Electron transport and ion clustering reactions in water vapor and deuterated water vapor

Abstract

The interaction of low energy electrons and negative ions with pure water vapor and deuterated water vapor has been studied in the following two experiments: (I) A time-of-flight electron swarm experiment has been used to determine longitudinal electron diffusion coefficients, DL, in H2O and D2O for electric-field-to-pressure ratios, E/P, from 1 to 25 V cm−1⋅Torr−1. The ratio of the longitudinal diffusion coefficient to mobility DT/μ for D2O is greater than that for H2O in the range of E/P from 5 to 20 V cm−1⋅Torr−1. Measurements of DT/μ for H2O are in agreement with the calculations of Lowke and Parker. Electron drift velocities have also been measured for values of E/P from 0.5 to 25 V cm−1⋅Torr−1, and the values for D2O are found to be considerably greater than for H2O in the range of E/P from 10 to 25 V cm−1⋅Torr−1. This result is justified on the basis of the energy dependence of the momentum transfer cross section for polar molecules. (II) Negative ions with masses in the range from 1 to 200 amu produced by electron swarm interactions and subsequent ion–molecule and ion–clustering reactions in H2O and D2O at room temperature have been recorded as a function of E/P from 0 to 70 V cm−1⋅Torr−1 and pressures from 0.1 to 5 Torr. The ions observed are H−(D−), OH− (OD−), and OH−⋅nH2O(OD−⋅nD2O) where n=1 to 7. The appearance of cluster ions from D2O occurred at a lower E/P than from H2O. This observation is compatible with our reported measurements of the characteristic electron energies (D/μ) for these vapors. The variation of the cluster ion distribution with pressure and E/P verifies the sequential clustering scheme suggested previously by Moruzzi and Phelps. For comparison purposes, negative ion products from electron swarm interactions with ammonia vapor are also presented. The ions observed are the primary ions H− and NH−2 and the cluster ions NH−2⋅nNH3 where n=1 to 4. These results are at variance with the two previous studies of ammonia.