Coupled-states calculations with large target-centred basis sets have been used to compute ionization probabilities and cross sections for at collision energies down to 10 keV. The results are in good agreement with recent theoretical calculations based on numerical solution of the time-dependent Schrödinger equation but there are no experimental data. Both an independent particle model and an independent event model were used to carry out calculations for . These results are in good agreement with experiment at 40 keV and above but differ at lower energies. The impact-parameter-dependent probabilities for these collisions were used in the independent event model to calculate helium double-ionization cross sections and excellent agreement was found with experimental measurements between 13 and 200 keV, but this simple way of treating correlation fails at higher collision energies.

We present multi-cut forced impulse method cross sections for the single ionization of helium by antiprotons in the energy range 12-200 keV amu −1 . Eight segmented time intervals, bracketed by collapse onto fully correlated states, are needed to establish convergence in the effects of intermediate dynamic correlation. At low energies the theoretical cross sections fail to reproduce experiments. We point out that a similar situation obtains for the antiprotonhydrogen system.We take great pleasure in dedicating this letter to Richard Lemmer on the occasion of his 65th birthday.

The independent event model (IEV) is used to calculate single and double electron removal cross sections for p + + He collisions. The single electron removal cross sections are in good agreement with experiment. In contrast to the model of Janev and co-workers, it is found that the single electron removal cross section for p + + He + is much larger than the double electron removal cross section for p + + He, since in small impact parameter collisions of protons with He the probability of removing the first electron is less than unity. Double electron removal cross sections for p + + He calculated in the IEV are found to be in poor agreement with experiment, in contrast to p − + He where good agreement was obtained. For p + + He, the IEV double ionization result is improved somewhat if approximate allowance is made for the screening of the projectile by the electron if it is captured in the first event. The difference in the agreement of the IEV double ionization results with experiment may also indicate a qualitative difference in the role played by electron correlation in these two collision systems.