Characteristics of close collisions of an antiproton with atoms

We discuss low-energy antiproton (p̄) collisions with H and He on the basis of adiabatic potentials and with the use of a molecular-orbital expansion method. Every electronic state for the p̄-H system merges into the continuum at a certain internuclear separation, and this separation is farther and farther out for higher and higher excited states. (This fact was first pointed out by E. Fermi and E. Teller [Phys. Rev. 72, 399 (1947)], who specifically considered the ground state of a system consisting of a meson and a hydrogen atom.) For the p̄-He system, as for to the p̄-H system, all electronic states cross over into the continuum of the He⁺+le^- state at a finite separation. These findings lead us to suggest that a close encounter of p̄ with an atom should result, at a high probability, in ionization with near-zero electron energy and a wide range of ejected-electron angle. This mechanism of ionization is absent for collisions of protons (p), and may account in part for the greater double-ionization cross section of He for p̄ than for p at the same energy, which was found in a recent experiment.