Emission spectroscopy of a pulsed helium-discharge plasma: Transition from the ionizing phase to the recombining phase

A pulsed helium-discharge plasma was found to have two peaks in the temporal development of its emission line intensities. At these peaks the population densities of the n ${\mathrm{}}^3$D state (n=3,4,. . .) were measured. For the first peak, the populations were found to be proportional to $n^{\mathrm{-}6}$, while the second peak showed an exp[-E(n)/${\mathrm{kT}}_e$] dependence. These distributions are identified with plasma in its ionizing and recombining phases, respectively. On the basis of a collisional-radiative model calculation, the transition from the ionizing phase to the recombining phase is demonstrated. This report is the first experimental demonstration of the $n^{\mathrm{-}6}$ distribution in the ionizing phase for conditions when the conventional criterion would indicate thermal populations. It is suggested that the apparent decrease in the emission line intensities of high-n members of series lines, or ‘‘the transparency window’’ that is observed in dense plasmas, may be attributed to this characteristic of ionizing plasma.