Positronium formation in the noble gases

We present results based upon our ab initio relativistic optical potential method for the simulation of positronium formation in the noble gases neon, argon, krypton and xenon. The method used in this simulation is based upon an approach, first suggested by Reid and Wadehra, whereby the ionization threshold of the atom is effectively reduced by 6.8 eV, the binding energy of positronium in its ground state. This procedure is then modified in order to account for the fact that positronium formation cross sections tend to zero much more rapidly than the corresponding direct ionization cross sections as the energy of the incident positron increases. The agreement of our positronium formation cross sections with experiment is quite good for argon, krypton and xenon over most of the energy region while for neon our cross sections are generally too small at all energies. Nonetheless, our method predicts positronium formation cross sections in better agreement with experiment than the much more sophisticated theoretical approaches used in the past. We also show that adding positronium formation in this way improves agreement with the measured elastic differential cross section at small scattering angles.