Thermal desorption of cold positronium from oxygen-treated Al(111) surfaces

We have measured the yields and energy spectra of positronium (Ps) emitted from Al(111) surfaces treated by exposure to oxygen gas at low temperatures. We find that the oxygen induces the emission of Ps with kinetic energies of a few tenths of an eV, in agreement with previous work. We also find that Ps is thermally desorbed at low temperatures and has a velocity distribution characterized by a temperature that is the same as that of the sample. The intensity of the thermal Ps component is about 12% of the positrons that reach the surface. The velocity distributions may be interpreted as indicating that the Ps has a sticking coefficient of 1 in the limit of zero velocity, unlike any other system studied to date.