Comparison of helium two-step plasma emission with that predicted from measured cross sections

Abstract

Plasma emission from the afterglow of a low-pressure, 100% modulated, radio frequency (rf) excited discharge can originate from collisions between metastable atoms and fast electrons. The fast electrons are generated by collisions between pairs of metastables (Penning ionization of one metastable by another) and collisions of metastables with slower electrons (superelastic collisions). Using time-resolved Langmuir probe data, measurements were made of the electron energy distribution function (EEDF) containing these fast electrons in a helium afterglow. The EEDF data were used, along with measured optical cross sections out of the 2s3S and 2s1S metastable levels, to predict intensities of various He emission lines seen in the afterglow. A comparison between the measured and predicted emission is presented.