Positronium formation at physisorbed monolayer surfaces of argon, nitrogen, and oxygen on graphite

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Abstract
Photon spectra arising from positrons annihilating at surfaces in exfoliated graphite have been measured over a range of temperatures. The role in the annihilation process of physisorbed equilibrium layers of argon, nitrogen, and oxygen has been observed. With argon and nitrogen the formation of orthopositronium is indicated by a three-to-two γ-ray ratio; for nitrogen maximum o-Ps was found to occur on completion of one monolayer. In contrast, a Doppler analysis shows that oxygen yields parapositronium. In the case of nitrogen, the results allow the estimation of the coverage, the binding energy of the molecule to the carbon substrate, and the activation energy for the process of Ps emission from the surface traps.