Positronium and Its Molecules

In 1928 Paul Dirac developed a wave equation that took into account Einstein’s special theory of relativity. The Dirac equation, as it is now known, was in excellent agreement with the known properties of the hydrogen atom, but also made a strange prediction; that, in addition to the well known electron, another particle should exist, that has a positive electric charge and a “negative energy” which he initially thought of as holes in a sea of electrons. After 3 years of consternation Dirac finally concluded that “a hole, if there were one, would be a new kind of particle, unknown to experimental physics, having the same mass and opposite charge to an electron. We may call such a particle an anti-electron”. Just such a particle was observed (apparently without knowledge of Dirac’s prediction) in a cloud chamber at Caltech by Carl Anderson in 1932. Just two years later an obscure Croatian physicist by the name of Stejphan Mohorovičić suggested that positrons and electrons might form bound states, having a spectrum similar to that of hydrogen but with wavelengths twice as long. This suggestion was all but ignored, however, until 1945 when two American physicists, Arthur Ruark and John Archibald Wheeler independently came up with the same idea. The bound state between the electron and its antiparticle (termed positronium by Ruark) was experimentally observed by another American, Martin Deutsch at MIT in 1951. This system is “stable” in that it forms a bound state with a well defined atomic structure. Being composed of a particle-antiparticle pair it will, however, self-annihilate in a fraction of a microsecond. Wheeler did more than consider simply a positron-electron state, but instead described systems with many electrons and positrons, which he called polyelectrons. We now know that the positronium atom (Ps) ions (Ps⁺, Ps^-) and molecule (Ps₂) are stable. The Ps^- ion was first observed by Allen Mills in 1981 at Bell Labs, and in 2007 evidence was obtained for the experimental production of Ps₂ at UC Riverside in California. In this presentation I shall describe some of the experimental methods that have been used to create and study Wheeler’s polyelectrons, and discuss the recent spectroscopic detection of the Ps₂ molecule via one of its excited states.