Search for adiabatic positronium emission from a metal surface

We measure by time of flight the energy distribution of positronium (Ps) emitted normally to a clean Al(111) surface that is bombarded with energetic (1.5–4.5 keV) positron pulses. The data are consistent with the predictions of a free-electron model for nonadiabatic Ps emission with less than ${10}^{\mathrm{-}3}$ of the Ps emitted normal to the surface in a monoenergetic peak at the maximum allowable energy. The Ps work function at 300 K is (-2.536±0.035) eV in agreement with the (-2.519±0.027) eV obtained from the sum of the electron and positron work functions minus the binding energy of Ps.