Collision Cross Section and Energy Loss of Slow Electrons in Hydrogen

Abstract

Values for the collision cross section for momentum transfer and the fractional energy lost by an electron on collision with a hydrogen molecule were obtained from measurements of the microwave conductivity of a gaseous plasma. The experiments were made in the afterglow of a pulsed discharge in a cavity resonating at a wavelength of 10 cm. The mean electron energy was varied from 0.04 ev (room temperature) to 0.08 ev by heating the gas, and from 0.04 ev to 1.6 ev by microwave agitation of the electrons alone. The collision probability in molecular hydrogen at electron energies in the neighborhood of 0.04 ev is found to be $28.5{\left(\frac{v}{v_0}\right)}^{0.55} {(\mathrm{c}\mathrm{m}-\mathrm{m}\mathrm{m}\mathrm{H}\mathrm{g})}^{-1\mathrm{}}\pm 3\%$, where $v$ is the electron velocity, and $v_0$ the most probable velocity at 300°K. Measurements in deuterium gas gave the same result. The collision probability in hydrogen increases to a peak value of 64 ${\mathrm{}(\mathrm{c}\mathrm{m}-\mathrm{m}\mathrm{m}\mathrm{H}\mathrm{g})\mathrm{}}^{-1\mathrm{}}$±7% at an electron energy of approximately 1.4 ev. The energy loss of electrons near room temperature was found by comparing the conductivity measurements obtained by thermal agitation with those obtained by microwave agitation of the electrons. The fraction of the excess electron energy lost on collision with the hydrogen molecule is (3.5±0.5)×${10}^{-3\mathrm{}}$. In the higher energy range, the loss and the collision probability were computed from a comparison between microwave conductivity measurements and electron drift velocity and diffusion experiments.