Experimental support for physisorbed positronium at the surface of quartz

We report temperature-dependent positronium (Ps) emission from a single crystal of SiO₂ using a monoenergetic positron beam. Slow positrons (e⁺) from an electrostatic beam system were injected with variable energy (0--1600 eV) into the SiO₂ target and Ps emission from the target surface was studied as a function of incident e⁺ energy (E) as well as target temperature (T). Our data suggest a physisorbed Ps surface state which is temperature activated into a ``slow'' Ps emission with an activation energy of {~}0.15 eV. In addition, a large Ps yield (40% of the incident positrons are emitted as Ps at 400 eV) is observed even for T{→}0, attributed to ``fast'' Ps produced by the bulk Ps formed within the SiO₂ target and diffusing to the surface. From the Ps yield vs E we find a bulk Ps diffusion constant of 0.047±{}0.013 cm²/sec. We also observe a slow e⁺ reemission yield of (15±{}2)% at 400-eV incident e⁺ energy.