Temperature dependence of thermal positron branching at metal surfaces

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 (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-e⁺-work-function surfaces is the reflection of the e⁺ wave from the surface potential. Using a simple one-dimensional model we obtain estimates for the transition rates for surface trapping, e⁺ , and Ps emission to be of the order of 10³ 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.