Positronium velocity spectroscopy of the electronic density of states at a metal surface

The velocity of positronium (Ps) formed when 1-2-keV positrons are implanted into an Al(111) target and diffuse to the clean surface has been measured. The time-of-flight distribution exhibits a sharp step at a Ps energy 2.62(4) eV in excellent agreement with the expected Ps work function Ps=-2.60(3) eV obtained from the known electron and positron work functions of Al(111). The presence of less than a monolayer of oxygen reduces the step amplitude by a factor of 2. It is shown that this spectrum is a measure of the surface density of states.