Observations of Positronium Lyman-αRadiation

The first observation of Lyman-$\alpha$ radiation from positronium is described. Positronium is formed in vacuum by a slow positron beam incident on a solid target. Single photons transmitted through an ≈ 20-Å-wide interference filter peaked near the 2430-Å Lyman-$\alpha$ ($2P\to 1S$) line of positronium are observed in coincidence with the annihilation $\gamma \text{'}\mathrm{s}$ of ground state positronium. The observed intensity corresponds to a production of one positronium "atom" in the excited state for every ${10}^4$ incident positrons.