Hole centres

As seen from Fig. 5, upon absoption of photons with the energy hv > Eg, an electron and hole centres are formed. They migrate to different sites on the PC surface, thus becoming spatially separated. Note, that what solid state physisists call surface electron and hole centers, in fact are some definite chemical species with strong reducing and oxidizing... 

The hole pitch (distance between the hole centres) l p should not be less than 2.0 hole diameters, and the normal range will be 2.5 to 4.0 diameters. Within this range the pitch can be selected to give the number of active holes required for the total hole area specified. 

From a chemist s viewpoint, the most important act of ionizing radiation (usually X-rays, y-rays or high energy electrons) is electron ejection. Initially the ejected electrons have sufficient energy to eject further electrons on interaction with other molecules, but the electrons ultimately become thermalised and then are able to interact "chemically". We consider first various reaction pathways for these electrons, and then consider the fate of the "hole" centres created by electron ejection. [We refer to electron-gain and electron-loss centres rather than to radical-anions and -cations since, of course, the substrate may comprise ions rather than neutral molecules. 

Specific Electron Capture. It is now customary to use solvents such as methanol (usually CD3OD) or methyl tetrahydrofuran (MTHF) as solvents if electron-capture by AB is required. These solvents form good glasses at 77 K, and for sufficiently dilute solutions of the substrate, AB, electron ejection occurs overwhelmingly from solvent molecules, so that AB+ centres are not formed. Electrons are fairly mobile, and hence AB radicals are formed provided AB has, effectively, a positive electron affinity. The hole centres, such as CD30D+, are not mobile because proton transfer to surrounding solvent molecules occurs rapidly at all temperatures. 

Optical bleaching sediment. Sunlight-reduced ESR signal intensity of the holes centres associated with A1 impurity and electron centres related to Ge impurities in quartz grains (sand). Whether sunlight bleached the optically sensitive centres completely is an issue in dating of sediments.45... 

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... 

Nonbonding oxygen hole centre (NBOHC) [Si04]+, Si043 ... 

A1 centre Al-related hole centre [A104]°, [Al3+h+]°, >Al-0 Ge centre Ge-related electron centre [Ge04 /Li]c°, [Ge4+e /Li+]c GeLi044 ... 

A series of papers has recently appeared in the literature concerning the dynamics of photo-induced formation of electron and hole centres at the surface of MgO powders.12 14 Monochromatic excitation of the sample with 282 nm photons leads to the creation of well-separated electron and hole centres at the surface which were monitored via EPR in term of a trapped hole (O ion) and a trapped electron according to the following equation ... 

Fig. 3.2. Two principal mechanisms of defect recombination in solids, (a) Complementary defect annihilation, r is the clear-cut (black sphere) radius, (b) distant tunnelling recombination due to overlap of wave functions of defects. Two principal kinds of hole centres - H and Vk...

As it is shown in Fig. 3.1(b) in Chapter 3, typical hole centres in alkali halides - H and centres being XJ - quasi-molecules oriented along the (110) axis are rather anisotropic which is observed experimentally, e.g., via polarized recombination luminescence [70, 71]. Their analog is a dumb-bell interstitial in many metals. 

Mobile H centres in alkali halides are known to aggregate in a form of complex hole centres [64] this process is stimulated by elastic attraction. It was estimated [65, 66] that for such similar defect attraction the elastic constant A is larger for a factor of 5 than that for dissimilar defects - F, H centres. Therefore, elastic interaction has to play a considerable role in the colloid formation in alkali halides observed at high temperatures [67]. In this Section following [68] we study effects of the elastic interaction in the kinetics of concentration decay whereas in Chapter 7 the concentration accumulation kinetics under permanent particle source will be discussed in detail. 

Kieffer et al., in 1971, observed a temperature-independent luminescence caused by the recombination of electron and hole centres in irradiated vitreous organic matrices [6], The authors suggested that the recombination proceeds via electron tunneling. 

