Observation of the Positronium Negative Ion

Abstract

A beam of 400-eV positrons is partially transmitted through a 40-\AA{} C film in vacuum. Measuring the $\ensuremath{\gamma}$-ray energy spectrum with a downstream Ge(Li) detector, a new $\ensuremath{\gamma}$-ray peak is observed, the energy of which can be changed by applying a voltage to a grid behind the film. Attributing the new peak to a Doppler-shifted annihilation $\ensuremath{\gamma}$ implies a mass-to-charge ratio $3.011(14)\frac{m}{e}$, thus signaling the presence of the ${e}^{\ensuremath{-}}{e}^{+}{e}^{\ensuremath{-}}$ positronium negative ion, ${\mathrm{Ps}}^{\ensuremath{-}}$. The ${\mathrm{Ps}}^{\ensuremath{-}}$ conversion efficiency of the C film is (2.8\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$.