Influence of Molecular Bonding on the Townsend Ionization Coefficients of Hydrocarbon Gases

Abstract

Measured values of Townsend's α and γ coefficients extending over 30≲E/p 0≲ 3000 for methane, ethylene, and acetylene are presented. The α values follow the Townsend equation α/p 0=A exp(—Bp 0/E) for 35≲E/p 0≲300, and the lower limit of E/p 0 is set by the experimental conditions. The molecular constants A and B increase with molecular weight and the degree of unsaturation and are in agreement with those obtained from earlier Vs—pd characteristics, provided the latter are obtained under space‐charge distortion‐free conditions, which was the case for our measurements. Comparison of calculated and experimental α values for ethylene favors a Maxwellian electron energy distribution. Due to the dissociative nature of the bonds, Townsend's γ—E/p curves do not exhibit a photoelectric peak at low E/p, and at high E/p the values are still some 102—104 times smaller than for the common diatomic gases. For a given E/p value, γ decreases with increase in molecular weight. Thus, in general, heavy and complex hydrocarbons are better conductors at high E/p and better insulators at low E/p. Measurements at e αd ≳ 106 show up the possible electronegative character of acetylene as compared with methane.