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 [1] and the resulting positronium lifetime spectra were measured using single shot positron annihilation lifetime spectroscopy (SSPALS) [2]. 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 [3]. 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.