Collisions Between Electrons and Gas Molecules

Using a simple method in which positive ion sheaths serve as perfect grids we have obtained data on the probability of various types of collisions occurring between Hg, H₂, N₂, He, Ne and A molecules and electrons of moderate velocity, i. e. , 30-250 volts. A theory has been developed and the mean angular deflection for elastic and inelastic collisions between electrons and gas molecules has been calculated. Evidence on the mechanism of ionization is presented. The current densities of the various Maxwellian groups of electrons and the average temperatures of these groups are calculated from the data and are shown to be in good agreement with data obtained by us using Langmuir and Mott Smith's probe wire method. The current density of positive ions has been measured by both plane and probe wire collectors and the space potential both measured and calculated. The evidence in support of negative anode drops has been pointed out and the ions flowing to the anode measured separately by means of a perforated collector. The maximum number of positive ions produced by an electron with a given velocity before losing its ionizing power has been measured and shown to be independent of current density and pressure being dependent only on the velocity of the electron and the nature of the gas. The mean free path of high velocity electrons (i. e. , above the ionizing potential) for various types of collision can be evaluated from the probability data. The mean free path for inelastic impact has been calculated separately and independently of the reflection of electrons from the collectors. The small angular scattering of electrons is discussed in detail. Phenomena, similar to Langmuir's high speed electrons in mercury vapor, have been observed and the curves showing the energy transfer between primary electrons in H₂ and A are given. The resonance and ionization potentials observed in the work are also tabulated.