Abstract
A perturbation series of the distorted-wave Born type is developed in such a way that the slow decrease of the Coulomb potential in which the incoming and scattered particles move plays no role. We apply this approach to triple-differential cross sections for electron-impact ionization of helium at low and intermediate energies, where we use Coulomb waves with an effective charge for the incoming, scattered, and ejected electron, and show that even though these wave functions are known to satisfy the wrong Coulomb boundary conditions at infinity, they give reasonably and unexpectedly good agreement with experiment.

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