Absolute measurement of ionization-excitation of helium at intermediate energies and their interpretation

Coincidence measurements in coplanar asymmetric geometry are reported for electron impact ionization of helium leaving the residual ion either in its ground state or in an n = 2 excited state. Cross sections are placed on an absolute scale by extrapolation of the generalized oscillator strength to the optical limit. Scattered electron energies of 570 and 1500 eV are considered with ejected electron energies of 10, 20 and 40 eV. Comparison is made with theoretical calculations in the first Born approximation using parametric potentials. Clearly, certain features of the experimental results can only be explained by multiple-scattering effects.