Electron Excitation of the Calcium 4227-Å Resonance Line

We have measured the optical-excitation function and polarization of the 4227-Å line, using crossed electron and calcium beams, for electron energies from threshold to 1400 eV. In the high-energy region, from 100-450 times the threshold energy, we have used the Bethe theory to normalize our experimental results. An excellent signal-to-noise ratio combined with ∼0.3-eV full width at half-maximum electron-energy resolution has enabled observation of structure in the excitation function and polarization curves below 5 eV. We observed a threshold polarization limit of (98±3)%, consistent with the expected + 100%. The data are also consistent with a logarithmic approach to a high-energy polarization of - 100%.