Interaction of Low-Energy Electrons with Water Vapor and with Other Polar Molecules

Abstract

Electron attachment coefficients and drift velocities have been measured in mixtures of H2O vapor with N2, CH4, C2H4, and with CO2. Electron capture takes place only in the H2O–CO2 mixture, and even in this case the attachment coefficient is quite small. Thus, low-energy (less than 1 eV) electrons do not, in general, form stable negative ions with H2O. Electron drift velocities are very sensitive to the addition of small quantities of H2O vapor in all the mixtures. When certain other polar molecules (acetone, heavy water, methyl alcohol, dimethyl ether, hydrogen sulfide, toluene and nitrous oxide) were mixed with C2H4, it was found that the decreases in drift velocity correlated well with the magnitude of the electrical dipole moments. The drift velocity data were analyzed to obtain the cross sections for momentum transfer. These cross sections are in fair agreement with the theory of Altshuler, except for H2O, D2O, and H2S, where the experimental values are a factor of 2 larger than theory. This discrepancy may be due to molecular rotational enhancement of the momentum transfer cross section, or it may be due to the temporary formation of negative ions, an effect which would decrease the observed drift velocity.