Recombination and Ionization in a Molecular-Ion-Dominated Helium Afterglow

Abstract

An intense source of free electrons has been observed to strongly influence the time history of the electron density in a high-pressure (> 15-Torr) helium afterglow. The electronic recombination coefficient $\ensuremath{\alpha}$ of molecular helium ions was measured by utilizing this source and was found to be over five times larger than the effective recombination coefficient obtained by equating the net loss rate of free electrons to the recombination loss rate in the usual manner. If $\ensuremath{\alpha}$ is assumed to be of the form $\ensuremath{\alpha}={\ensuremath{\alpha}}_{0}+{k}_{p}p$, then ${\ensuremath{\alpha}}_{0}=1.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}$ ${\mathrm{cm}}^{3}$ ${\mathrm{sec}}^{\ensuremath{-}1}$ and ${k}_{p}=4.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ ${\mathrm{cm}}^{3}$ ${\mathrm{sec}}^{\ensuremath{-}1}$ ${\mathrm{Torr}}^{\ensuremath{-}1}$ over the pressure range from 15 to 56 Torr. A model is presented which indicates that ionizing metastable-metastable collisions are responsible for the source of free electrons.