The mechanism of double ionization of helium by slow antiprotons

The double ionization of helium in slow collisions with antiprotons (p - ) is treated as a two-step sequential process, the first step of which takes place at relatively large internuclear distances (R approximately 1 a 0 ) and is governed by the Fermi-Teller mechanism, while the second step (removal of the remaining electron) takes place at R 0 approximately 0.2 a 0 and is governed by a direct superpromotion to the continuum via the S s sigma superseries of hidden crossings of the He + -p - system. It is shown that the double ionization cross section in the energy region below approximately 40 keV is determined by the probability of the second step only. The agreement of the calculated cross section with the experimental data in the region below 40 keV is within 20%.