Ionization and Scattering Accompanying Positive Ion Impact in Gases

The ionization produced by ${\mathrm{K}}^{+}$ ion impact in A, Ne, He, ${\mathrm{N}}_2$, CO, ${\mathrm{H}}_2$, and Hg was studied for voltages ranging from 0 to 4000 volts. These results check with the results of previous investigators wherever it was possible to make comparisons. The A curve saturates (i.e., the ionization in A approaches a constant maximum value as the voltage is increased) at 4000 volts. The Ne curve shows a tendency to saturate at 4000 volts. The ionization in He and ${\mathrm{H}}_2$ was very weak. ${\mathrm{N}}_2$ and CO ionize equally well and their curves fall just a little above the neon curve. The minimum potentials at which ionization sets in vary from 100 volts for A to 1600 for ${\mathrm{H}}_2$. Accurate measurement of these potentials was not attempted. It is shown that when the results are plotted against the kinetic energy of the impinging ion-molecule system (relative to its center of mass) that the various ionization potentials show a comparatively small range. The scattering experiments show that at low voltage a few of the ${\mathrm{K}}^{+}$ ions make direct impact in mercury and are reflected with $\frac{4}{9}$ of their original energy which is the value expected from conservation of momentum and energy. Small angle scattering was also observed in this case and for ${\mathrm{K}}^{+}$ and ${\mathrm{Na}}^{+}$ in argon. No ions are here observed with appreciable velocity in the reverse direction. The evidence indicates that the gases which ionize most readily are also most effective as a scattering agent. It is suggested that ionizing collisions are glancing collisions.