Effects of density and temperature on mobilities of electrons in vapors and liquids of pentane isomers: Molecular structure effects

Abstract

The mobility of electrons in low density gaseous n-pentane is independent of applied electric field strength up to E/n=2 Td. The mobility in isopentane increases at E/n≳0.4 Td, and in neopentane at ≳0.2 Td. The ratio of the threshold drift velocity for electron heating to the velocity of sound, vd(threshold)/co, increases as the molecules become less spherelike. The value of the ratio in low density xenon is ∼0.7, in neopentane ∼5, isopentane ∼15, and n-pentane ≳100. The high field mobilities in neopentane and isopentane vapors seem to approach the field independent value in n-pentane. Representing the velocity dependence of the scattering cross section as σv=e/mbjv1−j, the value of 1−j increases with increasing sphericity of the molecules. Values of 1−j and σav=〈v〉/〈v/σv〉 for thermal electrons in the dilute gases at 300 K are respectively: n-pentane, 1.1, 1.3×10−15 cm2; isopentane, 1.5, 1.8×10−15 cm2; neopentane, 3.2, 4.0×10−15 cm2. The variation of (1−j) and σav with sphericity for the isomeric pentanes can be interpreted in terms of either: a) a Ramsauer–Townsend effect, the σv minimum occurring at lower v for less spherelike molecules; or b) a transient negative ion state lying near zero energy, which makes a stronger contribution to scattering from more spherelike molecules. The apparent values of (1−j) and σv increase in the high density gases, due to multibody interactions. Mobilities in the liquids n-pentane and isopentane are interpreted in terms of thermally activated electron transitions between localized and extended states. The extent of localization is greater for less spherelike molecules. Behavior in neopentane is more complex. The structure factor S (0) does not have the influence suggested earlier on electron scattering in the fluid far removed from the triple point.