The polarization of electron impact radiation in helium

Abstract
The excitation and polarization of electron impact radiation have been studied as a function of helium pressure in order to determine conditions for which secondary processes do not cause significant depolarization of the observed radiation. Under these conditions the optical excitation functions, and the polarization as a function of electron energy have been determined for a number of transitions. The measurements give zero polarization at the threshold, whereas, in general, finite threshold polarization is predicted theoretically. There is some agreement between an extrapolated threshold and the theoretical prediction. Negative polarization has not been observed. Possible causes of the threshold discrepancy are discussed.