Helium Afterglow. I. Atomic Spectrum

Spectroscopic investigation of a flowing helium afterglow revealed the presence of strong lines of neutral helium. An examination of the population distribution of the excited atomic states revealed the presence of a Saha equilibrium between the upper states and the free electrons indicating that the primary process of populating these states was the collisional—radiative recombination of He+. A comparison of the axial variation and pressure dependence of the intensity of the emission with the He2+ and He+ concentrations strengthened this conclusion. Observations were made at times from 50–250 μsec into the afterglow (flow velocity ≈6×104 cm/sec) in a system having a diffusion half-life for He+ of the order of 100 μ sec, an ion density of 5×1012/cc and a maximum electron temperature corresponding to ≈0.15 eV.