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

We have studied the electron impact excitation of the 33D state of helium at incident energies near the well known negative ion resonances He-(2s22p)2P and He-(2s2p2) 2D at 57.2 and 58.3 eV respectively. This was done by measuring the polarization degree of the radiation emitted in the subsequent 33 D→23P transition both in a non-coincidence and a coincidence experiment. As the direct excitation process of 33D 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. In addition, we show that measurements of the circular polarization degree of the coincidence radiation as a function of the incident energy through a resonance facilitates a complete determination of the wavefunction of the 33D state through the interference. (author)