Positive Ions in the Afterglow of a Low Pressure Helium Discharge

Abstract

The production and loss of ${\mathrm{He}}^{+}$ and $\mathrm{He}_{2}^{+}$ ions in the afterglow of a low pressure helium discharge are studied using a mass spectrometer to analyze the positive ions and microwave techniques to determine the electron density. The change of positive ion and electron density with time is explained by considering three dominant processes in the afterglow: the production of ${\mathrm{He}}^{+}$ ions and electrons by collisions between pairs of metastable atoms, the ambipolar diffusion of the ions and electrons, and the conversion of ${\mathrm{He}}^{+}$ ions to $\mathrm{He}_{2}^{+}$ ions by three-body collisions with neutral atoms. The time constants for the decay of the electron density at low pressures yield the ambipolar diffusion coefficient for ${\mathrm{He}}^{+}$ ions of 560 ${\mathrm{cm}}^{2}$/sec and a frequency for the conversion of ${\mathrm{He}}^{+}$ ions into $\mathrm{He}_{2}^{+}$ ions of 65 ${\mathrm{sec}}^{\ensuremath{-}1}$, both at 1 mm pressure and 300\ifmmode^\circ\else\textdegree\fi{}K. The mobility coefficient for ${\mathrm{He}}^{+}$ ions in helium obtained from the measured ambipolar diffusion coefficient is 14 ${\mathrm{cm}}^{2}$/volt-sec at 300\ifmmode^\circ\else\textdegree\fi{}K and 760-mm pressure and agrees satisfactorily with the value of 12 ${\mathrm{cm}}^{2}$/volt-sec calculated using the quantum-mechanical interaction of the ${\mathrm{He}}^{+}$ ion and the neutral helium atom.