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

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 $A=\frac{({\sigma }^{\uparrow \downarrow }-{\sigma }^{\uparrow \uparrow })}{({\sigma }^{\downarrow \uparrow }+{\sigma }^{\uparrow \uparrow })}$ in the total ionization cross section for antiparallel and parallel incident- and atomic-electron spins are inconsistent with all theoretical calculations below 50 eV.