Atomic and Molecular Emission in the Negative Glow of a Helium Discharge

Abstract

The spatial distribution of helium atomic and molecular radiation has been investigated in the negative glow of a dc discharge. The atomic radiation peaks sharply at the cathode edge of the negative glow, the molecular radiation less sharply at a greater distance from the cathode. Microwave quenching indicates that most of the atomic and all of the molecular radiation results from electron-ion recombination processes. The decrease in atomic radiation reflects the decrease in atomic ion concentration which is controlled primarily by diffusion away from the cathode and by three-body conversion to molecular ions. The molecular ions are lost by diffusion and by collisional—radiative recombination to the neutral molecule, with collisional processes predominating. The negative glow can be characterized as a plasma steady in time but with a one-dimensional spatial decay away from the cathode.