Spectroscopic Study of the Early Afterglow of a Recombining Helium Plasma

Abstract

The afterglow of a 1.3-A 1.5-\ensuremath{\mu}sec-duration discharge in helium at 11 Torr was studied in some detail during the times 15 to 35 \ensuremath{\mu}sec after the discharge pulse. Spectroscopic measurements were used to obtain the number densities of excited atomic and molecular states, the conductance of the plasma column was determined from simultaneous electric field and current measurements, and a 10-\ensuremath{\mu}sec current pulse was used to selectively heat the electrons, thus disturbing some of the afterglow processes. The atomic and molecular ion densities and the electron temperature, obtained from the spectroscopic measurements, were in good agreement with the plasma conductance and field strengths. The inferred recombination rate favors the recent calculations of Mansbach and Keck over the Bates, Kingston, and McWhirter calculations of the collisional radiative-recombination rate. The rate of conversion of atomic into molecular ions was dominated by associative ionization of excited atomic states, and good agreement was obtained by including this process with other known processes.