Effects of ion speed distributions in flow-drift tube studies of ion-neutral reactions

The effects of non-Maxwellian ion speed distributions on ion-neutral reaction rate constants measured in drift tubes are examined experimentally and are compared to the predictions of recent theories. The rate constants of strongly kinetic-energy-dependent ion-molecule reactions of O⁺ with O₂, N₂, and NO are measured separately in helium and argon buffer gases, in which the O⁺ speed distributions are expected to be very different. The differences between the helium-buffered and argon-buffered rate constants are often substantial. When different, the argon-buffered values are generally larger than the helium-buffered values at the same mean energy, indicating that the O⁺-in-argon distribution has a larger high-energy "tail" than the O₊-in-helium distribution. The differences between the two sets of data are compared to predictions from (a) the Monte Carlo trajectory calculations of Lin and Bardsley, and (b) the moment solution of the Boltzmann equation of Viehland and Mason, both described in accompanying papers. The excellent agreement demonstrates that the non-Maxwellian ion speed distributions now pose no problems in the interpretation and application of the kinetic-energy aspects of the rate constants of atomic-ion reactions studied in rare-gas-buffered drift tubes.