Chemical modification of positronium formation at metal surfaces

We have studied the chemical modification of surface charge using positron charge capture. The positronium energy, intensity, and fraction were determined for oxidation of surfaces of Al(111) and Ni(100) at various oxide coverages. We found that increased positronium production at low oxygen coverage results from pickup of electrons from oxygen orbitals at the surface. From the energy of the extra positronium, the energy of the electron state can be determined relative to the Fermi energy.