Measurement of Spin-Exchange Effects in Electron-Hydrogen Collisions: Impact Ionization

Abstract

We have measured the interference between the direct and exchange scattering amplitudes in electron impact ionization of atomic hydrogen over the energy range 15 to 197 eV in a crossed-beams experiment utilizing spin-polarized electrons and spin-polarized hydrogen atoms. The experimental values for the asymmetry $A=\frac{({\ensuremath{\sigma}}^{\ensuremath{\uparrow}\ensuremath{\downarrow}}\ensuremath{-}{\ensuremath{\sigma}}^{\ensuremath{\uparrow}\ensuremath{\uparrow}})}{({\ensuremath{\sigma}}^{\ensuremath{\downarrow}\ensuremath{\uparrow}}+{\ensuremath{\sigma}}^{\ensuremath{\uparrow}\ensuremath{\uparrow}})}$ in the total ionization cross section for antiparallel and parallel incident- and atomic-electron spins are inconsistent with all theoretical calculations below 50 eV.