Positronium formation from valence and inner shells in noble gas atoms
  When recent experimental positronium (Ps) formation cross sections in noble gases have been compared with the most up-to date theoretical studies, the agreement is qualitative, but not quantitative. In this paper we re-examine this process and show that at low energies Ps formation must be treated nonperturbatively. We also look at Ps formation with inner shell electrons. 
L. J. M. Dunlop and G. F. Gribakin