Double ionization of helium differential in energy transfer, e, is studied for both high-energy Compton photons and charged particles. The ratios of double to single ionization, R C ͑e͒ for Compton scattering, and R Z ͑e͒ for charged particles, are found to display an unexpected behavior: For large e up to the two-body binary encounter (BE) limit, e BE , we find R C ͑e͒ R Z ͑e͒ Ӎ 0.86% in good agreement with recent experimental data. For even larger e . e BE , R C ͑e͒ and R Z ͑e͒ are modified by an "exchange shakeoff" mechanism. At e ¿ e BE ¿ 1, both differential (unlike integrated) ratios are predicted to approach the photoionization limit. [S0031-9007(96)00861-7]

The scaling behaviour of cross sections for electron capture from circular Rydberg states by protons is studied for a wide range of principal quantum numbers n = 4 to 100 at intermediate collision speeds. We find the convergence to n 4 -scaling to be slow and dependent on the initial orientation. The origin of the slow convergence is traced to the capture mechanisms that probe different parts of the electron's phase space distribution.

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