Mass Spectrometric Studies of Molecular Ions in the Noble Gases

Molecular ions of the rare gases (${\mathrm{He}}_2^{+}$, ${\mathrm{Ne}}_2^{+}$, ${\mathrm{A}}_2^{+}$, ${\mathrm{Kr}}_2^{+}$, and ${\mathrm{Xe}}_2^{+}$) produced by electron impact at gas pressures from ${10}^{-4\mathrm{}}$ to ${10}^{-2\mathrm{}}$ mm Hg have been studied with a small mass spectrometer. The ion intensity increased linearly with electron current and with the square of the gas pressure. The form of the ionization versus electron energy curves resembles closely curves of excitation probability by electron collision. The appearance potentials of the molecular ions were less than those of the atomic ions by ${1.4}_{-0.2\mathrm{}}^{+0.7\mathrm{}}$ volts in He, ${0.7}_{-0.3\mathrm{}}^{+0.7\mathrm{}}$ volt in Ne, ${0.7}_{-0.2\mathrm{}}^{+0.7\mathrm{}}$ volt in A, ${0.7}_{-0.3\mathrm{}}^{+0.7\mathrm{}}$ volt in Kr. These results can be interpreted, we believe, only by assuming that the process of formation of the molecular ions observed in this experiment is, using helium as an example, an excitation by electron impact, $\mathrm{He}+e+\mathrm{K}.\mathrm{E}.\mathrm{}\to {\mathrm{He}}^{*}+e$, followed by the collision process, ${\mathrm{He}}^{*}+\mathrm{He}\to \mathrm{He}_2^{}{}_{}{}^{+}+e$, where ${\mathrm{He}}^{*}$ stands for a helium atom raised to a high-lying excited state. Our results differ from those of Arnot and M'Ewen on helium particularly in that they reported the appearance potential low enough to permit metastable atoms to form molecular ions.