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4.4.5 Electron diffusion

The coefficient of electron diffusion D_e may be calculated from the equation D_e/μ = ηp/Ne, where μ is the electron mobility, N is the number of gas molecules per m³ at s.t.p., e is the electronic charge, p is the pressure in N m⁻² and η is the ratio of the mean agitation energy of an electron to that of a gas molecule. η is also known as the Townsend energy factor. Experimental values of η are given below as a function of E/p. For very low values of E/p or E/N, D_e/μ = kT/e, i.e. the electrons are in equilibrium with the gas.

Field strength per unit pressure (E/p)/(V m N⁻¹)

Most of the values in the above table were obtained in the 1920s and are from Healey and Reed (1941). Later determinations have shown only small changes for H₂, He, N₂ and air but the values for O₂ over a limited range are higher than those in the above table (see Huxley et al., 1959). D/μ values are quoted in modern literature.

J.W. Leake

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