Resonant charge exchange and the transport of ions at high electric-field to gas-density ratios (E/N) in argon, neon, and helium

Translational kinetic-energy distributions of singly and doubly charged ions have been measured at high electric-field to gas-density ratios (E/N) up to 5.0×${10}^{\mathrm{-}17}$ V ${\mathrm{m}}^2$ (50 kTd) in diffuse, parallel-plate Townsend discharges in Ar, Ne, and He using an ion energy analyzer-mass spectrometer. For ${\mathrm{Ar}}^{+}$ in Ar and ${\mathrm{Ne}}^{+}$ in Ne when E/N <2.0×${10}^{\mathrm{-}17}$ V ${\mathrm{m}}^2$ and for ${\mathrm{He}}^{+}$ in He when E/N<1.0×${10}^{\mathrm{-}17}$ V ${\mathrm{m}}^2$, the energy distributions are Maxwellian and consistent with predictions based on the assumption that resonant symmetric charge exchange is the dominant ion–neutral-species collision process. At higher E/N values, the kinetic-energy distributions for ${\mathrm{Ar}}^{+}$, ${\mathrm{Ne}}^{+}$, and ${\mathrm{He}}^{+}$ show departures from the Maxwellian form that are indicative of deviations from the charge-transfer model. The mean ion energies (effective ion temperatures) are consistent in the low E/N range with the available drift-velocity data, and in the case of ${\mathrm{Ar}}^{+}$ with recent results of Radovanov et al. [Phys. Rev. E 51, 6036 (1995)] from Townsend discharge experiments. The charge-exchange cross sections derived from Maxwellian fits to the energy distribution data for ${\mathrm{Ar}}^{+}$ + Ar, ${\mathrm{Ne}}^{+}$ + Ne, and ${\mathrm{He}}^{+}$ + He agree with available data. The relative contributions of the doubly charged ions ${\mathrm{Ar}}^{2+}$, ${\mathrm{Ne}}^{2+}$, and ${\mathrm{He}}^{2+}$ to the total ion flux were found to be small (less than 3%) and tend to decrease initially with increasing E/N. The mean energies of the doubly charged ions are higher than those for the corresponding singly charged ions, and the results suggest that double charge transfer could be the dominant process affecting the transport of ${\mathrm{Ar}}^{2+}$ and ${\mathrm{Ne}}^{2+}$ for E/N below about 1.5×${10}^{\mathrm{-}17}$ V ${\mathrm{m}}^2$. The observed ${\mathrm{He}}^{2+}$ kinetic-energy distributions are not consistent with a charge-transfer model.