Single and double electron removal from helium by protons

The independent event model (IEV) is used to calculate single and double electron removal cross sections for 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 is much larger than the double electron removal cross section for , 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 calculated in the IEV are found to be in poor agreement with experiment, in contrast to where good agreement was obtained. For , 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.