Velocity spectrum of positronium thermally desorbed from an Al(111) surface.

Time of flight measurements show that the spectrum of the positronium (Ps) thermally desorbed from a clean Al(111) surface is an exponentially decreasing function of the normal component of the Ps kinetic energy. The slope of the exponential obtained from samples at 443K and 692K gives Ps temperatures of (464+- 51)K and (636+-64)K, in excellent agreement with the expectation of a thermally activated process.