Triple-differential cross sections for electron-impact ionization of helium are calculated in the distorted-wave approximation. A general distorted-wave formalism for ionization is presented for two coordinate systems—one with the $z$ axis parallel to the direction of the incident electron and one with the $z$ axis parallel to the momentum-transfer direction. Cross sections in the scattering plane are compared with experimental results and with other theoretical calculations. Cross sections out of the scattering plane are presented and spatial symmetries are discussed.

• Received 10 March 1977

DOI:https://doi.org/10.1103/PhysRevA.16.552