Influence of the autoionising states on the linear polarisation of helium lines excited by electrons with energy between 56 and 64 eV

Data are presented for the linear polarisation of helium lines excited by electron impact in the energy range corresponding to the He - resonances lying near the thresholds of the first doubly excited states. These autoionising He - states are observed as resonance structures in the polarisation. Within the experimental uncertainties these structures correspond to a maximum polarisation effect in the emitted light. Correlated to this is the change of angular momentum with respect to the direction of the bombarding electron beam which is a minimum for the particles concerned with the resonant process. Post collisional interaction, associated with the neutral doubly excited states, give no or weak structures in the polarisation.