Excited Species from a Pulsed Discharge in Helium at One Atmosphere Pressure

Abstract

The relative intensities of atomic emission lines have been analyzed in regard to a Boltzmann distribution of the electronic levels in the pulsed discharge. The analysis confirms a Boltzmann distribution with an excitation temperature of 3200 ± 220 K, a relatively low temperature compared with that for other excitation sources, such as microwave and radio-frequency discharges. The analysis also suggests that little ionization occurs via direct excitation in the discharge. The emission spectra from excited diatomic helium states have been analyzed and confirm the formation of a32σu+) and the Hopfield emission He2( A1σu+ → 2He(11 S) continuum in the range 72 to 92 nm. Emission intensity-time profiles have been obtained for both atomic and diatomic helium emissions. Analysis of these profiles indicates that excited He2 states are obtained by two reactions: (1) an excited atomic helium reacting with a ground-state helium atom, and (2) recombination of He2+ with electrons. The study concludes that excitation in a discharge through helium at atmospheric pressure yields the following predominant species: He(23S), He2( a3σu+), Hopfield emission continuum 72–92 nm, and He2+.