Direct Measurement of the Dependence on Electron Density of the Recombination-Rate Coefficient ofHe2+with Electrons in a High-Pressure Helium Plasma

A computer-linked spectroscopic system with a high-sensitivity-resolution product has been used to measure the time-dependent net rate of production of neutral helium molecules resulting from the recombination of $\mathrm{He}_2^{}{}_{}{}^{+}$ with electrons in a pulsed helium afterglow at a neutral pressure of 44.6 Torr. The relative recombination-rate cofficient of $\mathrm{He}_2^{}{}_{}{}^{+}$ as a function of afterglow time has been obtained by dividing these data by the unscaled product of the electron and molecular ion density as determined from levels in Saha equilibrium with the free electrons. Simultaneous measurement of the electron density with a 35-Gc/sec microwave interferometer has given the unscaled rate coefficient as a continuous function of electron density over the range ${10}^{10}$-${10}^{12}$ ${\mathrm{cm}}^{-3\mathrm{}}$ without the need of a priori assumptions about the functional form. Scale has been established through integration of the rate equation for the loss of ions. Empirically, the rate coefficient can be represented as $\alpha =2.8\mathrm{}\times {10}^{-11\mathrm{}}{\mathrm{}\left[e\right]\mathrm{}}^{0.185}$ ${\mathrm{cm}}^3$/sec, where $e$ is the electron density in ${\mathrm{cm}}^{-3\mathrm{}}$.