Optical Spectroscopy of Molecular Positronium

We report optical spectroscopic measurements of molecular positronium (Ps₂), performed via a previously unobserved L=1 excited state. Ps₂ molecules created in a porous silica film, and also in vacuum from an Al(111) crystal, were resonantly excited and then photoionized by pulsed lasers, providing conclusive evidence for the production of this molecular matter-antimatter system and its excited state. Future experiments making use of the photoionized vacuum L=1 Ps₂ could provide a source of Ps⁺ ions, as well as other multipositronic systems, such as Ps₂H^- or Ps₂O.