Measurement of the Rate Coefficient for the Recombination ofHe+with Electrons

Abstract

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 atoms resulting from the recombination of ${\mathrm{He}}^{+}$ with electrons in a pulsed helium afterglow at neutral pressures of 1.86 and 44.6 Torr. The recombination-rate coefficient of ${\mathrm{He}}^{+}$ as a function of afterglow time has been obtained by dividing these data by the product of the electron and atomic-ion density as determined from intensity measurements of radiation from bound levels in Saha equilibrium with the free electrons. Simultaneous time-resolved measurement of the electron density with a 35-GHz Fabry-Perot resonator has given the recombination-rate coefficient as a continuous function of electron densities without the need of a priori assumptions about that functional form. The electron temperature as a function of electron density in the recombining afterglow was determined from analyses of time-resolved measurements of the intensity distribution of radiation from the continuum states above the $n^{3}P\ensuremath{\rightarrow}2^{3}S$ Rydberg series in atomic helium. For pressures below a nominal 2 Torr, the resulting parametrization of the recombination-rate coefficient for ${\mathrm{He}}^{+}$ with electrons can be represented as $\ensuremath{\alpha}=7.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}20}{(\frac{T}{300})}^{\ensuremath{-}4.3}[e]$ ${\mathrm{cm}}^{3}$/sec, where $T$ is the electron temperature and [$e$] the electron density. At higher pressures the rate coefficient is found to decrease as a consequence of associative ionization of the excited states.