Evidence for positronium molecule formation at a metal surface

We have observed a reduction in the amount of positronium emitted from an atomically clean Al(111) surface that depends on the incident positron beam density. We interpret this as evidence for the formation of molecular positronium, created following interactions between two pseudopositronium atoms trapped in a surface state. We find that this process is highly sensitive to the condition of the surface and is easily suppressed by changes thereupon. The implications of our data for planned spectroscopic studies of molecular positronium are discussed, as well as improvements to the experimental procedure that will allow more detailed measurements of the thermodynamics of the formation of this molecule from metal surfaces.