Evidence for phonon-assisted positronium emission from graphite

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Abstract
We observe positronium (Ps) emission from a graphite surface being bombarded by slow positrons. The band structure of graphite does not support the usual one-electron-hole process for Ps formation because of conservation of momentum parallel to the surface. A large temperature coefficient for the emission of energetic Ps suggests a new mechanism in which momentum conservation is satisfied by the emission and absorption of phonons. A simple theory including one-phonon processes explains the temperature dependence as well as the angular distribution of the Ps.