Evidence for quantum sticking of slow positronium

We have measured the energy spectrum of positronium (Ps) thermally desorbed from Al(111) surfaces. For sample temperatures as low as 84 K and Ps perpendicular energies between 5 and 50 meV the spectrum is a beam Maxwellian, i.e., a simple exponential. We argue that the surface must be essentially a blackbody for Ps emission, and that we thus have the first example of a system that fails to exhibit the expected perfect reflection of a very slow particle from a cold surface. We suggest that the effect is associated with the breakdown of the perturbation expansion for the Ps reflection probability.