Characteristics of close collisions of an antiproton with atoms

We discuss low-energy antiproton ($\overline{p}$) 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 $\overline{p}-\mathrm{H}$ 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 $\overline{p}-\mathrm{H}\mathrm{e}$ system, as for to the $\overline{p}-\mathrm{H}$ system, all electronic states cross over into the continuum of the ${\mathrm{He}}^{+}+{\mathrm{le}}^{-}$ state at a finite separation. These findings lead us to suggest that a close encounter of $\overline{p}$ 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 $\overline{p}$ than for $p$ at the same energy, which was found in a recent experiment.