Molecular Structure Effects on the Free-Ion Yields and Reaction Kinetics in the Radiolysis of the Methyl-Substituted Propanes and Liquid Argon: Electron and Ion Mobilities

Abstract

The free-ion yields in liquid alkanes increase with increasing molecular sphericity and increasing temperature, in agreement with earlier observations. The Arrhenius temperature coefficient of free-ion formation is 0.8 kcal/mole in propane and 0.7 kcal/mole in 2-methylpropane. The free-ion yield in purified argon at 87°K appears to be Gfi=2.0. Addition of oxygen to liquid argon decreased the free-ion yield, mainly because the efficiencies of electron energy loss processes with the diatomic oxygen molecules were much greater than those with monatomic argon molecules; the oxygen served to decrease the ion—electron separation distance more by de-energizing the electron than by scavenging it before it reached the end of its normal track. In pure liquid oxygen at 87°K, Gfi=0.013. The ion—electron neutralization rate constant equals 8 × 10−5 cm3/ion · sec in pure argon at 87°K and 3.1 × 10−4 cm3/ion · sec in liquid methane at 120°K. The mobility of electrons in liquid methane is 300 cm2/V · sec and decreases in other hydrocarbons with decreasing sphericity of the molecules. The Arrhenius temperature coefficient of electron mobility in 2-methylpropane is 3 kcal/mole. Ion—ion neutralization rate constants in the present liquids were all of the order of 10−9 cm3/ion·sec, and the ion mobilities were all in the vicinity of 10−3 cm2/V · sec.