Observation of Positronium Specular Reflection from LiF

Abstract

A monoenergetic positronium (Ps) beam of 0-60-eV energy and an angular width of \ifmmode\pm\else\textpm\fi{}5\ifmmode^\circ\else\textdegree\fi{} 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\ifmmode^\circ\else\textdegree\fi{} to 60\ifmmode^\circ\else\textdegree\fi{} we observe a specularly reflected beam with a maximum reflected fraction $R=(30\ifmmode\pm\else\textpm\fi{}5)%$ 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 $\ensuremath{\lambda}=(0.75\ifmmode\pm\else\textpm\fi{}0.15)$ \AA{}, and to a lesser extent a Ps inner potential of about 4 eV.