Desorption of Surface Positrons: A Source of Free Positronium at Thermal Velocities

Abstract

A direct measurement is reported of the velocity of positronium (Ps) ejected into a vacuum when 0- to 100-eV positrons (${e}^{+}$) strike a negatively biased Cu(111) surface. At 30\ifmmode^\circ\else\textdegree\fi{}C, about half the ${e}^{+}$ form Ps with normal energy component $\overline{E}=3.4(3)$ eV. At 790\ifmmode^\circ\else\textdegree\fi{}C, most of the remaining ${e}^{+}$ form Ps but with $\overline{E}=0.14(1)$ eV, and a non-Maxwellian thermal distribution. We infer that surface-bound ${e}^{+}$ are thermally desorbed to form the extra Ps. These low Ps velocities suggest exciting possibilities for experiments on free Ps.