F-centres

F centre An anionic sice in a crystal occupied only by an electron, e.g. NaCl plus Na vapour gives blue Naj, . C1 containing F centres. 

Size-selected palladium atoms were deposited on an in. sv /M-prepared MgO(lOO) thin film at 90 K the palladium surface concentration was about 1% of a monolayer. Comparison of ab initio calculations and FTIR studies of CO adsorption provided evidence for single Pd atoms bond to F centres of the MgO support with two CO molecules attached to each palladium atom.24... 

The chemistry of a carbon(-f) centre is best looked at through the chemistry of carbon in a ketone or an aldehyde which we can write... 

If the hfs of the nucleus under consideration is not resolved in the EPR spectrum, all nuclear spin states are simultaneously saturated and a sign determination using ENDOR line intensities is not possible. In this case the relative signs may sometimes be determined from second order hf contributions. This method has been applied by DuVarney and Spaeth74) to determine the sign of the 41K electric quadrupole moment using F centres in KC1. 

An extensive study of the Friedel-Crafts phenylations of a wide variety of aminophosphazenes have been conducted [230-233]. Reactions at the PCI2 centre is generally sluggish. With the bulky secondary amino derivatives, hydrocarbon formation is the competing process [232]. Aryl fluorophosphazenes undergo Friedel-Crafts arylation readily [182, 234-236]. These reactions have been used to convert a P(Ar)F centre to a P(Ar)(Ar ) centre (Eqs. 43,44) [234]. 

FIGURE 2.6 Two F-centred unit cells with the planes shaded. 

FIGURE 2.7 The i/7/7 reflection from an F-centred cubic lattice. 

Subsequently, it was found that F-centres can also be produced by heating a crystal in the vapour of an alkali metal this gives a clue to the nature of these defects. The excess alkali metal atoms diffuse into the crystal and settle on cation sites at the same time, an equivalent number of anion site vacancies are created, and ionisation gives an alkali metal cation with an electron trapped at the anion vacancy (Figure 5.24). In fact, it does not even matter which alkali-metal is used if NaCl is heated with potassium, the colour of the F-centre does not change because... 

FIGURE 5.24 (a) The F-centre, an electron trapped on an anion vacancy ... 

The trapped electron provides a classic example of an electron in a box . A series of energy levels are available for the electron, and the energy required to transfer from one level to another falls in the visible part of the electromagnetic spectrum, hence the colour of the F-centre. There is an interesting natural example of this phenomenon The mineral... 

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 ... 

A crystal containing F-centres contains anion vacancies. We would expect, therefore, that the density would be lower i w. that of the colourless crystal. 

Trapped electron et A self-trapped electron like an F centre in alkali halides is observed at g = 2.001 with a linewidth of 4.3 mT mostly in glassy ice doped with alkaline ions. 

In ionic crystals showing semiconducting or insulating properties, other defect structures such as F centres, V centres, and Koch-Wagner type are well known, but descriptions of these defect structures are not included here. 

Within the pulse duration, polarons and bipolarons dissociate into Drude-type electrons and the subsequent bleach recovery at various probe wavelengths indicates the reformation of those species. On the other hand, the equilibrated absorption spectrum of Drude-type electrons is substantially red-shifted [7] compared to the F-centre absorption band. According to our mechanism polarons and bipolarons form Drude-type electrons after ultrafast excitation which leads to an increase of the transient absorption in the NIR region. 

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. 

As it is known, I centres are the most mobile radiation-induced radiation defects in alkali halides and therefore they play an essential role in low-temperature defect annealing. It is known, in particular, from thermally-stimulated conductivity and thermally-stimulated luminescence measurements, that these centres recombine with the F and F electron centres which results in an electron release from anion vacancy. This electron participates in a number of secondary reactions, e.g., in recombination with hole (H, Vk) centres. Results of the calculations of the correlated annealing of the close pairs of I, F centres are presented in Fig. 3.11. The conclusion could be drawn that even simultaneous annealing of three kinds of pairs (Inn, 2nn and 3nn in equal concentrations) results in the step-structure of concentration decay in complete agreement with the experimental data [82]. 

If half the Bohr radius of the electron centre ro is known (e.g., for F-centres in alkali halides from ESR experiments, see, e.g., [56]), the pre-exponential diffusion coefficient Do could be found. 

Fig. 4.6. Schematic pattern of the tunnelling recombination of positively changed Vk centre with neutral electron centre (e.g., F centre) (a) and with oppositely charged activator atom (e.g., Tl°) (b). In the second case the Coulomb field traps Vk at long distance R n, but electron transfer itself occurs at much shorter distance.

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