Positronium Molecule Formation on Al (111)a
A single crystal Al (111) sample cleaned by repeated cycles of ion bombardment and annealing was irradiated by a subnanosecond high density positron beam  and the resulting positronium lifetime spectra were measured using single shot positron annihilation lifetime spectroscopy (SSPALS) . We observed the amount of positronium emitted dependence on the incident positron beam density, which indicates the formation of positronium molecules (Ps2) at the Al (111) surface . The Ps2 formation probability appears to be extremely sensitive to surface contamination and further experiments under improved vacuum conditions are planned to clarify this effect.
S. J. Wang, Z. Q. Chen, B. Wang and Y. C. Jean
S.H.M. Deng et al., "Positronium Molecule Formation on Al (111)a", Materials Science Forum, Vol. 607, pp. 22-24, 2009