ELECTRON THERMALIZATION DISTANCES AND FREE-ION YIELDS IN DENSE GASEOUS AND LIQUID BENZENE

Electron thermalization has been studied in gaseous and liquid benzene at 4.1 less-than-or-equal-to d/kg m-3 less-than-or-equal-to 878 (temperatures 295-560 K) using measurements of the free-ion yield G(fi)E as a function of electric field strength E and temperature T. The measured (G(fi)E, E) values at each T were compared to those calculated using an extended Onsager model. Assumption of a three-dimensional Gaussian distribution of secondary electron thermalization distances YG resulted in too large a field dependence. The Gaussian with the small added tail, YGP, gave the correct dependence. Values of the yield extrapolated to zero field, G(fi)0, and of the most probable thermalization distance b(GP) were obtained. Variation of the density-normalized distance b(Gp)d with reduced density d/d(c) (d(c) = critical fluid density) was expected to be similar to that in ethene, due to the pi-electrons in the two compounds. Instead, it was similar to that in ethane. Throughout the liquid range, epithermal electrons were de-energized less efficiently than in the gas at d < 0.5 d(c) where the benzene molecules are further apart. As the density increases above 2 d(c) the values of b(Gp)d decreased as in other hydrocarbons, rather than like those in hexafluorobenzene, which increased sharply. Dielectric constants were also measured up to 560 K.