Electron mobilities and ranges in methyl substituted pentanes through the liquid and critical regions

Abstract

Density normalized penetration ranges bGPd of secondary electrons and mobilities μe of thermal electrons vary in similar manners as the temperature of the solvent hydrocarbon is increased toward the critical. These quantities pass through maxima in 3,3-dimethylpentane, 2,2,4-trimethylpentane, and 2,2,4,4-tetramethylpentane, but not in 3-methylpentane. In 2,3-dimethylbutane μe has a small maximum just below the critical point while bGPd reaches a short plateau. As the molecules in a series become more spherelike, the maxima grow and the temperature at which they occur moves farther below the critical. Changes of the range and mobility are caused by changing scattering properties of the fluids. The decrease of μe on the high temperature side of the maximum is attributed to a decrease of mobility in the conduction band μ0h rather than to an increase in electron trap depth that might have been caused by the greater density fluctuations that occur as the critical region is approached. The results support the earlier conclusion that the major part of the secondary electron range bGP is attained while the electron has only a few tenths of an eV energy and is skittering through the liquid in the conduction band. The mobilities in all the liquids are independent of field strength up to 30 kV/cm, except in 2,2,4,4-tetramethylpentane under conditions where the drift velocity exceeds 14 km/s. The mobilities in the five critical fluids are all between 19 and 31 cm3/V s, a factor of 1.6, whereas those in the liquids at 293 K range over a factor of 130. This demonstrates the importance of short range forces in governing electron behavior in dense matter.