Elastic Electron Scattering in Helium

Abstract

The scattering of electrons by helium atoms was measured experimentally over an angular range from 15° to 150° for electrons varying in speed from 25 to 700 volts. The scattering curves all fall steeply with angle, especially so for the smaller angles, provided that the speed is above 100 volts. For speeds below 100 volts, the steep fall at small angles is followed by a gentle rise beginning near 90°. Within the range cited, no maxima in the angular scattering curves were found. The theory of Mott accounted for the 700 and 500 volt curves accurately over the whole range, and for the 350 volt curve, over the range 20° to 100°. Below 20° the experimental curve was steeper than the Mott curve, and above 100° the experimental curve was flatter than the Mott curve. As we go to lower energies, the departures from the Mott curve increase progressively. For small angles the theoretical curve is not steep enough and for large angles it is too steep. The results were also compared with the theory developed by Massey and Mohr taking into account certain factors omitted by Mott. While the newer theory accounts qualitatively for the upturn in the angular scattering curve, it fails to give its correct shape. It is shown how atomic structure factors, or $F$ values, which are employed in the theory of x-ray scattering, can be inferred from these experiments in electron scattering.