H-centre

With the exception of adamantane and few related compounds [1-5] in which dichlorocarbene reacts at the tertiary C-H centre, the yields for the majority of insertion reactions into hydrocarbons are low and of little synthetic value (Table 7.1). Reaction also occurs in low yield at benzylic C-H sites [1, 6, 7] and, in the case of simple alkanes, the insertion reaction is promoted by alkoxy groups [1,6-14], Thus, whereas methylcyclohexane produces only 4% yield of the l-dichloromethyl-l-methylcyclohexane, the corresponding yield with 1-methoxycyclohexane is 13% [6], Similarly, the low yielding reaction of 1-methoxyadamantane with dichlorocarbene produces l-dichloromethyl-3-methoxyadamantane by insertion into the tertiary C-H site and (2,2-dichioroethoxy)adamantane by reaction at the primary C-H site, which is activated by the methoxy group. No reaction occurs at the secondary C-H sites [2],... 

Many other colour centres have now been characterized in alkali halide crystals. The H-centre is formed by heating, for instance, NaCl in CI2 gas. In this case, a [CI2] ion is formed and occupies a single anion site (Figure 5.24(b)). F-centres and H-centres are perfectly complementary—if they meet, they cancel one another out ... 

Irradiation of all kinds of solids (metals, semiconductors, insulators) is known to produce pairs of the point Frenkel defects - vacancies, v, and interstitial atoms, i, which are most often spatially well-correlated [1-9]. In many ionic crystals these Frenkel defects form the so-called F and H centres (anion vacancy with trapped electron and interstitial halide atom X° forming the chemical bonding in a form of quasimolecule X2 with some of the nearest regular anions, X-) - Fig. 3.1. In metals the analog of the latter is called the dumbbell interstitial. 

This interaction arises from the overlap of the deformation fields around both defects. For weakly anisotropic cubic crystals and isotropic point defects, the long-range (dipole-dipole) contribution obeys equation (3.1.4) with a(, ip) oc [04] (i.e., the cubic harmonic with l = 4). In other words, the elastic interaction is anisotropic. If defects are also anisotropic, which is the case for an H centre (XJ molecule), in alkali halides or crowdions in metals, there is little hope of getting an analytical expression for a [35]. The calculation of U (r) for F, H pairs in a KBr crystal has demonstrated [36] that their attraction energy has a maximum along an (001) axis with (110) orientation of the H centre reaching for 1 nn the value -0.043 eV. However, in other directions their elastic interaction was found to be repulsive. 

Fig. 3.4. Processes defending the survival probability of F centres in alkali halide crystals 1 -tunnelling recombination of close F, H defects, 2 - their annihilation, 3 - trapping of mobile H centre at impurity, 4 - formation of immobile dimer centre, 5 - H-centre leaves its geminate partner in random walks on a lattice.

During their diffusive walks, H centres can either approach their own F centres to within the distance r ro and recombine with them in the course of the so-called geminate (monomolecular) reaction or leave them behind in their random walks. Some of these H centres recombine with foreign F centres, thus participating in bimolecular reactions. The rest of the H centres become trapped by impurities, dislocations, or aggregate in the form of immobile dimer H2 centres thus going out of the secondary reactions as shown in Fig. 3.4. In other words, the survival probability of the geminate pairs (F centres) directly defines the defect accumulation efficiency and thus, a material s sensitivity to radiation. 

They have calculated the continuous diffusion equation (3.2.30) with U(r) = -a/r3 for several kinds of nn F, H centres in the crystalline lattice. Figure 3.9 demonstrates well that both defect initial separation and an elastic interaction are of primary importance for geminate pair recombination kinetics. The 3nn defects are only expected to have noticeable survival probability. Its magnitude agrees well with equation (3.2.60). 

In a more realistic and complicated case, defects at several relative distances are produced. As is easily seen from Fig. 3.10, the step-structure on the annealing curve is pronounced to be worse, the greater the distance of an H centre from a vacancy it is no longer easily seen even for 2nn and 3nn, despite their elastic interaction. A comparison of theoretical calculations with actual experimental data as well as a more detailed treatment of the F, H annealing kinetics is given in [77]. 

Making use of the elastic constant entering equation (3.1.4) for F, H centres in KBr a = 3 eVA3 [69], one can estimate easily that the effective radius of annihilation stimulated by elastic interaction, (equation (4.2.29)) varies... 

In the case of F, H centres in alkali halides their maximum attraction and repulsion correspond to the configurations 1 and 2 in Fig. 4.8(b). Even for the nearest F, H pair in KBr, the attraction energy is rather small ss 0.04 eV. 

Fig. 4.8. Elastic interaction of of crowdion with vacancy in metals (a) and F, H centres in alkali halides (b). Configuration 1 is energetically the most favourable with Eml = -0.043 eV.

Here, heating the samples above 120 K leads to the release of H centres earlier trapped by the substitutional Na+ ions (Ha centres [97]). These mobile H centres are believed to destroy F centres which leads to the irreversible decrease of the I starting from the very moment of the temperature increase (Fig. 4.16(b)) after the temperature again decreases, the luminescence intensity does not stand back in line with the basic reference dependence of I(t) vs. t observed before the temperature cycle. The influence of excitation time r duration upon the tunnelling luminescence decay kinetics has been analysed in [88]. It is shown that after t 3r, 7(f) oc t a holds quite well. This is why in actual experiments the temperature stimulation was only imposed only at t 800 s. 

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. 

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. 

Making use of the elastic constant entering equation (3.1.4) for F, H centres inKBr a = 3eVA3 [69], one can estimate easily that the effective radius of annihilation stimulated by elastic interaction, (equation (4.2.29)) varies from 11 A down to 7 A as the temperature increases from 40 K to 200 K (and then is independent of the annihilation radius R 4 A). On the other hand, the effective radius of tunnelling recombination, equation (4.2.17), decreases from 10 A (at 40 K) down to 5 A (60 K). It coincides with the elastic radius, 7 eh at37 K, where diffusion is very slow and the binary approximation, equation (4.2.19), does not hold any longer. 

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