The triple-differential cross section for electron-impact ionization of argon has been investigated for the primary electron energies near 100 eV and scattering angle 15°. The in-plane and out-of-plane measurements show that the forward lobe has a true minimum in the full three-dimensional sense when the ejected electron energy is 5 eV. However, such a minimum is not observed when the ejected electron energy is 20 eV. The cross section has much more structure than that of helium. No simple symmetry has been found common to all the cases for argon.

• Received 1 December 1977

DOI:https://doi.org/10.1103/PhysRevA.17.1829