The positron and positronium emission yields are measured as a function of temperature at Cu(111) and Al(110) surfaces having a negative positron work function, and at the positive-work-function surfaces Ag(100) and Ag(111). At the Cu(111) positron (${\mathit{e}}^{+}$) emission yield is reduced at low temperatures, vanishing as T→0 K. The positronium (Ps) emission exhibits a similar temperature dependence at Cu(111) and Al(110). The dominating temperature-dependent feature at these negative-${\mathit{e}}^{+}$-work-function surfaces is the reflection of the ${\mathit{e}}^{+}$ wave from the surface potential. Using a simple one-dimensional model we obtain estimates for the transition rates for surface trapping, ${\mathit{e}}^{+}$ , and Ps emission to be of the order of ${10}^3$ m/s. Ps-emission probabilities for Ag(100) and Ag(111) surfaces show drastically different temperature behavior. In order to explain these results we propose a new surface-trapping mechanism, the acoustic-phonon-mediated trapping. This would imply a weak temperature dependence of Ps emission at Ag surfaces.

• Received 20 February 1990

DOI:https://doi.org/10.1103/PhysRevB.42.1560