Reaction kinetics of a high pressure helium fast discharge afterglow

This work reports the study of elementary processes which govern the relaxation of the afterglow of a 3 kA 7 ns duration discharge in helium at atmospheric pressure. Populations of the dominant energy storing species were monitored as functions of time through measurements of the emission asnd absorption of radiation from the plasma. The atomic helium spectral lines, radiated during the discharge pulse, were found to rapidly transform into molecular helium bands in the visible and far ultraviolet (600--1000 A) regions during the afterglow. The observed features of this He2 radiation were consistent with a collisional-radiative scheme of electronic recombination of molecular He2+ ions, the concentration of which was inferred from H/sub β/ line broadening. A significant fraction of the He+2 recombination events were found to result in helium atoms in the metastable 2 3S state, and the time dependence of this dissociation process correlated with that of He2 populations characterized by quantum number n = 4. The concentration of 2 3S atoms was comparable to the electron density, ranging from 1014 to 1015 cm-3. The concentration of molecular a 3Σ+/sub u/ metastables was found to be one order of magnitude lower, suggesting a collisional coupling to the radiating A 1Σ+/sub u/ state. From the data rate coefficients have been extracted for the processes of electron--ion recombination, ionizing collisions between atomic metastables, and superelastic electron--metastable collisions