Positronium binding to metal surfaces

Positron binding to a metal surface is investigated in the hydrodynamic model. A variational calculation is presented. Some of the short-range forces neglected in previous calculations are included through the introduction of a single, distinguishable electron. The surface attractive forces are derived from the Van der Waals potential seen by the positronium-like composite. Calculations predict surface bound states with binding energies of the correct magnitude. These states are stable against positronium emission, in contrast to previous calculations. The dipole moment of the system is also calculated. It is concluded that the positronium channel cannot be ignored in calculations of positron surface bound states using the hydrodynamic model.