Oxygen Hole Centres

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

Apart from other published experiments [4.57] related to EURECA, mission radiation and recording of EPR spectra have been done at room temperature (see spectrum 3 in Fig. 4.61). Spectra 1 and 2 result from samples irradiated at 77 K, recorded at 170 K and irradiated at 77 K, annealed at 440 K, and recorded at 170 K, respectively (published in [4.57]). The main, quite complicated signal around gfree consists basically of a variety of the so-called oxygen hole centres (OHCs) close to aluminium or silicon. Aluminium has a natural abundance of 100% AP with 1 = 5/2, yielding a hyperfine coupling. In accordance with other publications it has been shown that all the A1 OHCs are situated in the h-quartz s.s. crystal phase of Zerodur . The computer simulation of the spectrum needed no distribution in the spin Hamiltonians as is necessary in random networks. 

The photon absorption is caused by different effects in glasses and is still not fully understood. Nevertheless, three main possibilities for the absorption are considered and explained for OH-group free and OH-group containing silica glass [138]. Deep UV (DUV) absorption can take place at defect centres, such as the E -centre, the non-bridging oxygen hole centre (NBOHC) and on peroxy radicals. 

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... 

Other groups assume that the trapped hole is an oxygen radical centred at the surface of the titanium dioxide particle, having an energy state lower than the valence band edge of the semiconductor (reaction 9) [32],... 

Typical point defects present at the Si02 surface are the so called E centres, holes trapped at oxygen vacancies, and Si dangling bonds. These latter defects are particularly important when present at the Si/SiOz interface because they markedly affect the electrical properties of electronic devices. These defects, which are also known as Pb centres, have been widely investigated in the past. Recently however, the microscopic origin of these defects has been unravelled by means of a sophisticated UHV-ESR system by Futako et al, 178 who elucidated the formation processes of interface dangling bonds (Pb centres) during the initial oxidation of a clean Si(lll) surface. After oxidation of one or two Si layer(s), the... 

We saw in Fig. 6-30 the conversion of ethylene oxide to crown ethers upon reaction with appropriate metal salts, and demonstrated that the hole sizes of the products corresponded to the ionic radius of the template ion. However, lest we become over-confident, it should be pointed out that the major product from the reaction of ethylene oxide with caesium salts (r = 1.67 A) is not the expected 21-crown-7 with a hole size of about 1.7 A) but 18-crown-6 (hole size, 1.4 A) (Fig. 6-34). The reason for this lies in the structure of the complex formed. We have always assumed that the metal ion will try to lie in the middle of the bonding cavity of the macrocycle. There is no real reason why this should be. Caesium could form a complex with 21-crown-7 in which all of the oxygen atoms lie approximately planar with the metal in the centre of the cavity. It is also apparent that caesium could not occupy the middle of the cavity in 18-crown-6. However, a different type of complex can be formed with 18-crown-6, in which a caesium ion is sandwiched bet-... 

Reaction 13 represents the production of a paramagnetic defect by capture of a photogenerated hole. The paramagnetic centres may be anihilated by the reverse of reaction 12 by capture either of photoelectrons from the conduction band or of electrons liberated during oxygen desorption. 

Alkali metals can occur in the atomic state in the vapour phase and they show a very high activity towards all electron acceptors. The introduction of alkali metals on to oxide surfaces involves their reaction with all surface electron acceptor centres. Such acceptor centres are anionic vacancies, the holes trapped on oxygen anions near the cationic vacancies, and surface hydroxyl groups. Oxide surfaces possessing these defects can react with alkali metal in accordance with equations (l)-(4). 

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