Electron-Temperature Dependence of Electron-Ion Recombination in Argon

A 3-mode microwave apparatus is used to study the dependence of the dissociative recombination coefficient α(Ar₂^_⁺) on electron temperature Tₑ over the range 300^∘K<~Tₑ<~10000^∘K while the ion and gas temperatures remain at 300^∘{}K. A high-Q cavity mode is used to ionize the gas periodically, a second high-Q mode is used to determine the decay of electron density during the afterglow from the change of resonant frequency of the cavity, and a nonresonant waveguide mode is used to apply a constant microwave heating field to the electrons. At Tₑ=300^∘ K, a value α(Ar₂^_⁺)=(8.5±0.8)×10^-7 cm³/sec is obtained from analysis of recombination-controlled electron-density decays using a computer solution of the electron continuity equation which includes recombination and ambipolar diffusion terms. Over the range 300^∘K<~Tₑ<~10000^∘K, the coefficient α(Ar₂^_⁺) is found to follow a simple Tₑ^_^-0.67 variation. This result is reconciled with Fox and Hobson's T^-1.3 variation, obtained under conditions where Tₑ=Tᵢ=T, on the basis of their changing Ar₂^_⁺ vibrational-state populations with changing temperature. Finally, it is noted that our Tₑ^_^-0.67 variation may be evidence for appreciable electron capture via the indirect as well as the direct dissociative process.