Hidden Crossing Theory of Threshold Ionization of Atoms by Electron Impact

The hidden crossing theory, familiar in the context of ion-atom collisions, is adapted to the computation of ionization cross sections and spin asymmetries for electron impact on atomic hydrogen. Computed ionization cross sections and spin asymmetry agree within 10% with measurements. A rapid variation of the spin asymmetry near threshold is found, and its origin is traced to anharmonic corrections to the Wannier threshold law for triplet spin states.