Electron-impact ionization of helium by fast electrons at small momentum transfer: A quantum-defect analysis of experimental data

Triple-differential cross sections for the ionization of helium by 500-eV electrons at small momentum transfer have been analyzed in terms of the exchange of one single virtual photon between the helium atom and the scattering electron. The helium form factor has been described by quantum-defect parameters. The variation of the asymmetric binary-recoil angular distributions (i) with the momentum transfer in the collision, and (ii) with the energy of the ejected electron have been obtained in good agreement with experiment. Previous theoretical work seems to suffer primarily from precision in the target wave functions