The influence of negative ion resonances on the electron impact excitation of the 33D state of helium near 60 eV

The authors have studied the electron impact excitation of the 3 3 D state of helium at incident energies near the well known negative ion resonances He - (2s 2 2p) 2 P and He-(2s2p 2 ) 2 D at 57.2 and 58.3 eV respectively. This was done by measuring the polarization degree of the radiation emitted in the subsequent 3 3 D to 2 3 P transition both in a non-coincidence and a coincidence experiment. As the direct excitation process of 3 3 D and the indirect excitation process via a negative ion resonance are indistinguishable, these processes interfere. Analysis of the present and previous results indicate that the effects of interference play a minor role for the polarization degree as measured in the non-coincidence experiment. For the coincidence experiment the effects of interference are important.