Spin-tagged electron-hydrogen scattering: Ionization in the near-threshold region

We use beams of polarized electrons and polarized hydrogen atoms to measure the ionization-rate asymmetry, ${\mathrm{\Delta }}_{\mathit{I}}$=[R(↑↓)-R(↑↑)]/[R(↑↓)+R(↑↑)], in the near-threshold region, where R(↑↓) and R(↑↑) are the ionization rates when the electron spin is antiparallel and parallel, respectively, to the atomic spins. Within 1.7 eV above threshold, our results reveal the presence of structure, which heretofore has not been predicted by any conventional theoretical calculation.