Lifetime of helium metastable spin-states in a helium discharge
Walters, G. King
Master of Arts
The lifetime of a helium 23s1 metastable atom electronic spin-state is measured in helium gas using optical pumping techniques. The metastable atoms are created by an RF electrical discharge. The spin-state lifetime is found to vary with the -pressure of the helium and the intensity of the discharge. Also, the lifetime of the metastable atom is measured in the afterglow of a helium discharge. The metastable spin-state lifetime in a discharge is found to be much shorter than the metastable afterglow lifetime at the same pressure. At 1 mm Hg pressure, the afterglow lifetime of the metastable atom is about 1 as. At the same pressure in a discharge the metastable spin-state lifetime is found to be about 0.2 ms. The mechanism pro- posed to explain this difference is collisions of metastable atoms with the hot (~4 ev) electrons present in the dis-charge. The total cross-section for this process is large, and presumably a large fraction of the collisions will be effective in relaxing the spin-states.