The polarisation degree of some prominent helium lines after collisional excitation by fast ions

The polarisation degree π of some prominent helium lines has been measured after collisional excitation by H+, H+2, H+3 and He+ ions in an energy range from 100-1000 keV. It was the aim of this experiment to investigate the influence of the different ion structures of the relative population of magnetic substates at high impact velocities. The proton data are compared with the first Born approximation. The effect of resonance-radiation capture on the measurement of π(31P-21S) have been reinvestigated. It is concluded that the intensities of the emitted radiation, polarised parallel and perpendicular, respectively, to the beam axis when measured at an angle of 900 with respect to the beam, which enter into the definition of the polarisation degree are differently affected by resonance trapping at low gas densities. (author)