Transient ion currents following an initiating electron pulse have been studied in oxygen over a range of $\mathrm{pd}$ from 7 to 26 cm mm Hg and of $\frac{E}{p}$ from 9 to 50 volts/cm mm. Analysis of oscillographic records yields information regarding ionic drift velocities and the electron attachment coefficient. Three distinct and pressure-independent velocities are found consistent with zero field mobilities of 3.4, 2.6, and 1.95 ${\mathrm{cm}}^2$/v sec. The multiplicity of velocities is ascribed to clustering reactions through which a single initially formed species of negative ion is converted into two slower species, although initially formed ${\mathrm{O}}_2^{+}$ ions retain their identity. It is then possible to compute the attachment coefficient from measured ion transient areas. The attachment coefficient thus obtained varies gently in the region $10\le \frac{E}{p}\le 25$ and joins at each end with curves previously reported. Consideration of the mobilities of the ions as well as other evidence leads to the identification of the initial ion as ${\mathrm{O}}^{-}$, while the clustered species appear to be ${\mathrm{O}}_3^{-}$ and ${\mathrm{O}}_2^{-}$ corresponding, respectively, to the above mobilities. No difference is detected between the mobilities of ${\mathrm{O}}_2^{+}$ and ${\mathrm{O}}_2^{-}$ ions.

• Received 8 October 1956

DOI:https://doi.org/10.1103/PhysRev.106.183