多年前科學家就已經(jīng)知道,，一個電子和它的反粒子，即正電子,，能夠結(jié)合形成一個亞穩(wěn)態(tài)的,、像氫原子一樣的原子，被稱為positronium (Ps),。據(jù)預測,，兩個Ps原子可以結(jié)合形成di-positronium (Ps2)分子，但此前這種分子一直沒有被無可爭議地觀測到,。當強正電子束照射進一個多孔二氧化硅薄膜時,，Ps2就會在內(nèi)部的有孔表面上形成。采用一個更強的正電子源,，也許有可能形成由Ps2分子構(gòu)成的一個玻色-愛因斯坦凝聚態(tài),，這對于了解物質(zhì)的基本組成將具有重要意義，并且也將成為制造湮滅伽瑪射線激光器的道路上的一個里程碑,。
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.
