High resolution studies of electron excitation III. Polarization near threshold of light from the 4D states of helium

We have measured the polarization of the helium lines at 492.2 nm (4 $^{1}$D-2 $^{1}$P) and 447.2 nm (4 $^{3}$D-2 $^{3}$P) over an energy range of about 0.8 eV above threshold using velocity selected electrons. The measurements are free from the effects of instrumental polarization. In both cases we find that the value of the polarization at the lowest energies for which we can detect a signal is quite consistent with the threshold value predicted by conservation of angular momentum. At higher energies we find that the polarization falls rapidly. We suggest that this rapid fall is a consequence of a 1s 4d$^{2}$ $^{2}$S resonance just below the threshold.