Mobilities and Reaction Rates of Ions in Helium

Abstract

The mobility of ${\mathrm{He}}^{+}$ ions in helium gas was measured, the result being 10.40\ifmmode\pm\else\textpm\fi{}0.10 (${\mathrm{cm}}^{2}$/V-sec). The ${\mathrm{He}}^{+}$ ions were observed to react with the helium gas, the attachment frequency being [1.08\ifmmode\pm\else\textpm\fi{}0.05]\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}31}$ ${n}^{2}(invsec)$, where $n$ is the gas density in atoms/${\mathrm{cm}}^{3}$. This value of the attachment frequency is roughly compatible with the theoretical estimate of Bates and with the mass-spectrometric measurement of Phelps and Brown for the rate of the three-body attachment reaction producing ${\mathrm{He}}_{2}^{+}$. The mobility of the production, ${\mathrm{He}}_{2}^{+}$, was found to be 16.70\ifmmode\pm\else\textpm\fi{}0.17. This mobility is compatible with recent ambipolar diffusion coefficient measurements on cold afterglows, where the identification of the ion is confirmed by the spectroscopic observations of Kerr. This mobility is inconsistent with the widely quoted value of Biondi and Chanin and with the theoretical analyses of Geltman and Dalgarno. Mobility determinations were also made on another ion, which is possibly what Biondi and Chanin called ${\mathrm{He}}_{2}^{+}$. The actual identity is unknown, the ion possibly being ${\mathrm{He}}^{2+}$, possibly a contaminant.