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°C, about half the $e^{+}$ form Ps with normal energy component $\overline{E}=3.4\mathrm{}\left(3\right)\mathrm{}$ eV. At 790°C, most of the remaining $e^{+}$ form Ps but with $\overline{E}=0.14\mathrm{}\left(1\right)\mathrm{}$ 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.