Polarization of the 31D state of helium excited by electron impact

The polarization fraction of the 31D-21P (667.8 nm) line in helium following electron impact excitation of the 31D state has been measured over the incident electron energy range from threshold to 300 eV. Serious discrepancies are observed with the only previous experimental measurement over an extended energy range, particularly at energies under 200 eV. All calculations underestimate the polarization fraction at incident electron energies above 30 eV, but a theoretical prediction of lack of dependency on the principal quantum number n is confirmed. (author)