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

摘要

The mobility of electrons in low density gaseousnhyphen;pentane is independent of applied electric field strength up toE/n=2 Td. The mobility in isopentane increases atE/ngsim;0.4 Td, and in neopentane at gsim;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 sim;0.7, in neopentane sim;5, isopentane sim;15, andnhyphen;pentane gsim;100. The high field mobilities in neopentane and isopentane vapors seem to approach the field independent value innhyphen;pentane. Representing the velocity dependence of the scattering cross section as sgr;v=e/mbjv1minus;j, the value of 1minus;jincreases with increasing sphericity of the molecules. Values of 1minus;jand sgr;av=lang;vrang;/lang;v/sgr;vrang; for thermal electrons in the dilute gases at 300 K are respectively:nhyphen;pentane, 1.1, 1.3times;10minus;15cm2; isopentane, 1.5, 1.8times;10minus;15cm2; neopentane, 3.2, 4.0times;10minus;15cm2. The variation of (1minus;j) and sgr;avwith sphericity for the isomeric pentanes can be interpreted in terms of either: a) a Ramsauerndash;Townsend effect, the sgr;vminimum occurring at lowervfor 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 (1minus;j) and sgr;vincrease in the high density gases, due to multibody interactions. Mobilities in the liquidsnhyphen;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 factorS(0) does not have the influence suggested earlier on electron scattering in the fluid far removed from the triple point.