Thermodynamics of positronium thermal desorption from surfaces

A formula equivalent to the Richardson-Dushman equation for thermionic emission is derived for positronium emission from metal surfaces. Data on clean copper surfaces agree with the thermodynamic equation if one takes an effective reflection coefficient of thermal Ps incident on the copper surfaces to be $\simeq =0.9\mathrm{}$. For submonolayer converage of sulfur on copper, the positronium emission exceeds the simple thermodynamic limit, and the implications of this result are explored.