Quasifree electron mobility

Figure 35 Electron transport, (a) Quasifree electrons, band mobility (b) trap-controlled band mobility (c) hopping transport.

In the nonpolar liquids exhibiting a high electron drift mobility, the condition Pef Pet is fulfilled. The trapped electrons can be considered as temporary negative ions and ions always have a much smaller mobility as compared to quasifree electrons. We may then neglect the second term of the r.h.s. of Equation 74 and obtain... 

Solutions of biphenyl in tetramethylsilane represent an example where the electron mobility is modulated by the formation of temporary negative ions due to electron attachment to the solute molecules. The electron mobility in tetramethylsilane at room temperature is about 100 cm V s The electrons are considered to be quasifree and to move in a conduction band. If biphenyl, Ph2, is dissolved electron attachment and subsequent thermally activated detachment occurs ... 

Later, Berlin et al. (1978) refined their theoretical model and derived different quasifree mobilities for each individual hydrocarbon, ranging from 27 cm V %" for n-hexane to 440 cm V" s" for neopentane. It should be mentioned that the electron mobility observed at room temperature in single crystals of paraffins is of the order of 1 cm V s" (see Section 10.2). The melting process changes the mobility only by a factor of 2 to 3 (see Section 1.1). From these divergent results it seems questionable if the simple two-state model could explain in a general form all the intricacies of the excess electron mobility in liquid hydrocarbons. 

Charge-separated states (electrons, positive ions, negative ions) and physical quantities that describe their behavior and reactions are seriously affected by the density and nature of the medium. Thus, the electron energy 8, the quasifree (excess) electron "ground state" energy V, the electron drift velocity w, the electron mobility y, the cross sections for electron scattering electron capture [Pg.284]

Carbon disulfide is isovalent to carbon dioxide and it also has a bent monomer anion. While gas-phase CO2 has negative EAg of —0.6 eV [24], for CS2, EAg is +0.8 eV [34]. Despite this very different electron affinity, Gee and Freeman [34] observed long-lived electrons in CS2 (with lifetime > 500 psec) with mobility ca. 8 times greater than that of solvent cations. Over time, these electrons converted to secondary anions whose mobility was within 30% of the cation mobility. Between 163 and 500 K, the two ion mobilities scaled linearly with the solvent viscosity, as would be expected for regular ions. Of course, Gee and Freeman s identification of the long-lived high-mobility solvent anions as electron is just a manner of speech Obviously, quasifree or solvated electrons cannot survive for over a millisecond in a positive-EAg liquid. 

The electron can be treated as quasifree if X Aj, where Aj denotes the mean free path for scattering obtained from the mobility formula. 

Tp is the time in the quasifree state, while denotes the time the electron is localized. As long as ppXp p, x, the second term of the r.h.s. of Equation 22 can be neglected, and the effective mobility becomes... 

Berlin, Y. A., Nyikos, L., and Schiller, R., Mobility of localized and quasifree excess electrons in liquid hydrocarbons, J. Chem. Phys., 69, 2401, 1978. 

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