Let us now consider the data about the spatial distribution of particles generated in vitreous matrices under the influence of radiolysis and photolysis. According to the data obtained with the help of the electron spin echo method [12], the character of this distribution substantially depends on the energy of electron and quanta by which the sample is irradiated. Thus, the positively charged hole centres which are formed in concentrated vitrified... 

The possibility of long-range electron tunneling in reactions of etr was first suggested for recombination reactions of e with hole centres. Direct experimental proof of the reality of this phenomenon was also obtained in studying this type of reaction. 

A theoretical model of the low-temperature decay of etr in MTHF discovered in ref. 30 was suggested in ref. 31. According to this model, the disappearance of et in y-irradiated MTHF at 77 K is due to electron tunneling from a trap to a hole centre. The form of the potential barrier for electron tunneling used in ref. 31 to analyze the curves of the decay of etr is represented schematically in Fig. 9(a). To evaluate the probability of tunneling per unit of time, the Gamow formula... 

Recombination reactions of trapped electrons with hole centres were the first chemical processes for which chemists succeeded in getting reliable experimental evidence for their occurring via electron tunneling over a large distance. Unfortunately, however, the initial distribution over distances between the reacting particles in the electron-hole centre pairs is, as a rule, known only approximately. This circumstance hinders considerably the detailed quantitative comparison of the kinetics observed with that theoretically expected for tunneling reactions. 

The results set forth in the present section indicate that in water-containing matrices the reactions of etr with acceptor additives, just as the reactions of the recombination of et with hole centres discussed above, proceed via the mechanism of temperature-independent electron tunneling only at sufficiently low temperatures. With increasing temperature the main contribution to the reaction rate starts to be made by temperature-dependent channels of tunneling. 

Ideas about the tunneling mechanism of the recombination of donor acceptor pairs in crystals seem to be first used in ref. 51 to explain the low-temperature of photo-bleaching (i.e. decay on illumination) of F-centres in single crystals of KBr. F-centres are electrons located in anion vacancies and are generated simultaneously with hole centres (centres of the Br3 type which are called H-centres) via radiolysis of alkali halide crystals. 

Numerous data about the processes of the tunneling recombination of radiation defects have been obtained in studies on tunneling recombination luminescence. The recombination luminescence of y-irradiated alkali halide crystals was discovered in the mid-1960s [58, 59] in studying the transfer of electrons from Ag and T1 atoms (electron donors) to Cl2 particles (electron acceptor). The Ag and T1 atoms are formed as a result of the action of irradiation on alkali halide crystals which contain Ag+ or Tl+ additives in amounts of about 10 3M. The electrons generated by the irradiation reduce the Ag4 or TU ions to Ag° or Tl° while the hole centres are stabilized in the form of the Cl2 ion occupying two anion positions in the lattice. The hole centres of this kind, whose structure is depicted schematically in Fig. 16, are referred to as Vk-centres. 

Similar results on the kinetics and temperature dependence of the recombination of electron and hole centres have been obtained [68-71] in studying highly dispersed samples of magnesium oxide MgO. As distinct from CaO, however, in MgO the hole centres are mainly stabilized on the surface (so-called Vs -centres) while the electron centres are stabilized both on the surface (Fs+ -centres) and in the volume (F1 -centres). After irradiation is over a slow recombination of radiation defects is observed... 

The kinetics of these processes in the time range 102-106s is well described by equations of the type illustrated by eqn. (30) of Chap. 4 [69, 70]. With a decrease in temperature from 77 to 4.2 K the rate of recombination of electron and hole centres decreases by less than a factor of 100, which corresponds to a formal activation energy of less than 40 cal mol i.e. the reaction proceeds practically without activation. In the absence of contact between the F+-centres stabilized in the volume and the Yr-centres stabilized on the surface these data point to the tunneling mechanism of recom-... 

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