Scavengers, electron

Electrica.1 Properties. The electrical properties of SF stem primarily from its effectiveness as an electron scavenger. To accomplish electrical breakdown in a dielectric gas, primary electrons must gain sufficient energy to generate appreciable numbers of secondary electrons on molecular impact. Sulfur hexafluoride interferes with this process by capturing the primary electrons, resulting in the formation of SF or SF ions and F atoms (29) ... 

Impurity-produced plasmas in semiconductors do not have to be compensated by charges of the opposite sign. These plasmas can be produced by introduction of either electron donors or electron scavengers, ie, hole producers, into semiconductor lattices. Thek densities range from a lower limit set by the abihty to produce pure crystals particles/cm ) to values in excess of 10 particles/cm. Plasmas in semiconductors generally are dilute, so that... 

Edwards two-parameter equation 549 Electrochemical oxidation of sulphides 76, 252, 253 of sulphoxides 968, 987, 1043 Electrochemical reduction of sulphones 962, 963, 1002-1041 of sulphoxides 933, 1041, 1042 Electronegativity, of the sulphur atom 584 Electronic effects 390, 484-535 Electron scavengers 892, 896 Electron spin resonance spectroscopy 874, 890-895, 1050-1055, 1082, 1083, 1090-1093... 

The practical importance of Reaction 4 stems from the fact that HC1 in rocket exhausts behaves as an electron scavenger, rapidly reducing electron concentrations. This fact, together with the above discussion, emphasizes the need for a more thorough investigation of the values of 4 and k 4. Detachment of electrons from Cl- through... 

Kemp and coworkers employed the pulse radiolysis technique to study the radiolysis of liquid dimethyl sulfoxide (DMSO) with several amines as solutes [triphenylamine, and N, A, A, N -tetramethyl-p-phenylenediamine (TMPD)]. The radiolysis led to the formation of transient, intense absorptions closely resembling those of the corresponding amine radical cations. Pulse radiolysis studies determine only the product Ge, where G is the radiolytic yield and e is the molar absorption. Michaelis and coworkers measured e for TMPD as 1.19 X 10 m s and from this a G value of 1.7 is obtained for TMPD in DMSO. The insensitivity of the yield to the addition of electron scavenger (N2O) and excited triplet state scavenger (naphthalene) proved that this absorption spectrum belonged to the cation. 

Emission of electrons from the electrode into the solution with formation of solvated electrons and the subsequent reaction between the solvated electrons and the electron scavenger in solution. 

Operationally, a procedure may be based on measuring the yield of a reaction traceable to ionization, usually giving a lower limit to the ionization yield. Thus, in the radiation chemistry of hydrocarbon liquids, the product of an electron scavenging reaction (for example, C2H3- radical from the scavenger C2H5Br)... 

Oxides of nitrogen are popular electron scavengers. The final product of reaction of eh with N20 is N2 and OH, but the mechanism is not well understood. The rate approaches diffusion control. A possible reactions scheme is... 

Four observation were thought to be in disagreement with the diffusion model (1) the lack of a proportional relationship between the electron scavenging product and the decrease of H2 yield (2) the lack of significant acid effect on the molecular yield of H2 (3) the relative independence from pH of the isotope separation factor for H2 yield and (4) the fact that with certain solutes the scavenging curves for H2 are about the same for neutral and acid solutions. Schwarz s reconciliation follows. 

Burns and Curtiss (1972) and Burns et al. (1984) have used the Facsimile program developed at AERE, Harwell to obtain a numerical solution of simultaneous partial differential equations of diffusion kinetics (see Eq. 7.1). In this procedure, the changes in the number of reactant species in concentric shells (spherical or cylindrical) by diffusion and reaction are calculated by a march of steps method. A very similar procedure has been adopted by Pimblott and La Verne (1990 La Verne and Pimblott, 1991). Later, Pimblott et al. (1996) analyzed carefully the relationship between the electron scavenging yield and the time dependence of eh yield through the Laplace transform, an idea first suggested by Balkas et al. (1970). These authors corrected for the artifactual effects of the experiments on eh decay and took into account the more recent data of Chernovitz and Jonah (1988). Their analysis raises the yield of eh at 100 ps to 4.8, in conformity with the value of Sumiyoshi et al. (1985). They also conclude that the time dependence of the eh yield and the yield of electron scavenging conform to each other through Laplace transform, but that neither is predicted correctly by the diffusion-kinetic model of water radiolysis. 

FIGURE 7.6 Effect of hydrated electron scavenger on the scavenging yield. See text for explanation. Reproduced from Pimblott and Green,1995, with permission of Elsevier . 

To evaluate the absolute kinetics of recombination from Eq. (7.27), it is necessary to obtain a reliable value of fes in addition to a measured from scavenging experiments. For electron scavenging in cyclohexane by biphenyl, Rzad et al. (1970) used fes = 3.0 x 10u M s-1 on the basis of the a value of scavenging... 

It has been shown by Mozumder and Tachiya (1975) that, within the context of the diffusion model, the probability of generation of free ions is independent of postthermal electron scavenging, both in the absence and presence of an external field. Thus, the experimental finding—that the free-ion yield is reduced in neopentane (NP) by the addition of electron attaching solutes SF6,... 

In a later report, Schmidt and Allen (1970) extended their measurement to 38 pure liquids and mixtures at room temperature and to 5 liquids as a function of temperature. The free-ion yields are arranged by the alkanes and their isomeric and cyclic counterparts, which show considerable differences in the results. Thus, the free-ion yield in neopentane (NP) is about seven times that in n-pentane. Some of the results are shown in Table 9.1. In mixtures of NP with CC1, or CS, the observed decrease of Gf with the additive concentration has been interpreted by Mozumder and Tachiya (1975) as due to epithermal electron scavenging (vide infra). 

In applying the quasi-ballistic model to electron scavenging, Mozumder (1995b) makes the plausible assumption that the electron reacts with the scavenger only in the quasi-free state with a specific rate fesf. Denoting the existence... 

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