Ionization of Gases as a Function of the Energy of Electron Impacts

Ionization of various gases by electrons of energy up to 300 volts.—Electrons were accelerated from a tungsten wire to a plane perforated electrode $G$, through which a small fraction passed into a space where they were all stopped by a retarding field between the perforated electrode and a plane parallel plate $P$ connected to an electrometer, which collected all the positive ions formed on collision between the electrons and the gas molecules. The fraction of the collisions resulting in ionization as a function of the energy of impact is shown to be $f\left(V^{\prime }\right)=\left(\frac{1}{2apD}\right)\left[\frac{d\left(\frac{V_{0}P}{E}\right)}{d{V}_0}\right]$, where $\frac{P}{E}$ is the ratio of positive ions produced to the number of electrons passing through the perforated electrode $G$, $V_0$ is the retarding potential between $P$ and $G$ and is somewhat greater than $V^{\prime }$ the maximum energy of impact, $p$ is the pressure, $D$ the distance apart of $P$ and $G$, and $a$ is the chance of collision per cm for 1 mm pressure which is assumed to have the Kinetic Theory value. For helium, neon, argon, hydrogen, nitrogen, and methane, $f\left(V^{\prime }\right)$ was found to increase very rapidly to a maximum value, after which there was a less rapid, though marked, decrease. The maxima occur at 147, 157, 80, 74, 101, and 80 volts for the gases in the order named, the maximum values being respectively.11,.14,.35,.21,.32, and.28. Even for the most favorable velocities, then, less than half of the collisions result in ionization.