Attractive atom-surface interaction: Saturation effects in positronium

The attractive interaction between an atom and a surface is examined within the general framework of a self-energy formalism. Considered in detail are deviations from the van der Waals force due to recoil effects and finite velocity of the particle. Calculations for positronium moving near a metal surface show that for such systems recoil and velocity effects are significant even at very low energies.