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 443 K and 692 K gives Ps temperatures of (464 +- 51) K and (636 +- 64) K, in excellent agreement with the expectation of a thermally activated process. The energy spectra are consistent with the surface having a velocity-independent Ps reflection coefficient, implying there is no activation barrier. Further, the temperature dependence of the total Ps yield is consistent with the surface Ps model of Platzman and Tzoar if the reflection coefficient is zero. We confirm Lynn's observation that exposure to O2 causes Ps to be thermally desorbed from Al at lower temperatures