The Rate of Formation of Negative Ions by Electron Attachment

Abstract

By a new method, the fraction $h$ of the collisions between electrons and molecules which result in the attachment of the electron to form a negative ion has been measured in air and oxygen as a function of the average electron energy, the gas pressure, and the moisture content. In oxygen $h$ had a minimum at 0.9 volts average electron energy, and increased with either increase or decrease of the energy from this value. In air $h$ increased as the average electron energy was reduced below 0.9 volts. The absolute value was about 0.4 of that predicted from the value in ${\mathrm{O}}_2$ assuming simple additivity, but this difference is possibly within the limits of error. The variation with electron energy and the order of magnitude of $h$ are in agreement with Bailey's results for air. On the other hand, a rapid increase in $h$ with increase in pressure at constant electron energy was found at low energy, while Bailey's results, which seem more reliable, show no variation with pressure. In ${\mathrm{H}}_2$O $h$ is of the same order of magnitude as in ${\mathrm{O}}_2$, but in mixtures of ${\mathrm{H}}_2$O and ${\mathrm{O}}_2$ $h$ is much larger than in either alone, showing that here at least the attachment process is complex.