Positronium

Positronium (Ps) is a system consisting of an electron and its anti-particle, a positron, bound together into an "exotic atom". The system is unstable: the two particles annihilate each other to produce two gamma ray photons after an average lifetime of 125 picoseconds or three gamma ray photons after 142 nanoseconds in vacuum, depending on the relative spin states of the positron and electron. The orbit of the two particles and the set of energy levels is similar to that of the hydrogen atom (electron and proton). However, because of the reduced mass, the frequencies associated with the spectral lines are less than half of those of the corresponding hydrogen lines.