Electron mobilities in alkanes through the liquid and critical regions

Abstract

Three properties of electrons in liquids are governed by scattering at low energies and show similar dependence upon liquid density and molecular shape. They are the mobility, the dependence of the mobility upon electric field strength, and the penetration range of low energy (<10 eV) electrons. The scattering cross sections of hydrocarbons in the liquid phase (near or below the normal boiling point) are smaller when the molecules are more sphere-like. The degree of sphericity of the molecules in the present series decreases in the order 2,2-dimethylpropane (DMPr) > 2,2-dimethylbutane (DMB) > cyclopentane (cP) > n-pentane (nP).Electron mobilities in DMPr and DMB measured as functions of the liquid density pass through maxima similar to those observed earlier in argon and xenon. The magnitudes of the maxima decrease in the order Xe > Ar > DMPr > DMB. The maxima occur at densities approximately double the critical density dc. There is a small maximum for electrons in cP, but none for those in nP.The mobilities in the supercritical gases are similar for the four compounds. The Arrhenius temperature coefficients are all 7–10 kcal/mol (0.3–0.4 eV/electron) for temperatures and densities near the critical values. This implies that electrons form localized states in the gases. The extent of localization in supercritical DMPr appears to be greater than that in the normal liquid. The localization is not the critical phenomenon predicted by Lekner.The mobility in liquid DMPr decreases at high electric field strengths. The magnitude of the field dependence changes with the liquid density and passes through a maximum similar to that of the mobility itself. The field dependence in liquid DMB is smaller than that in DMPr, but it also passes through a maximum at a density about double dc.Equations are given that describe the mobility in nP at all temperatures from the triple point through to the supercritical gas. The electrons reside mainly in localized states over the entire temperature range, but transport occurs mainly in extended states to which the electrons are thermally excited.The penetration range parameter bGP of secondary electrons in these liquids is normalized for comparison by multiplying by the liquid density d. The values of bGPd plotted against d form curves similar in shape to the μ against d curves, but the relative variations in bGPd are much smaller.