THE INFLUENCE OF NEGATIVE-ION RESONANCES ON THE ELECTRON-IMPACT EXCITATION OF THE 3-3D STATE OF HELIUM NEAR 60 EV

We 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-(2s2 2p)2P and He-(2s 2p2)2D 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 --> 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. 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 3(3)D state through the interference.