Electron Removal in Helium Afterglows

Observations of the electron density, visible and near ultraviolet light intensity, and spectrum associated with a 3000 Mc/sec. pulsed electrodeless discharge have been made. Our electron density data indicate recombination-type electron removal with a constant $\alpha =1.0\mathrm{}\times {10}^{-8\mathrm{}}$ ${\mathrm{cm}}^3$/ion-sec. in reasonably good agreement with previous data. During the discharge, the spectrum of atomic helium predominates. In the afterglow, the band spectrum of ${\mathrm{He}}_2$ predominates. The total energy radiated in the wave-length range 2000 to 8000A is estimated as 0.5 electron volts per electron removed. A maximum in the light intensity occurs a few hundred microseconds into the afterglow. The following mechanism is quantitatively consistent with all of our data: ${\mathrm{He}}^{+}$ forms ${\mathrm{He}}_2^{+}$ by undergoing a triple collision with two neutral atoms, after which the molecular helium ions combine with electrons.