Monte Carlo simulation of ion motion in drift tubes

Abstract

The motion of a swarm of ions in a uniform electric field is studied by simulating the motion of a single ion through many collisions with neutral atoms in order to obtain the drift velocity, average energy, and velocity distribution for the ions. For K⁺ ions in He at low field strengths, the results agree well with the solutions of the Boltzmann equation by Kumar and Robson; and for K⁺ in Ar at all field strengths, the computed mobilities demonstrate that the Viehland–Mason moment method can give useful results, especially if carried through to third order. The velocity distributions computed for O⁺ ions in He and Ar are used in the accompanying paper by Albritton et al. to analyze drift tube measurements of O⁺ reaction rates. Significant deviations from the Maxwell–Boltzmann form have been found and are seen to have important effects in that application. Velocity distributions have also been obtained for Li⁺ in He. The sensitivity of ionic mobilities to changes in the ion–atom interaction potential is examined with particular reference to K⁺ ions in Ar.