Experimental support for physisorbed positronium at the surface of quartz

Abstract
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 (01600 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.