Pressure and Electron Density Dependence of the Electron-Ion Recombination Coefficient in Helium

Abstract

The dependence of the recombination coefficient $\ensuremath{\alpha}$ of ${\mathrm{He}}_{2}^{+}$ ions and electrons on electron density and gas pressure is measured in helium afterglow plasmas where electron and gas temperatures are equal to 300\ifmmode^\circ\else\textdegree\fi{} K, gas pressure ranges from 10 to 100 Torr, and electron density from ${10}^{9}$ to 5\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\ensuremath{-}3}$. Electron density decay measured by microwave interferometry is compared with computer solutions of a continuity equation for electrons which takes into account ambipolar diffusion and recombination effects. One finds $\ensuremath{\alpha}={\ensuremath{\alpha}}_{2}+{k}_{e}{n}_{e}+{k}_{\mathrm{He}}{n}_{\mathrm{He}}$, where ${n}_{e}$ and ${n}_{\mathrm{He}}$ are the electron and neutral densities, respectively, and ${\ensuremath{\alpha}}_{2}\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ ${\mathrm{cm}}^{3}$ ${\mathrm{sec}}^{\ensuremath{-}1}$, ${k}_{e}=(2\ifmmode\pm\else\textpm\fi{}0.7)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}20}$ ${\mathrm{cm}}^{6}$ ${\mathrm{sec}}^{\ensuremath{-}1}$, ${k}_{\mathrm{He}}=(2\ifmmode\pm\else\textpm\fi{}0.5)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}27}$ ${\mathrm{cm}}^{6}$ ${\mathrm{sec}}^{\ensuremath{-}1}$. These values compare satisfactorily with results of theoretical computations for a collisional-radiative recombination mechanism including both collisions with electrons and neutrals. The effect of neutrals is particularly noteworthy as far as future experimental work on weakly ionized gases is concerned.