Photon-photon scattering cross-sections increase up to an energy of about the rest-mass of a positron-electron pair and then begins to drop. If the center-of-mass energy is higher than the about 1022 keV, it's possible to produce an electron-positron pair.
But suppose you could map this spectrum out with very high frequency resolution.
If the energy was 6.8 eV below the pair-production threshold, it could produce positronium in the 1S state. If it is 1.7 eV below, you could get the 2S or 2P state, and so-on.
Would an extremly close examination of the cross-section show resonance peaks at these ("1022" keV-6.8/n^2 eV) energy locations? Resonance peaks correspond to creating a particle, and positronium is a (composite) particle.
