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

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 $G_{\mathrm{fi}}\text{=2.0}$ . 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, $G_{\mathrm{fi}}\text{=0.013}$ . The ion—electron neutralization rate constant equals ${\text{8 × 10}}^{\text{−5}}\hspace{0.17em}{\mathrm{cm}}^3/\mathrm{ion}·\sec$ in pure argon at 87°K and ${\text{3.1 × 10}}^{\text{−4}}\hspace{0.17em}{\mathrm{cm}}^3/\mathrm{ion}·\sec$ in liquid methane at 120°K. The mobility of electrons in liquid methane is $\text{300\hspace{0.17em}}{\mathrm{cm}}^2/\mathrm{V}·\sec$ 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 ${10}^{\text{−3}}\hspace{0.17em}{\mathrm{cm}}^2/\mathrm{V}·\sec$ .