EPJ D Topical review - Review of experimental and theoretical research on positronium ions and molecules

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Published on 02 July 2021

The relativistic quantum theory developed by Dirac in the 1930’s is the cornerstone of Quantum Electrodynamics (QED), which has proved to be one of the most successful theories in physics. For example, measurements and QED calculations of the anomalous electron magnetic moment agree to 10 significant figures. Physicists now believe that QED can fully account for all effects that are mediated by electromagnetic interactions.

The work of Dirac also resulted in an unexpected prediction: the existence of antiparticles, which was confirmed by the discovery of positrons, the electron-antiparticle, in cosmic rays by C. Anderson in 1932. It was quickly realized that electrons and positrons could form metastable bound states, now known as positronium atoms. In 1946 J. A. Wheeler suggested that more complex electron-positron systems, which he called polyelectrons, would also form stable bound states, albeit with short lifetimes against self-annihilation. The simplest three-body polyelectron is the positronium ion, composed of two electrons and one positron (or one electron and two positrons). The simplest four-body polyelectron is the positronium molecule, composed of two electrons and two positrons. As they contain only leptons these systems are fully described by QED theory, with only extremely small contributions from weak interactions. Since the work of Wheeler much theoretical effort has been devoted to polyelectron systems. Larger polyelectrons are not thought to form stable bound states.

The positronium negative ion was first observed experimentally by A. P. Mills in 1981, while the positronium molecule was not observed until 2007 by Cassidy and Mills. Subsequently, measurements of the Ps ion decay rate have been performed, as well as spectroscopic measurements of the Ps ion binding energy and photodetatchment threshold, and the first Ps molecular excited state. The positronium negative ion and the positronium molecule are of particular interest as pure three-, and four-body QED systems with which to test bound-state QED theory. In this review article published in EPJ D, authors Mohsen Emami-Razavi (Islamic Azad University, Iran) and Jurij W. Darewych (York University, Canada) discuss the current status of research involving positronium ions and molecules from both theoretical and experimental points of view.