DIFFERENTIAL EXCITATION FUNCTIONS OF N=2 STATES OF HELIUM

Energy and angular dependences for the electron impact excitation of the Helium 23S, 21S, 23P, and 21P states have been measured using monochromatic (0.05 eV) electrons in the energy range from 19 to 23 eV and for scattering angles ranging from 0 to 110°. The structures near the 23S-threshold have been analyzed in terms of resonant and non resonant i.e. direct scattering phase shifts. The first peak in this excitation function at 19.95 eV is mostly due to the rapid change with collision energy of the direct s-wave phase of the inelastic scattering, which in turn is a consequence of the existence of the 22S-resonance at 19.3 eV. The energy position and behaviour with collision energy of the first peak at 20.7 eV in the 21S-channel is in agreement with the predictions for the existence of a virtual resonance state of Burke et al. A term sceme of the He- resonances with assignments of configurations is presented. © 1968 Springer-Verlag.