Review of experimental and theoretical research on positronium ions and molecules

Abstract

 In this review we discuss the status of the research for the three-body positronium negative ion ($\hbox {Ps}^{-}$, $e^{-}e^{+}e^{-}$) and the four-body positronium molecule ($\hbox {Ps}_{{2}}$, $e^{-}e^{+}e^{-}e^{+}$) from both theoretical and experimental points of view. Experimental results are included to compare with theoretical predictions. J. A. Wheeler, during the 1940s, proved that $\hbox {Ps}^{-}$ has a stable bound state and he also speculated that larger systems such as $\hbox {Ps}_{{2}}$ can exist. There have been several theoretical works starting from the 1940s about the properties of $\hbox {Ps}^{-}$ and $\hbox {Ps}_{{2}}$. However, experimental observation of such exotic systems proved to be challenging. Thus, $\hbox {Ps}^{-}$ was first observed in 1981 and Ps$_{2}$ was only seen in 2007. During recent decades, there have been major advances in determining the properties of the positronium negative ion and the positronium molecule. These studies shed light on the properties of such exotic systems and allow us to better understand and test the theory of quantum electrodynamics (QED).

Graphic abstract