Cross Section for Formation of Doubly-Ionized Helium by Electron Impact

The cross section for the formation of ${\mathrm{He}}^{++}$ by electron impact has been measured relative to that of ${\mathrm{He}}^{+}$ for electron energies between 100 ev and 2400 ev. A relative minimum in the measured ratio of the yields of ${\mathrm{He}}^{+}$ to ${\mathrm{He}}^{++}$ at an energy of about 600 ev is believed to be real. For incident energies above 1400 ev the results are consistent with a constant value of 145 for this ratio.