Excess Electrons in Liquid Hydrocarbons

Stable electronic charge carriers have been observed in n-hexane, n-pentane, benzene, and methylbutene with room temperature mobilities of 0.07, 0.07, 0.6, and 3.6 cm2 V−1·sec−1, respectively. The charge carriers were injected into the highly purified hydrocarbon liquids by photoemission of electrons from a low work function surface. Mobilities increase rapidly with temperature and obey an Arhennius expression with activation energies of 4.3 kcal mole−1 in hexane and 2.6 kcal mole−1 in methylbutene. The carriers in hexane could be drawn out of the liquid into the vapor and had a higher mobility in the solid than in the liquid at temperatures near the melting point. Current versus voltage measurements indicate that the injection process is controlled by transmission over the image barrier at the cathode. The mobilities of the injected charges in these fluids are more than 100 times ionic mobilities in the same fluids but considerably less than the mobilities of electrons in the rare-gas liquids. The free electron theory of Cohen and Lekner, somewhat successful in explaining electron mobilities in the rare-gas liquids, does not give a satisfactory explanation of the hydrocarbon mobilities. A proposed generalization of the Cohen–Lekner theory to account for the effect of internal configurations of the hydrocarbon molecules on electron scattering also seems unable to explain the magnitudes and temperature dependence of the observed mobilities. A trapping model, in which the electron is frequently trapped for short times and otherwise moves as a free electron, appears to explain the mobility properties observed.