The rate coefficient ${\beta }_1$ 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 ${\beta }_1$ is also determined with a perturbation technique. All the values of ${\beta }_1$ determined by these techniques lie within the range (4.5±1.0)×${10}^{-9\mathrm{}}$ ${\mathrm{cm}}^3$ ${\sec }^{-1\mathrm{}}$. 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.

• Received 29 September 1972

DOI:https://doi.org/10.1103/PhysRevA.7.925