Cross-section for the reaction O^- + O₂ → O₂^- at relative kinetic energies from threshold to 3 eV

The cross-section for the charge-transfer reaction of O^- with O₂ is determined in the relative kinetic energy range from 1 eV (threshold) to about 3 eV, using a combination of experimental and theoretical ion-swarm techniques. Reaction rate constants are measured in a helium-buffered flow-drift tube in the range 50 ≤ E/N ≤ 110 Td, where E is the DC electric field strength, N is the helium gas number density and 1 Townsend (Td) 10^-17 V cm². Velocity distributions are computed using Monte Carlo techniques for O^- ions drifting in helium at E/N values in this range. These distributions and the drift-tube rate constants define the reaction cross-section at low collision energies, a region in which it has been difficult to make reliable ion-beam measurements.