Investigation of Negative Ion Mobility and Ion-Molecule Reactions in Atmospheric O₂ with a Small Amount of H₂O Based on Ion Mobility Measurement

A study of transport coefficients such as a mobility and a diffusion coefficient has been carried out in low pressure gases for collecting the fundamental data of discharge plasmas. Recently, the fundamental data are required for investigating atmospheric pressure plasmas. In this study, we carried out a Monte Carlo simulation using mobility of electrons, that of the ions obtained in the measurements, and rate coefficients of ion-molecule reactions. Then, we compared the experimental result with that of the simulation and estimated the ion mobility and rate coefficients of ion molecule reactions. The negative ion mobility was measured in O₂ at pressures between 840 and 900 Torr with a little amount of H₂O concentration between 15 and 17000 ppb. As the results, the mobility value 2.39 cm²/V∙s of O₄ˉ is obtained in ultrahigh purity O₂ of which the H₂O concentration is between 15 to 100 ppb. Moreover, the mobility decreases with the H₂O concentrations at which the ion species are considered to be O₂ˉ∙(H₂O)_n (n = 1, 2, 3). Then we investigated the influence of each mobility of ions and rate coefficients of ion-molecule reactions on observed ion mobility in the measurement.