Cause of the Observed Polarization of Electron-Induced Radiation from Helium

Recent improved measurements of threshold excitation and polarization of radiation from helium excited by electrons have led to the conclusion that previously reported "anomalies" are of experimental origin. Helium lines, $\lambda =4922,4388, \mathrm{and} 5016$ Å, exhibit polarization energy dependencies similar to that previously reported for $\lambda =3889\mathrm{}$ Å. Helium lines $\lambda =4922\mathrm{} \mathrm{and} 4388$ Å have excitation probability energy dependencies through limited energy ranges as predicted by theory. A tentative explanation of earlier results involves the contribution to both the excitation probability and the polarization of radiation caused by radially directed electrons elastically scattered from the electron beam. Attempts to reduce the radial travel of electrons have resulted in threshold and near-threshold polarization measurements which approach expected magnitudes.