Electrons, trapped

Since solids do not exist as truly infinite systems, there are issues related to their temiination (i.e. surfaces). However, in most cases, the existence of a surface does not strongly affect the properties of the crystal as a whole. The number of atoms in the interior of a cluster scale as the cube of the size of the specimen while the number of surface atoms scale as the square of the size of the specimen. For a sample of macroscopic size, the number of interior atoms vastly exceeds the number of atoms at the surface. On the other hand, there are interesting properties of the surface of condensed matter systems that have no analogue in atomic or molecular systems. For example, electronic states can exist that trap electrons at the interface between a solid and the vacuum [1]. 

The electron can be trapped, for example by an interstitial which is converted to an H atom. The AlO is the hole color center which absorbs light and gives the color to smoky quart2. Bleaching is the result of thermal energy releasing the trapped electron, which then produces the reverse of reaction 1. The amethyst color center in quart2 is exactly like the smoky, except that Fe " replaces. ... 

The Fe(III)0 4 hole color center gives the purple color. On being heated, the trapped electron is released and the reverse of reaction 2 occurs producing Fe(III)0 4, which provides the pale yellow color of citrine. 

The absorption band at 300 nm may also be associated with alkah ions, possibly the result of a trapped electron stabili2ed by an alkah ion. The band shifts to longer wavelengths when heavier alkah ions are present, and growth rates for the band show a definite dependence on the type of alkah (205,209). 

The Na is 555 pm from the nearest N and 516 pm from the nearest O, indicating that it is a separate entity in the structure. Potas-sides, rubidides and caesides have similarly been prepared. The same technique has been used to prepare solutions and even crystals of electrides, in which trapped electrons can play the role of anion. Typical examples are [K(cryptand)]+e and [Cs(18-crown-... 

Plaskett, J. S., Phil. Mag. 45, 1255, "Self-trapped electrons in metals."... 

Thermoluminescence. Thermoluminescence is a property of some solids in which excitation by light or particle radiation is frozen in as trapped electrons and holes or a crystal defect. Subsequent heating allows relaxation of the excited state and emission... 

In addition, the results of adsorption experiment in Fig. 4 revealed that H2O2 promotes the adsorption of 4-NP on the Cr-Ti-MCM-41 surface. From considering above results, it can be said that H2O2 increases the reaction rate by the promotion of adsorption of reactant and the removing of surface-trapped electrons. 

The recombination of trapped electrons and holes produces the fluorescence. Adsorbed oxygen scavenges electrons producing O2" which also is adsorbed. OJ is a much better quencher than Oj. Its accumulation under illumination therefore leads to the decrease in fluorescence intensity. During the dark period disappears. During the illumination in the presence of oxygen, the colloid undergoes photoanodic dissolution (see Sect. 3.2). The ZnS particles become smaller in this way, and this finally leads to an increase in fluorescence yield as already described for CdS. 

Gratzel and Serpone and co-workers recently reported on a picosecond laser flash photolysis study of TiO. They observed the absorption spectrum immediately after the 30 ps flash and attributed it to electrons trapped on Ti" " ions at the surface of the colloidal particles. The absorption decayed within nanoseconds, the rate being faster as the number of photons absorbed per colloidal particle increased. This decay was attributed to the recombination of the trapped electrons with holes. 

Electron spin resonance (ESR) measures the trapped electron population in a lattice, which is directly related to the amount of ionizing radiation received by the sample since its formation. The total radiation dose received by the sample is estimated from the ESR... 

In Sections III(l) and 111(2) the lability principle has been illustrated for processes involving the transfer of weakly bound electrons, including the reactions of solvated and trapped electrons and F-centers and processes of electrochemical generation of solvated electrons. In Sections IV and V, it will be illustrated also by atom transfer reactions and, in particular, by reactions involving adsorbed atoms. 

By capturing an electron or a hole the chemisorbed particle passes from the electrically neutral to the charged state. It is very important that the trapped electron or hole is forced to take part in the chemisorption bonding. 

Recently the spectrum of O- has been observed on MgO by Lunsford and co-workers (50, 51). The species was formed by adsorption of N20 at low temperatures onto MgO which contained trapped electrons. By using N2170 it was shown that the species was indeed 0 The spectrum shown in Fig. 21 is characterized by gx = 2.042 and g = 2.0013 with = 19.5 and an = 103 G. From the hyperfine coupling it may be shown that the unpaired electron is localized mainly in one 2p orbital. Both the g values and the hyperfine coupling constants are consistent with the energy level diagram of Fig. 20a. The results are also consistent with the spectrum of... 

Gillis, 1969 Baxendale and Rasburn, 1974 Baxendale and Wardman, 1971 Baxendale et al, 1971, 1973). However, it may be best to consider these cases as extensions of the trapped electron in low-temperature matrices. 

FIGURE 6.4 A typical trapped electron absorption spectrum in ethanol at 4 K and the corresponding solvated electron spectrum at 77 K. The irradiation is at 4 K in both cases. Reproduced from Hase et a1. (1972a), with permission from Am. Inst. Phys.O... 

Kevan (1974) has exhaustively reviewed ein organic glasses, to which the reader s attention is drawn. He points out that the effective spur radius r for trapped electrons may be operationally given in angstroms as... 

The first subnanosecond experiments on the eh yield were performed at Toronto (Hunt et al., 1973 Wolff et al., 1973). These were followed by the subnanosecond work of Jonah et al. (1976) and the subpicosecond works of Migus et al. (1987) and of Lu et al. (1989). Summarizing, we may note the following (1) the initial (-100 ps) yield of the hydrated electron is 4.6 0.2, which, together with the yield of 0.8 for dry neutralization, gives the total ionization yield in liquid water as 5.4 (2) there is -17% decay of the eh yield at 3 ns, of which about half occurs at 700 ps and (3) there is a relatively fast decay of the yield between 1 and 10 ns. Of these, items (1) and (3) are consistent with the Schwarz form of the diffusion model, but item (2) is not. In the time scale of 0.1-10 ns, the experimental yield is consistently greater than the calculated value. The subpicosecond experiments corroborated this finding and determined the evolution of the absorption spectrum of the trapped electron as well. 

The authors assume different and statistically independent mechanisms of electron-ion recombination in the quasi-free and trapped states. Thus P = w, where and wt are respectively the probabilities of escaping recombination in the quasi-free and trapped states. Based on some heuristic and not entirely plausible arguments, wq( is approximately equated to 1/2. The probability of finding a trapped electron at a distance between r and r + dr from the positive ion is given by (crtP dr/v) exp(-crPr/v), where P, is again the probability of finding an... 

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