Time-of-flight determinations of electron diffusion coefficients and electron drift velocities in ethylene, water vapor, and in hydrogen

The time-of-flight method for the determination of electron diffusion coefficients D and drift velocities ω of electrons in gases has been improved in two essential ways. First, an accurate method of measuring all fluctuation in the time-of-flight distribution, except that due to the diffusion process itself, has been developed. Second, a data analysis procedure has been worked out in which the effects of instrumental fluctuations are removed and accurate values of the parameters D and ω are found from a generalized least-squares procedure making use of a digital computer. Distortions in the electron time-of-flight distributions arising out of electron losses during detector dead times were calculated by assuming that single-electron detection follows Poisson statistics. This assumption was examined experimentally and was not found to be in error. Application of the improved method has been made to ethylene, ethylene-water vapor mixtures, and hydrogen over the range of E/P from 0.01 to 1.0 V cm^-1- torr^-1. Independent measurements of the quantities D and ω were made; the ratios D/ω were compared with theory in the region of thermal electron energies. The ratio of the momentum transfer cross sections for H₂0 compared to ethylene was essentially the same whether use was made of D(E/P) or ω(E/P) data in the region of thermal energies. In the cases of ethylene and ethylene-water vapor mixtures, it was found that the value of D/ω for thermal electrons was 12% higher than the theoretical value.