Absolute double differential cross sections for electron-impact ionization of helium

We present a combined experimental and theoretical study of double differential cross sections (DDCS) for electron-impact ionization of helium. Relative DDCS have been measured in a crossed-beam apparatus for primary energies 200, 100, 64.6, 50, 40, 32.6 and 28.6 eV. These are put on the absolute scale using the convergent close-coupling (CCC) calculations at 100 eV. The absolute data are found to be in good agreement with the CCC calculations except for the cases where the secondary electron energy is close to the half excess energy. In these cases, particularly at smaller energies, the CCC theory still predicts similar profiles, but gives substantially smaller magnitudes than the present measurements, which are consistent with the earlier measurements of Pichou et al.