Electron thermalization ranges in dielectric fluids: Effects of molecular structure and fluid density

Values of thermalization ranges bGp were estimated from measured free ion yields using the extended Onsager model in ethene, propene, cyclopropane, propane, n-butane and i-butane at 0.15 ≲ d/d_c <; 3 (d_c = critical fluid density). At d/dc <; 0.3 the density normalized range b_GPd was (5.5 ± 0.2) 10^-6 kg/m² for the present alkanes and methane and ethane. The double bond in ethene, and the double bond and permanent dipole combination in propene led to greater thermalizing ability and hence lower b_GPd. As d/d_c was increased towards unity the partial π-bond character of cyclopropane became effective. In the normal liquid at d/d_c > 1.5 the effects of molecular shape gave b_GPd in methane > i-butane > ethane > propane > n-butane. The effect of π-bonding gave n-butane > cyclopropane > ethene ~ propene; the permanent dipole of propene had negligible effect in the dense liquid.