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.

• Received 23 September 1987

DOI:https://doi.org/10.1103/PhysRevLett.60.538