Classical trajectory Monte Carlo calculations have been used to elucidate the collision mechanisms responsible for the differences in the single- and double-ionization cross sections of He atoms by protons and antiprotons in the energy range from 250 to 1000 keV. The calculations employed the Bohr helium-atom model which includes the $\frac{1}{r_{12}}$ electron-electron interaction. The unexpected large observed difference [Andersen et al., Phys. Rev. Lett. 57, 2149 (1986)] in the ratio $R$ of double to single ionization for proton and antiproton collisions is reproduced by the calculations. The calculations indicate that the antiproton double-ionization cross section is larger than that for protons because of two effects. The first effect is that the antiproton can push one electron into the other from larger impact parameters than for protons which must be between the nucleus and the first electron in order to pull it into a trajectory which collides with the second electron. The most important effect, however, is that at small impact parameters the antiproton screens the helium nucleus causing a Coulomb explosion while the proton increases the binding of the two electrons. The calculations also show a component of the difference in the ratio $R$ at low energies ($E<~500$ keV) is due to the single-ionization cross sections with protons predicted to have a larger cross section than antiprotons (36% difference at 250 keV).

• Received 18 May 1987

DOI:https://doi.org/10.1103/PhysRevA.36.1519