Two-electron atoms excluding He

Known data is shown in Table 2.4. The relationship between the ionization potentials and positron affinities of neutral atoms shown in Fig. 2.1 confirms the conjectures of several that atoms with ionization potentials near 6.803 eV should have a large positron affinity. For an atom with this exact ionization potential, there would be an accidental degeneracy between the thresholds (e+ + atom) and (Ps + cation), giving the largest quantum mechanical resonance effect. The atoms whose ionization are closest to 6.803 eV are hafnium (IP = 6.825), and titanium (IP = 6.828 eV). e+Ti and e+Hf can be treated as 5-particle systems, but they have not yet been studied for their positron affinities, to our knowledge. 

Atoms in our tables that do not bind a positron only corroborate this notion Their ionization potentials are either less than that of the left-most atom (23S He - 4.77 eV), or greater than the right-most (Hg - 10.44 eV). The lone exception is gold, Au, a bound state for which was not found by the Cl method. 

Clearly, positron and positronium binding follow quite different trends. 

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