Ionizing collisions of two metastable helium atoms (23S)

The rate coefficient β₁ for the ionizing collision of two metastable helium atoms ($2^{3}S) is evaluated from measurements in a steadily decaying helium afterglow plasma with three different techniques. The value of{β}_{1}$ is also determined with a perturbation technique. All the values of β₁ determined by these techniques lie within the range (4.5±1.0)×10⁻⁹ cm³ sec⁻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 He₂⁺ is a significantly large source of atomic metastables.