Positronium desorption of positrons bound to alkali-metal-covered Ni surfaces

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Abstract
Both positrons and electrons can be bound to metal surfaces by image-correlation interactions. For electrons, the image potential dominates, preventing the electron from escaping into vacuum. For positrons, a consistent picture of the trapping process has not been developed. We investigate the effect of alkali-metal-atom adsorption on the binding of positrons to Ni surfaces. Thermal desorption of positronium is observed for temperatures down to 325 K and cryogenic desorption is predicted. The slow variation of the activation energy for positronium desorption with alkali-metal-atom coverage indicates that correlation effects dominate the positron surface state. The physical picture considered is that of a composite particle trapped in a correlation well of the screened positron with the well eventually joined to the 1s positronium bound state.