Different electron-helium collision processes which can lead to a joint change of state of both target electrons are reviewed. They are examined with the help of a deliberately simplified plane wave Born series model. Certain terms which are second order in the interaction potentials are shown to correspond to contributions obtainable in first order models using distorted wave descriptions of the projetile and/or wavefunctions for twoelectron subsystems which take account of the Coulomb interaction between the electrons. The remaining terms are related to double-hit mechanisms where the projectile interacts first with one and then with the other target electron. Applications to double excitation (autoionization), ionization with excitation and double ionization are discussed.

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.

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