Time-Dependent Study of the Emitted Light and Electron Density in a Low-Pressure Helium Afterglow

The relationship between the emitted light and decay of electron density in a pulsed helium afterglow at 1 Torr has been studied using a 0.75-meter tandem monochromator and 36-Gc/sec microwave interferometer. Substantial signal-to-noise enhancement was obtained with an automatic data-processing system enabling populations of the $n{}_{}{}^3{}_{}{}^{}D$ levels ($n\le 25$) to be measured as a function of time to 10 msec in the afterglow. In addition, the populations of low quantum levels and the free-electron concentration were measured to 20 msec. It was found that the time-dependent behavior of the levels divided them naturally into three groups, only the lowest-lying of which had been measured in previous afterglow experiments. The levels of this lowest group were found to decay exponentially with the same lifetime for better than three orders of magnitude. Levels of the middle group were found to decay exponentially for 2.5 decades, the lifetimes increasing monotonically with quantum number. Levels of the third group were identified to be in Saha equilibrium with the free electrons, and uniformly had lifetimes equal to the sum of the calculated atomic and experimentally measured electron lifetimes.