Positronium

Positronium is a quasi-stable system consisting of a positron and an electron bound together into an "exotic atom." Its set of energy levels is similar to that of the hydrogen atom (electron and proton). However, because of the different reduced mass, the frequencies associated with the spectral lines are less than half of those of the corresponding hydrogen lines. The half-life of positronium is at most about 10^-7 seconds, its existence being terminated by electron-positron annihilation[?].