Low-Temperature Measurements of the Three-Body Electron-Attachment Coefficient inO2

The three-body electron-attachment coefficient for ${\mathrm{O}}_2$ has been determined by microwavecavity techniques to be 2.1 × ${10}^{-30\mathrm{}}$ ${\mathrm{cm}}^6$/sec±10% at 300 °K, 1.5 × ${10}^{-30\mathrm{}}$ ${\mathrm{cm}}^6$/sec±15% at 200 °K, and 7.2 × ${10}^{-31\mathrm{}}$ ${\mathrm{cm}}^6$/sec±20% at 113 °K. Initial electron densities were produced by singlepulse photoionization of small amounts of NO in the ${\mathrm{O}}_2$. For a given ${\mathrm{O}}_2$ pressure the electron density decay rates were found to be independent of the NO concentration used in this study. Mass analysis of ions in the afterglow shows that N${\mathrm{O}}^{+}$ is the predominant initial positive ion. Although ${\mathrm{O}}_2^{-}$ is the initially predominant negative ion observed at 300 °K, it reacts rapidly at low temperature to form ${\mathrm{O}}_4^{-}$ and N${\mathrm{O}}_3^{-}$.