Electron impact excitation of helium: absolute differential cross sections of the n = 2 and 33S states from threshold to 3.6 eV above

The absolute differential cross sections of the n = 2 and 33S states of helium are obtained from threshold up to 3.6 eV above at 30, 60, 90 and 120 degrees scattering angles. The energetically-dependent transmission of the post-collision analyser of the electron impact spectrometer is deduced from the known characteristics of the ionization process near threshold. The results are made absolute by normalization to accurate elastic cross sections of helium. Integral cross sections for the excitation of the 23S and 21S states, and of the 23P and 21P states are given at 1.6 and 2 eV above threshold, respectively. Recently calculated cross sections are in very good agreement with these measurements. (author)