Behavior ofHe(2S3)Metastable Atoms in Weakly Ionized Helium Plasmas

Abstract

Investigations concerning the behavior of the $\mathrm{He}\left(2\mathrm{}{}_{}{}^3{}_{}{}^{}S\right)$ population in active-discharge and after-glow helium plasmas are described. Good quantitative agreement is obtained between measured $\mathrm{He}\left(2\mathrm{}{}_{}{}^3{}_{}{}^{}S\right)$ densities in active discharges and predicted values based on the solution of a nonlinear rate equation. The rate coefficient for ionizing triplet-metastable-metastable collisions is measured in afterglow helium plasmas for the pressure range 10-40 Torr; close agreement is obtained with the value reported by Phelps and Molnar, though consideration of effects due to electrons suggests a downward revision of about 20%. The electron temperature decay in the afterglow is measured and found to agree closely with the decay expected on the basis of calculated energy source and loss processes which affect the electron gas. In this work, measurement of the $\mathrm{He}\left(2\mathrm{}{}_{}{}^3{}_{}{}^{}S\right)$ metastable density is accomplished by the novel use of a He-Ne laser operating at 1. 0798 $\mu$ in an interferometer as well as by the conventional optical-absorption method. Advantages and disadvantages of this interferometer technique are discussed.