Three-Body Attachment inO2Using Electron Beams

The three-body attachment reaction in ${\mathrm{O}}_2$, $e+2\mathrm{}{\mathrm{O}}_2\to {{\mathrm{O}}_2}^{-}+{\mathrm{O}}_2$, is studied by electron-beam techniques. Whereas swarm experiments show a smooth variation of the attachment coefficient with energy, the present electron-beam experiment shows pronounced structure. The peaks of this structure occur at the positions of the vibrational levels of the ${\mathrm{O}}_2^{-}$ system, which serve as compound states and have been identified in a previous experiment. These compound states dominate the low-energy cross sections in ${\mathrm{O}}_2$. They can decay by autodetachment, thus accounting for the bulk of the vibrational cross sections in ${\mathrm{O}}_2$ at low energy; alternatively, they can be stabilized by a subsequent collision, thus leading to the formation of stable ${\mathrm{O}}_2^{-}$. Thus we establish clearly that three-body attachment in ${\mathrm{O}}_2$ is a two-stage process and that the intermediate is a vibrationally excited state of ${\mathrm{O}}_2^{-}$. We also obtain the temperature dependence of the three-body attachment coefficient and we discuss the anomalous behavior at higher pressures. The experimental observations are in excellent agreement with the theory of Chapman and Herzenberg.