Desorption of surface positrons: A source of free positronium at thermal velocities

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 Ē=3.4(3) eV. At 790 °C, most of the remaining e+ form Ps but with Ē=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.