Ionizing collisions of two metastable helium atoms (2$sup 3$S)

The rate coefficient ${\ensuremath{\beta}}_{1}$ for the ionizing collision of two metastable helium atoms ($2^{3}S$) is evaluated from measurements in a steadily decaying helium afterglow plasma with three different techniques. The value of ${\ensuremath{\beta}}_{1}$ is also determined with a perturbation technique. All the values of ${\ensuremath{\beta}}_{1}$ determined by these techniques lie within the range (4.5\ifmmode\pm\else\textpm\fi{}1.0)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}9}$ ${\mathrm{cm}}^{3}$ ${\mathrm{sec}}^{\ensuremath{-}1}$. This value is larger than that observed directly from the decay of the density of the triplet helium metastable atom because the dissociative recombination of $\mathrm{He}_{2}^{+}$ is a significantly large source of atomic metastables.