Threshold ionization of hydrogen by antiproton impact

Antiproton impact ionization near threshold is analysed from the point of view of an unstable dynamical equilibrium in the slow three-body break-up configuration. The instability is shown to be displaced from the Coulomb saddle-the centre of charge of the antiproton and the electron-and hence with the proton on the line between the negative pair but about sir times closer to the antiproton than to the electron. The Wannier threshold law is derived, and possible negative-ion probes of these underlying Coulomb instabilities considered.