Cross Section and Polarization of the ${\mathrm{H}}_{\alpha }$ Line by Electron Impact Excitation

The modulated cross-beam technique has been used to measure the excitation function and the polarization of the Balmer-$\alpha$ line of atomic hydrogen excited by electron impact. Normalization of the ${\mathrm{H}}_{\alpha }$ excitation function to the Born approximation above 300 eV provides cross sections in the energy range from the threshold up to 350 eV. The polarization of the ${\mathrm{H}}_{\alpha }$ line shows an abrupt decrease, which is contrary to the "normal" behavior of increasing polarization up to the threshold observed for the alkali atom resonance line.