Electron impact excitation of the spectral resolved Na-D lines

Measurements of the optical excitation function and the polarisation of the spectral resolved Na-D lines (5890/5896 AA) have been carried out in the electron impact excitation of ²³Na for energies from threshold (2.1 eV) up to 500 eV by means of the crossed-beam technique. A Lyotfilter of high transmission as monochromator and a sensitive photon-counting technique made it possible to detect the small intensities of the resolved Na-D doublet with sufficiently high efficiency. An unpolarised Na-D1 line for all energies of the impinging electrons was obtained, while the threshold polarisation of the Na-D2 line was found to be about 17%, in agreement with theory.