Measurement of differential cross sections for excitation of helium by electron impact within the first 4 eV above threshold

Absolute differential cross sections for excitation of helium by electron impact are reinvestigated within the first 4 eV above threshold at scattering angles of 20, 60, 90 and 120' with improved resolution and statistical accuracy. Cross sections are reported for the n=2, n=3, 4 3 S and 5 3 S states. The new results permit a more accurate assessment of the widths and shapes of the threshold features, in particular for the 2 3 S and the 2 1 S states, which are in very good agreement with the published results of R-matrix calculations. Structures caused by doubly excited Wannier ridge resonances are observed in all cross sections. Their intensities in the individual decay channels are informative of the energy partition of the excited electron pair and their shapes exhibit interesting regularities.