April 23, 2009
Scientists Create Elusive Molecule In Lab Experiments
Scientists say the Rydberg molecule, a molecule that until now existed only in theory, has finally been created, BBC News reported.
The molecule is formed through an elusive and extremely weak chemical bond between two atoms.Researchers reported in the journal Nature that the new type of bonding occurs when one of the two atoms in the molecule has an electron very far from its nucleus or center.
Nobel prize-winning physicist Enrico Fermi developed fundamental quantum theories about how electrons behave and interact. Experts say the new molecule discovery reinforces Fermi's ideas.
The scientists formed the Rydberg molecules in question 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 center.
But what makes a Rydberg atom unique is that it has one electron alone in an outermost orbit, which is, atomically, very far from its nucleus.
Fermi predicted back in 1934 that if another atom were to "find" that lone, wandering electron, it might interact with it.
However, Chris Greene, a theoretical physicist from the University of Colorado who first predicted that Rydberg molecules could exist, said Fermi never imagined that molecules could be formed.
"We recognized, 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," Greene said.
Vera Bendkowsky from the University of Stuttgart, who led the research, explained that unimaginably cold temperatures are needed to create the molecules and 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.
"We use an ultracold cloud of rubidium - as you cool it, the atoms in the gas move closer together," she said.
She said that with temperatures very close to absolute zero, a "critical distance" of about 100nm (nanometers - 1nm = one millionth of a millimeter) 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.
Greene likened the Rydberg electron to a sheepdog that keeps its flock together by roaming speedily to the outermost periphery of the flock, and nudging them back towards the center any member that might begin to drift away.
But it requires energy to push the electron out to its lonely periphery in order to make a Rydberg atom.
Bendkowsky said they excite the atoms to the Rydberg stage with a laser and if they have a gas at the critical density and with two atoms at the correct distance that are able to form the molecule, they excite one to the Rydberg state in order to form a molecule.
The researchers said, however, that 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.
Helen Fielding, a physical chemist from University College London, called it a "very exciting set of experiments," adding that it shows that this approach is feasible.
"It will be interesting to see what other fundamental physics we'll be able to test with it," she said.
A Nobel prize-winning piece of physics research by Indian physicist Satyendra Nath Bose was the catalyst for Professor Greene's prediction that Rydberg molecules could exist.
Bose sent some theoretical calculations about particles to Albert Einstein in 1924 and Einstein made a prediction 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.
Experts say this is analogous to when a gas suddenly condenses into drops of liquid.
Greene realized that when scientists reached the goal of Bose-Einstein condensation, by cooling and trapping alkali atoms, ultracold physics could be used to form molecules that simply would not exist in any other conditions.
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