Abstract
Absolute angular differential cross sections for the elastic scattering of electrons on argon atoms have been measured for incident electron energies of 40.002 and 50.128 eV over the angular range 20 to 150 degrees. An angular resolution of 0.02 radians has revealed two deep minima in the cross sections at about 67 and 140 degrees. The cross section values generally agree well in angular dependence, except for angles less than 30 degrees, with an exchange plus polarization approxi- mation calculation by Thompson and a relativistic calculation by Walker. However, both theoretical approximations yield cross section values which are about a factor of 1 a 3 higher than experimental values. All previous experimental measurements have not revealed the true depth of the cross section minima. The present measurements have been made with crossed electron, modulated atom beams. Cross section values have been made absolute by measuring elastic scattering cross section ratios for argon, atomic hydrogen and helium and using a phase shift analysis of the scattering around the 19.355 eV resonance in helium.