The Production of Molecular Positronium –
Cassidy, D. B.; Mills, Jr., A. P.; Nature, 2007, 449, 195-197.
From the abstract:
It has been known for many years that an electron and its antiparticle, the positron, may together form a metastable hydrogenlike atom, known as positronium or Ps. In 1946, Wheeler speculated that two Ps atoms may combine to form the dipositronium molecule (Ps2). … Because Ps has a short lifetime and it is difficult to obtain low energy positrons in large numbers, Ps2 has not previously been observed unambiguously. 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. This result experimentally confirms the existence of the Ps2 molecule and paves the way for further multipositronium work.
I just thought it was amazing that positronium has existed and had an elemental symbol (Ps) for the last 60 years and yet, as a fairly advanced student of chemistry, I had never heard of it. These Ps2 molecules only exist for 0.4 nanoseconds before the electrons and positrons annihilate each other, so it’s not the most stable of systems, but it’s still pretty cool that its possible to create an atom that has a weight less than 1/1000th of a hydrogen atom.