Abstract
The 3s-3p impact excitation of spin-polarised Na atoms by unpolarised electrons has been studied. The fluorescence light emitted in the decay of the excited state exhibits linear polarisation caused by the collisional induced 3p alignment, as well as circular polarisation resulting from the spin orientation of the 3p electron at the time of the decay. Since the exchange interaction causes a spin flip, the degree of circular light polarisation is a measure for the contribution of the exchange channel to the total excitation cross section. At small impact energies where exchange becomes important, the authors found a decrease of the circular polarisation. They analysed the hyperfine coupling between the excited electron and the polarised nucleus, and calculated the light polarisation from theoretical excitation cross sections. Comparison of experiment and theory shows reasonable agreement.