Pressure and Electron Density Dependence of the Electron-Ion Recombination Coefficient in Helium

The dependence of the recombination coefficient α of He₂⁺ ions and electrons on electron density and gas pressure is measured in helium afterglow plasmas where electron and gas temperatures are equal to 300^∘{} K, gas pressure ranges from 10 to 100 Torr, and electron density from 10⁹ to 5×{}10¹¹ cm^-3. Electron density decay measured by microwave interferometry is compared with computer solutions of a continuity equation for electrons which takes into account ambipolar diffusion and recombination effects. One finds α=α₂+kₑnₑ+kHenHe, where nₑ and nHe are the electron and neutral densities, respectively, and α₂~5×10^-10 cm³ sec^-1, kₑ=(2±0.7)×10^-20 cm⁶ sec^-1, kHe=(2±0.5)×10^-27 cm⁶ sec^-1. These values compare satisfactorily with results of theoretical computations for a collisional-radiative recombination mechanism including both collisions with electrons and neutrals. The effect of neutrals is particularly noteworthy as far as future experimental work on weakly ionized gases is concerned.