schemes based on the general scheme of AB3 molecules with 25,24 and 23 electrons for atoms A (B, C, Si, N, P, As and S) and B (O) characterize their specific -factor. Hence, the anisotropic -factor of these radicals estimated from the powder spectrum has been to identify the radical species.1... 

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. 

The commonly used scheme of energy relaxation in RGS includes some stages (Fig.2d, solid arrows). Primary excitation by VUV photons or low energy electrons creates electron-hole pairs. Secondary electrons are scattered inelastically and create free excitons, which are self-trapped into atomic or molecular type centers due to strong exciton-phonon interaction. 

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... 

Excitons. Localization of the excitons occurs via the process of self-trapping to produce so-called Self Trapped Excitons (STE). For a description of STE s we refer to Figure 2 in which are sketched three typical configurations for STE s in an M+X crystal. Toyozawa (L5) discusses the formation of STE s in which the electron and hole are localized concentrically (STE 1 and STE 2) or eccentrically (STE 3). In types 2 and 3 the hole is trapped on an X2 molecule and the strong coulombic repulsion between it and the trapped electron make this type of STE highly unstable. 

When irradiated at room temperature the non-locally compensated centers trap electrons to yield Ce + in 0 symmetry, while the holes become self-trapped producing perturbed centers (24), thus ... 

Si and Fe3+---0 -Si centers in quartz. However, they may also form without attendant ionizing radiation. One possiblity is the redox conversion of OH or Si-OH pairs into molecular H2 plus peroxy. Such OH or Si-OH pairs derive from traces of dissolved "water . Peroxy anions, 022 , or peroxy entities, X/°° Y (X, Y=Si, Al,Fe3+...) are equivalent to self-trapped positive hole pairs. 

An electron or a hole injected on such a chain cannot then be present as a charged soliton. However, here again the electron-phonon interaction is important. If a charge is put on a true one-dimensional system, it always becomes dressed by a lattice distortion that is, it will self-trap and form a polaron [68], an extension that depends on the ratio of the electron-phonon coupling to the electronic intersite coupling t. Presumably, the time needed to relax the one-dimensional lattice around the charge is very short, on the order of one vibrational period or 100 fs. 

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. 

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 ... 

Electronic point defects, displaced electrons, almost always exist in connection with atomic point defects. A purely electronic defect, the so-called self-trapped electron trapped by induced polarization in a solid, has been suggested by Landau 29) but never found. If an incoming quantum imparts enough energy to an electron of one of the atoms of a solid, the electron will be freed from the atom and can wander through the solid. If it is not to be recaptured by the radiation-produced positive ion, it must be trapped at some other point in the solid, one with an effective positive charge. This will almost always be an atomic defect, specifically a negative ion vacancy or an impurity of suitable electron affinity relative to that of the host solid. When an electron is thus removed from an atom, the vacancy in the electronic structure is termed a positive hole. Such a hole has mobility like that of an electron... 

Holes are rapidly self-trapped in AgCl and their effective masses are thought to be very large. 

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