Electron avalanches in oxygen and in mixtures ofO2andH2: Determination of the first Townsend coefficientα

We have studied current transients of pre-breakdown electron avalanches in uniform fields in pure oxygen, and in mixtures of oxygen and hydrogen. The goal of this work is to determine an effective first Townsend coefficient $\alpha$, which accounts for instantaneous ionization processes by electron impact. Following the reasoning of Price, Lucas, and Moruzzi, the mixtures were chosen to remove the primary negative ion ${\mathrm{O}}^{-}$ by a rapid associative detachment reaction with ${\mathrm{H}}_2$, thus effectively making oxygen behave as a nonattaching gas. This simplifies the analysis of the transients. A simple extrapolation to zero hydrogen pressure is used to determine $\frac{\alpha }{p}$, the ratio of the ionization coefficient to pure oxygen pressure. The values of $\frac{\alpha }{p}$ thus obtained are in agreement with pulse measurements in pure oxygen and supplement the pure oxygen data at lower field-strength-to-pressure ratios (between 30 to 40 V/cm Torr) and at higher pressures (16 to 32 Torr). Because of the observed presence of a detachment reaction in the mixture, the role of associative detachment in making oxygen behave as a nonattaching gas is reexamined.