The Drift Velocities of Molecular and Atomic Ions in Helium, Neon, and Argon

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Drift velocity measurements as a function of $\frac{E}{{p}_{0}}$ the ratio of field strength to normalized gas pressure, are presented for atomic and molecular ions of He, Ne, and A in their respective parent gases. Identification of the molecular ions is based upon the time resolution of the apparatus and the dependence of ion concentration on pressure, applied voltage, and gas purity. Extrapolation of the low field measurements to zero field yields mobility values for atomic ions, ${\ensuremath{\mu}}_{0}({\mathrm{He}}^{+})=10.8$ ${\mathrm{cm}}^{2}$/volt sec, ${\ensuremath{\mu}}_{0}({\mathrm{Ne}}^{+})=4.4$, and ${\ensuremath{\mu}}_{0}({\mathrm{A}}^{+})=1.63$ in good agreement with theory: Massey and Mohr compute ${\ensuremath{\mu}}_{0}({\mathrm{He}}^{+})=11$, and Holstein gives ${\ensuremath{\mu}}_{0}({\mathrm{Ne}}^{+})=4.1$ and ${\ensuremath{\mu}}_{0}({\mathrm{A}}^{+})=1.64$. Drift velocity data at low field for the molecular ions agree within experimental error with data of Tyndall and Powell (He), and Munson and Tyndall (Ne and A), which they assigned to atomic ions. A qualitative description in terms of ion-atom interaction forces is given for the observed field variation of the atomic ion drift velocities up to high $\frac{E}{{p}_{0}}$.