Density and temperature effects on electron mobilities in gaseous, critical and liquid n-hexane, cyclohexane, and cyclopentane

Abstract

In the low density vapors the density normalized mobilities μn of thermal electrons decreased in the order n-hexane > cyclopentane > cyclohexane, although the differences were only ∼ 10%. The mobilities began to increase at electric field strengths E/n > 0.4 Td in cyclopentane, > 0.5 Td in cyclohexane and remained independent of field strength up to 1.5 Td in n-hexane. The ratio of the threshold drift velocity for electron heating to the speed of low frequency sound in the gas, υd(threshold)/c0 = 11 in cyclopentane, 14 in cyclohexane, and is > 35 in n-hexane; it increases with decreasing sphericity of the molecules. The electrons are cooled mainly by inelastic collisions with the hydrocarbon molecules. The temperature coefficients of mobilities in the low density gases increase as the molecules become more globular, which could reflect the participation of either low lying transient negative ion states or a Ramsauer–Townsend effect in the scattering processes. The normalized mobilities μn in the saturated vapors began to decrease at n = 4 × 1019 molecules/cm3 in n-hexane, 13 × 1019 in cyclohexane, and 30 × 1019 in cyclopentane. It appears that the minimum size of molecular cluster required for electron quasilocalization is smaller in n-hexane than in cyclohexane, and larger in cyclopentane. Electron localization interactions are weaker for more globular molecules.Mobilities in the critical fluids were 16 cm2/V s in n-hexane, 23 in cyclohexane, and 22 in cyclopentane.Mobilities in the liquids were independent of field up to the highest value used, which was 1.5 Td in the hexanes and 0.9 Td in cyclopentane. The mobilities and their temperature dependences were interpreted in terms of a model.