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+ H2O⇌O2˙ 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.