Observation of Positronium Specular Reflection from LiF

A monoenergetic positronium (Ps) beam of 0-60-eV energy and an angular width of ±{}5^∘{} is created by charge-exchange collisions of a slow positron beam passing through an Ar gas cell. The Ps beam is directed at a Lif(100) crystal, and reflected Ps atoms are detected at an annihilation target. With angles of incidence of 50^∘{} to 60^∘{} we observe a specularly reflected beam with a maximum reflected fraction R=(30±5)% at a Ps energy of 7 eV. At higher energies (20-60 eV) the reflectivity ($R=0.5%$ at 60 eV) can be ascribed principally to a short Ps mean free path λ=(0.75±0.15) {}, and to a lesser extent a Ps inner potential of about 4 eV.