High resolution measurements of the 2³S cusp and the 2²S resonance in elastic electron-helium scattering

Abstract

The cusp in the cross section for the elastic scattering of electrons by helium atoms at the energy of the 23S threshold has been measured at scattering angles from 25 degrees to 100 degrees . High resolution measurements at 90 degrees have shown that the s wave phase shift is 107 degrees +or-11 degrees at this energy. The energy of the 22S resonance has been accurately calibrated from the position of the cusp and it has been found to be at 19.367+or-0.009 eV, with a natural width of 9+or-1 meV. The s wave phase shift at the resonance energy has been found to be 106 degrees +or-3 degrees . The 21S cusp has also been studied.