Positronium formation in graphene and graphite

Positronium (Ps) formation on the surface of clean polycrystalline copper (Cu), highly oriented pyrolytic graphite (HOPG) and multi layer graphene (MLG) grown on a polycrystalline copper substrate has been investigated as a function of incident positron kinetic energy (1.5eV to 1keV). Measurments on Cu indicate that as the kinetic energy of the incident positrons increases from 1.5eV to 900eV, the fraction of positrons that form Ps ($f_{Ps}$) decreases from ~0.5 to ~0.3. However, in HOPG and MLG, instead of a monotonic decrease of $f_{Ps}$ with positron kinetic energy, a sharp peak is observed at ~ 5eV and above ~200eV,remains nearly constant in HOPG and MLG. We propose that in HOPG and MLG, at low incident positron energies the Ps formation is dominated either by a surface Plasmon assisted electron pick up process or by an energy dependent back scattering process. Both these processes can explain the peak observed and the present data can help to augment the understanding of Ps formation from layered materials.