Electron Excitation of the SodiumDLines

The electron excitation of the sodium resonance lines ($D$ lines) has been measured in the energy range from threshold to 1000 eV. The electron-beam full width at half-maximum was $\sim \frac{1}{3}$ eV, and the sodium-beam optical depth was small and varied. After correction for minor cascade contributions and the measured polarization, the excitation function has been normalized to the Born theory in a high-energy limit where the energy dependence converges to the theoretical behavior. The resulting normalized cross section and the polarization are in excellent agreement with recent close-coupling calculations for the energy region from threshold to 5 eV.