Angle-resolved positronium emission spectroscopy

The momentum distribution of the positronium (Ps) emitted from an Al(100) surface being bombarded by low-energy positrons shows anisotropic lobes that gradually disappear as the initially clean surface is exposed to small amounts of oxygen. A clean Al(111) surface yields a more isotropic spectrum. The simplest model suggests that the anisotropy stems from the energy gaps in the projected nearly-free-electron band structure of Al. We conclude that inelastic many-body effects are weak and that the Ps spectra reflect the electron density of states near the surface.