Threshold electron-impact ionization mechanism for hydrogen atoms.

The near-threshold evolution of electron-impact ionization of hydrogen is revealed with measurements of the angular and energy correlations of the outgoing electrons down to 0.05 eV. The single-, double-, and triple-differential cross sections in the perpendicular plane are measured simultaneously using a dual wedge-and-strip detector on a single-toroidal energy analyzer, avoiding many experimental problems. The experimental and calculated data are in excellent agreement, within the experimental precision of +/-10%, and provide further evidence that the accurate solution of the Schrödinger equation provides a complete description of the reaction dynamics of near-threshold ionization.