Recombination of Electrons and Molecular Helium Ions

The recombination coefficient $\alpha (N_e, T_e)$ of ${\mathrm{He}}_2^{+}$ is measured in helium afterglow plasmas ($12 \mathrm{Torr}\le p\le 20 \mathrm{Torr}$; $t\gtrsim 1$ msec) as a function of electron density and gas temperature under conditions where the temperature of electrons and ions equals that of the neutral gas ($2.5\times {10}^{12}{\mathrm{cm}}^{-3\mathrm{}}\le N_e\le 2\times {10}^{13}$ ${\mathrm{cm}}^{-3\mathrm{}}$; $900°\mathrm{K}\le T\le 2200°\mathrm{K}$). The recombination coefficient is found equal to the theoretical collisional-radiative recombination rate of ${\mathrm{He}}^{+}$. In spite of the high vibrational excitation of the recombining ${\mathrm{He}}_2^{+}$, corresponding to the high gas temperatures, there is no evidence for dissociative processes. In particular, the time decay of the recombination light cannot be reconciled with collisional-dissociative recombination.