Determination of Diffusion Coefficients of Thermal Electrons with a Time-of-Flight Swarm Experiment

A typical time-of-flight electron-swarm experiment was modified by replacing the electric-field–ring assembly with a conducting paper cylinder in order to (1) increase the lateral dimension of the swarm region, and (2) remove nonconducting surfaces which become charged with electrons and thereby provide the zero-field condition necessary for thermal electron diffusion and capture studies in gases. A new drift–dwell–drift technique was devised whereby electron swarms were (1) drifted to the center of the swarm region, (2) allowed to dwell there for varying periods of time under field-free conditions, and then (3) drifted to the anode where proper sampling with a differentially pumped electron-multiplier detection system allowed the reconstruction of the swarm time-of-arrival distribution. Diffusion coefficients were determined for thermal electron swarms in nine gases. The mean thermal diffusion-pressure (DtP) values were 0.214, 2.01, 0.885, 0.133, 0.294, 0.178, 0.0151, 0.253, and 0.281 cm2 μsec−1 torr for He, Ne, Ar, H2, N2, CO, CO2, CH4, and C2H4, respectively.