Examinations of Charge Transfer between Ionized He and P, S, and Cl in a Microwave-Induced Plasma

Abstract

Charge transfer between monomer helium ions and nonmetals has been postulated to be the source of high nonmetal ion populations in high electron density (> 1014 e−/cm3) discharges. In this study, overpopulations of excited Cl, P, and S ion energy levels in a helium microwave-induced plasma are examined using ion and atom line emission intensity data and charge transfer theory. The resonance emission lines from phosphorous, sulfur, and chlorine were examined. To examine these highly energetic lines, wavelengths in the vacuum-ultraviolet and the ultraviolet-visible (UV-visible) spectral regions were monitored. Energy-level population calculations were performed on similar UV-visible emission data from the literature. Charge transfer ionization cross sections are calculated for these elements. To obtain an indication of the significance of charge transfer, these parameters are compared to the behavior that would be expected if the plasma were in local thermodynamic equilibrium, and kinetically dominant processes such as CT were not considered. These evaluations, with the aid of energy-level diagrams, help to explain the spectroscopic behavior of these nonmetals in the helium discharge.