Electron avalanches in oxygen: Detachment from the diatomic ionO2−

The current transients of prebreakdown electron avalanches in uniform electric fields in oxygen are studied with a bakable drift tube and a time resolution of about 1% of the ion transit time involved. At this low time resolution, electronic-current components are discovered which are attributed to detachment from the diatomic ion ${\mathrm{O}}_2^{-}$. Detachment rates of ${\mathrm{O}}_2^{-}$ in ${\mathrm{O}}_2$ are measured over a range of pressures from 1 to 32 Torr, and over a range of field strength to pressure ratios from 35 to 130 V/cm Torr, corresponding to mean ion swarm energies for ${\mathrm{O}}_2^{-}$ between about 0.2 and 1.5 eV. The detachment rates observed depend linearly on pressure, indicating a two-body detachment reaction, and are consistent with a detachment cross section near threshold (0.44 eV) of approximately 0.7 × ${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$. Also obtained are effective electron attachment rates for ${\mathrm{O}}_2^{-}$ and ${\mathrm{O}}_3^{-}$ in oxygen, which show a characteristic pressure dependence consistent with attributing the observed detachment to diatomic ${\mathrm{O}}_2^{-}$. These rates reflect the formation of the diatomic ion ${\mathrm{O}}_2^{-}$ by charge transfer from the primary atomic ion ${\mathrm{O}}^{-}$. The pressure dependence observed, however, is inconsistent with the trial assumptions that either the atomic ion ${\mathrm{O}}^{-}$, or the triatomic ion ${\mathrm{O}}_3^{-}$, detach their electrons at the rates reported here. The mass-spectrometric study undertaken supports these conclusions.