Relative Production of Negative and Positive Ions by Electron Collisions

In the tube used, either positive or negative ions produced by an axial beam of electrons accelerated from a filament by a voltage $V$, were collected on a concentric cylindrical electrode $D$ by maintaining it at a potential slightly lower or higher than that of the beam, the primary electrons being prevented from reaching $D$ by means of a solenoidal magnetic field of 100 gauss parallel to the axis. While saturation was not always reached, the relative values of the positive and negative ion currents are significant. Contrary to the ordinary theory, the polar compounds mercuric chloride and hydrogen chloride gave results quite similar to those for the elements mercury and iodine. With electron currents of the order of ${10}^{-6\mathrm{}}$ amp. and pressures of about.001 mm, the negative ion current is in all cases small compared with the positive ion current above the ionization potential. With larger currents the negative current increases sharply at the ionization potential and the ratio to positive current is greater. For vapors other than mercury there are some negative ions produced at all voltages, the negative ion currents each having a maximum at zero and increasing again at certain critical potentials, the voltages being 4 for Hg${\mathrm{Cl}}_2$, 6.5 for HCl and 2.4, 4.6, 8.4 for ${\mathrm{I}}_2$. As far as is known, these potentials are equal to potentials of inelastic collision. These experiments give no support to the theory that electron collisions dissociate polar molecules into positive and negative ions. In agreement with the results of the magnetic analysis of positive ions, they indicate that the primary effect of an electron impact is the production of a positive molecule ion. The negative ion curves obtained can be explained by the hypothesis that only slow moving electrons attach themselves to molecules to form ions, but the absence of low voltage negative ions in the case of Hg is not understood.