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April 25, 2009
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World first for strange molecule
Opinion
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 center.
It reinforces fundamental quantum theories, developed by
Nobel prize-winning physicist Enrico Fermi, about how
electrons behave and interact.
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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 center.
“It will be interesting to see what other fundamental
physics we will be able to test with this approach “
A Rydberg atom is special because it has one electron alone
in an outermost orbit - very far, in atomic terms, from its
nucleus.
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Back in 1934 Enrico Fermi predicted that if another atom
were to “find” that lone, wandering electron, it might
interact with it.
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“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.
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“We recognized, in our work in the 1970s and 80s, the
potential for a sort of forcefield between a Rydberg atom
and a groundstate
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[or normal] atom.
“It's only now that you can get systems so cold, that you
can actually make them.”
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.
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“We use an ultracold cloud of rubidium - as you cool it, the
atoms in the gas move closer together.”
At temperatures very close to absolute zero - minus 273C this “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.
“The Rydberg electron resembles a sheepdog that keeps its
27.04.2009 13:42
tehran times : World first for strange molecule
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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.
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 realized that ultracold physics could be
used to form molecules that simply would not exist in any
other conditions.
(Source: BBC)
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