多年前科学家就已经知道，一个电子和它的反粒子，即正电子，能够结合形成一个亚稳态的、像氢原子一样的原子，被称为positronium (Ps)。据预测，两个Ps原子可以结合形成di-positronium (Ps2)分子，但此前这种分子一直没有被无可争议地观测到。当强正电子束照射进一个多孔二氧化硅薄膜时，Ps2就会在内部的有孔表面上形成。采用一个更强的正电子源，也许有可能形成由Ps2分子构成的一个玻色-爱因斯坦凝聚态，这对于了解物质的基本组成将具有重要意义，并且也将成为制造湮灭伽玛射线激光器的道路上的一个里程碑。
英文原文：
Nature 449, 195-197 (13 September 2007) | doi:10.1038/nature06094; Received 11 June 2007; Accepted 17 July 2007
The production of molecular positronium
D. B. Cassidy1 & A. P. Mills, Jr1
Department of Physics and Astronomy, University of California, Riverside, California 92521-0413, USA
Correspondence to: D. B. Cassidy1 Correspondence and requests for materials should be addressed to D.B.C. (Email: cassidy@physics.ucr.edu).
It has been known for many years that an electron and its antiparticle, the positron, may together form a metastable hydrogen-like atom, known as positronium or Ps (ref. 1). In 1946, Wheeler speculated2 that two Ps atoms may combine to form the di-positronium molecule (Ps2), with a binding energy3 of 0.4 eV. More recently, this molecule has been studied theoretically4; however, because Ps has a short lifetime and it is difficult to obtain low-energy positrons in large numbers, Ps2 has not previously been observed unambiguously5. Here we show that when intense positron bursts are implanted into a thin film of porous silica, Ps2 is created on the internal pore surfaces. We found that molecule formation occurs much more efficiently than the competing process of spin exchange quenching, which appears to be suppressed in the confined pore geometry. This result experimentally confirms the existence of the Ps2 molecule and paves the way for further multi-positronium work. Using similar techniques, but with a more intense positron source, we expect to increase the Ps density to the point where many thousands of atoms interact and can undergo a phase transition to form a Bose–Einstein condensate6. As a purely leptonic, macroscopic quantum matter–antimatter system this would be of interest in its own right, but it would also represent a milestone on the path to produce an annihilation gamma-ray laser7.
