Low-Energy-Electron Attachment to Oxygen Clusters Produced by Nozzle Expansion

Electron attachment to ${\mathrm{O}}_2$ clusters gives two homologous series, ${{\left({\mathrm{O}}_2\right)}_n}^{-}$ and ${\left({\mathrm{O}}_2\right)}_n{\mathrm{O}}^{-}$. Measured relative attachment-cross-section functions show significant differences, i.e., ${\left({\mathrm{O}}_2\right)}_n{\mathrm{O}}^{-}$ ions show a similar behavior to ${\mathrm{O}}^{-}$ from ${\mathrm{O}}_2$ (a single peak at ∼7 eV), whereas ${{\left({\mathrm{O}}_2\right)}_n}^{-}$ ions show an additional peak at ∼0 eV. The latter result explains the large thermal-electron attachment rates observed in high-pressure and low-temperature swarm experiments.