First Townsend Ionization Coefficient in Hydrogen

Rose's measurements of the first Townsend ionization coefficient η in hydrogen for Ep₀100 V (cmTorr)^-1 have been questioned by a number of authors principally either because of a supposed neglect of the second Townsend ionization coefficient γ, or because electrons allegedly do not have an energy distribution which is only characteristic of Ep₀, or because of gas purity. In order to resolve these questions, new measurements of η in hydrogen have been carried out for 50≤Ep₀≤800 in an ultrahigh-vacuum system. The present work takes γ into account, and the values of η obtained are in agreement with those of Rose over the whole range of Ep₀ studied. Various methods of evaluating η from prebreakdown ionization currents are critically discussed. The present results show that prebreakdown ionization-current data in hydrogen can be represented in the usual way, at least up to Ep₀=800.