Polarisation of radiation in low-energy He+-He collisions

We calculated the polarisations of the emission cross sections of the 21S-31P and 21P-31D transitions of HeI by the two-level model for the collision energy below 20 keV, attributing the cause of alignment of the magnetic sublevels to the excitation mechanism assigned in a previous paper. Qualitative agreement between the calculated polarisations and the measured ones shows that noticeable variation of the observed polarisation with increasing impact energy is ascribed to rotation of the molecular axis following the change of scattering angle. The contribution of the secondary processes and the influence of the deviation of the symmetry axis of the atom from the internuclear axis of the separating particles, to the polarisations were discussed. (author)