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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.
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Cassidy et al. (2012) studied this question.
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