Formation of Molecular Helium Ions by Associative Ionization in a Recombining Plasma

The afterglow of a 1.3-A 1.5-μsec-duration discharge in helium at 11 Torr was studied during the time 15 to 35 μsec after the discharge pulse. A 10-μsec current pulse was used to selectively heat the electrons, and it was found that the differences in the atomic and molecular intensities subsequent to this heating pulse could be explained by assuming a process of associative ionization of excited atomic helium states resulting in molecular helium ions, ${\mathrm{He}}^n+\mathrm{He}\to {{\mathrm{He}}_2}^{+}+e^{-}$. The rate of this process is determined. It is further shown, by comparison of excited-atomic-state densities with previous measurements at much lower pressures, that there must be a source, rather than a loss, of lower-lying atomic states. A model is proposed in which associative ionization of highly excited atomic states results in vibrationally and rotationally excited molecular ions, which subsequently relax by neutral helium collisions to the ground-state level. Dissociative recombination of these excited molecular ions while in the process of going to the ground-state level then forms a source of additional lower excited atomic states. Extremely rapid vibrational and rotational relaxation rates are required.