Ionisation of Atomic Hydrogen and Helium by Low Energy Antiprotons

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Experimental studies of collision processes, in which fundamental charged particles ionize simple atoms and molecules, provide the data necessary for our understanding of the many-body interaction and the development a theoretical description of this fundamental yet unsolved problem in collision dynamics. The ionization process plays an important role in astrophysical and technological plasmas and in the dissociation of molecules in the Earth's atmosphere. A comparison of single and multiple ionization of simple atomic and molecular targets by equi-velocity protons, electrons and their corresponding antiproton and positron antiparticles makes it possible to determine the effects of projectile mass and charge. The simplest collision system for testing theory is the single ionization of atomic hydrogen by antiprotons. The system has only one active electron, a precisely known wave function and, unlike in the electron impact case, there are no exchange effects. In addition, the antiproton cannot capture the hydrogen electron and straight-line constant velocity projectile paths may be used. The cross section for single ionization of atomic deuterium (no isotope effects expected) was measured in the energy range 30 keV / 1 MeV at the Low Energy Antiproton Ring (LEAR) at CERN by the PS194 collaborative grouping lead by Knudsen and co-workers from the University of Aarhus and including Prof. M Charlton (one of the present applicants). It was found that for antiproton velocities larger than the target electron velocity (2.2 x 108 cm s-1 or 25 keV antiproton energy) the experimental data and all theories are in agreement, as might be expected. However, at lower energies there are significant discrepancies. Most of the theories show an almost energy independent trend, as predicted by the Fermi-Teller mechanism, in which the collision is of a quasi-molecular nature and the target electron binding energy vanishes at a certain critical distance. Experimental measurements are required urgently in the energy regime below 30 keV to determine the magnitude and energy dependence of the cross section for this most fundamental of collision systems