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

The dependence of the recombination coefficient $\alpha$ of ${\mathrm{He}}_2^{+}$ 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 ${10}^9$ to 5×${10}^{11}$ ${\mathrm{cm}}^{-3\mathrm{}}$. 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 $\alpha ={\alpha }_2+k_e{n}_e+k_{\mathrm{He}}{n}_{\mathrm{He}}$, where $n_e$ and $n_{\mathrm{He}}$ are the electron and neutral densities, respectively, and ${\alpha }_2\sim 5\times {10}^{-10\mathrm{}}$ ${\mathrm{cm}}^3$ ${\sec }^{-1\mathrm{}}$, $k_e=(2\mathrm{}\pm 0.7)\times {10}^{-20\mathrm{}}$ ${\mathrm{cm}}^6$ ${\sec }^{-1\mathrm{}}$, $k_{\mathrm{He}}=(2\mathrm{}\pm 0.5)\times {10}^{-27\mathrm{}}$ ${\mathrm{cm}}^6$ ${\sec }^{-1\mathrm{}}$. 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.