Electron attachment and negative ion-molecule reactions in pure oxygen

Abstract

The formation of negative ions in oxygen by electron attachment and by subsequent ion-molecule reactions has been studied in the gas phase, using a drift-tube and mass filter. Pressures ranged between 0.65 and 13 mbar (0.5–10 Torr) and reduced fields (field/density) between 3 × 10^–18 and 6 × 10^–16 V cm² part.^–1. At reduced fields below 10^–16 V cm² part.^–1 masses 32 (O^– ₂), 64 (O^– ₄) and 50 (O₂˙H₂O^–) predominate. The O^– ₂ formed initially is converted to O^– ₄ in a three-body reaction, O^– ₂+ 2O₂ O^– ₄+ O₂, with a thermal rate constant of 3 ± 0.5 × 10^–31 part.^–2 ml² s^–1 at 298 K. The back reaction has a rate constant of 7 ± 1 × 10^–15 part.^–1 ml s^–1. The hydrate concentration is related to that of the dimer via the reactions O^– ₄+ H₂OO₂˙ H₂O^–+ O₂. The forward rate constant is estimated to be of the order of 10^–9 part.^–1 ml s^–1, and the back measured as 2.5 ± 1 × 10^–15 part.^–1ml s^–1. At higher fields the initially formed mass 16 (O^–) is converted irreversibly to mass 48 (O^– ₃): O^–+ 2O₂→O^– ₃+ O₂. The thermal rate constant is measured as 1.05 ± 0.1 × 10^–30 part. ^–2 ml s^–1 up to pressures of at least 13 mbar, without any significant change in order. The change in the rate constants with ion collisional energy has also been followed.