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A molecule that until now existed only in theory has finally been made.
Known as a Rydberg molecule, it is formed through an elusive and extremely weak chemical bond between two atoms.
The new type of bonding, reported in Nature, occurs because one of the two atoms in the molecule has an electron very far from its nucleus or centre.
It reinforces fundamental quantum theories, developed by Nobel prize-winning physicist Enrico Fermi, about how electrons behave and interact.
The Rydberg molecules in question were formed from two atoms of rubidium - one a Rydberg atom, and one a "normal" atom.
"We use an ultracold cloud of rubidium - as you cool it, the atoms in the gas move closer together."
The researchers excite an atom to the "Rydberg state" using a laser
At temperatures very close to absolute zero - minus 273C - this "critical distance" of about 100nm (nanometres - 1nm = one millionth of a millimetre) between the atoms is reached.
When one is a Rydberg atom, the two atoms form a Rydberg molecule. This 100nm gap is vast compared to ordinary molecules.
"The Rydberg electron resembles a sheepdog that keeps its flock together by roaming speedily to the outermost periphery of the flock, and nudging back towards the centre any member that might begin to drift away," said Professor Greene.
Pushing this electron out to its lonely periphery - and make a Rydberg atom - requires energy.
"We excite the atoms to the Rydberg stage with a laser," explained Dr Bendkowsky.
"If we have a gas at the critical density, with two atoms at the correct distance that are able to form the molecule, and we excite one to the Rydberg state, then we can form a molecule."
This ultracold experiment is also ultra-fast - the longest lived Rydberg molecule survives for just 18 microseconds.
But the fact that the molecules can be made and seen confirms long-held fundamental atomic theories.