The relationship between electron—molecule collision cross‐sections, experimental Townsend primary and secondary ionization coefficients and constants, electric strength and molecular structure of gaseous hydrocarbons

Abstract

Experimental Townsend primary ionization coefficients are concisely presented for 24 gaseous hydrocarbons starting with methane. Applying the traditional Townsend formula for the primary ionization coefficient enables the saturation ionization constant, A , and the inelastic barrier constant, B , to be obtained for each hydrocarbon. Making use of the Lewis formula for B allows this constant to be further analysed in terms of available excitation and ionization potentials and total electron‐molecule collision cross‐sections, and enables the as‐yet unknown‐except for methaneinelastic collision cross‐sections to be found for each hydrocarbon. Using the inelastic collision cross‐section data for methane, the only hydrocarbon for which this is available, vindicates for the first time the Lewis formula and shows that the vibrational cross‐sections have to be included in the inelastic one for these molecular gases. Inserting the ionization coefficients and/or constants into Townsend's sparking criterion allows Paschen characteristics to be presented up to 500 kV. Each of these sets of data is linked in turn to the number of hydrogen atoms/carbon‐hydrogen bonds rather than the carbon number and thus this electrical classification differs from the chemical one. This is further corroborated by the hydrogen molecule being the natural precursor to methane in its electrical characteristics and by the total collision cross‐section in the relevant electron energy range being determined by the hydrogen atoms in the hydrocarbon molecule. For the alkanes tested, propane is the first member of the series; for the 1‐alkenes, propene‐1 is, while for the methyl‐alkenes, ethene‐1 is the first.