Free-Ion Yield and Electron Mobility in Liquid Hydrocarbons: A Consistent Correlation

Abstract

A correlation is sought between the free-ion yield and electron mobility in liquid hydrocarbons in terms of the elastic and the inelastic scattering mean free paths of epithermal (≤0.2 eV) electrons. These determine the thermalization distance distribution and consequently the free-ion yield. The thermal quasi-free electron mobility, μqf, can also be obtained from the same cross sections. Finally, the effective mobility is derived from μqf using the electron trap concentration and binding energy, thereby establishing a relationship between free-ion yield and mobility. Thus, given the input data for trapping and the elastic and inelastic cross sections, both the free-ion yield and the effective mobility may be obtained from the interactions of epithermal electrons. In very low mobility liquids (μ < 0.1 cm2 v-1 s-1), transport is governed by trapping and detrapping rates, being relatively independent of μqf. As found by Jay-Gerin et al. (Can. J. Chem. 1993 , 71, 287) the free-ion yield in such cases is virtually independent of mobility, a phenomenon which is naturally explained in the quasiballistic model of the author (Chem. Phys. Lett. 1993, 207, 245 and 1995, 233, 167) but not in the usual trapping model. For low mobility liquids, the elastic mean free path, L, is found to be ca. 1−5 A and there is a probability of ∼0.1−0.3 inelastic scatterings per elastic mean free path. These values increase progressively with mobility, where, in the high mobility cases, L is on the order of a few tens of angstroms and inelastic collisions outnumber elastic ones by a factor of ∼2−4. The situation is reminiscent of liquefied rare gases.