By Victoria Gill
Science reporter, BBC News
Electrons can be pictured as orbiting around a central nucleus
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.
The movement and position of electrons within an atom can be described as orbiting around a central nucleus - with each shell of orbiting electrons further from the centre.
A Rydberg atom is special because it has one electron alone in an outermost orbit - very far, in atomic terms, from its nucleus.
Back in 1934 Enrico Fermi predicted that if another atom were to "find" that lone, wandering electron, it might interact with it.
"But Fermi never imagined that molecules could be formed," explained Chris Greene, the theoretical physicist from the University of Colorado who first predicted that Rydberg molecules could exist.
"We recognised, in our work in the 1970s and 80s, the potential for a sort of forcefield between a Rydberg atom and a groundstate [or normal] atom.
"It's only now that you can get systems so cold, that you can actually make them."
Right place, right time
Unimaginably cold temperatures are needed to create the molecules, as Vera Bendkowsky from the University of Stuttgart who led the research explained.
"The nuclei of the atoms have to be at the correct distance from each other for the electron fields to find each other and interact," she said.
"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.
"This is a very exciting set of experiments," added Helen Fielding, a physical chemist from University College London.
"It shows that this approach is feasible, and it will be interesting to see what other fundamental physics we'll be able to test with it."
Professor Greene's prediction that Rydberg molecules could exist was inspired by another Nobel prize-winning piece of physics research.
The Bose-Einstein condensation was a new way of thinking about matter
When, in 1924 the Indian physicist Satyendra Nath Bose sent some theoretical calculations about particles to Albert Einstein, Einstein made a prediction.
He said that if a gas was cooled to a very low temperature, the atoms would all suddenly collapse into their "lowest possible energy state", so they would be almost frozen and behave in an identical and predictable way.
In a sense this is analogous to when a gas suddenly condenses into drops of liquid.
When scientists reached the goal of Bose-Einstein condensation, by cooling and trapping alkali atoms, Professor Greene realised that ultracold physics could be used to form molecules that simply would not exist in any other conditions.