On the formation of excited state positronium in vacuum by positron impact on untreated surfaces

The formation of excited state (2P) positronium following the bombardment of untreated beryllium, copper and gold surfaces by positrons in the energy range 10-500 eV is reported. This investigation finds emission efficiencies in the low per cent region, in accord with one other study of similar samples. Timing spectra between the de-excitation Lyman-a photons and annihilation gamma rays with a statistical accuracy superior to that of other studies have been generated. The interpretation of the data, including the production mechanism of the excited atoms, is discussed.