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Ultracold polar molecules
Paul S. Julienne
Simple models for ultracold molecular collisions
Physicists at the 2010 March Meeting announced the first observation of chemical reactions at temperatures near absolute zero. It is the first time that the quantum state of the molecules significantly affected a chemical reaction.
Researchers at the University of Colorado cooled a gas of potassium-rubidium molecules in an optical lattice to a few nanoKelvins above absolute zero and observed the atoms break and reform molecular bonds.
“What’s going on here is chemistry,” said Deborah Jin from JILA at the University of Colorado and one of the team that conducted the experiments, “This is the first time in a chemical reaction the quantum state plays a role.”
The cooled molecules reacted with each other over distances much greater than they normally would at room temperatures. At these ultra-low energy levels, the quantum wavelength of each molecule expands out to over 100 nanometers, much greater than the 1 nanometer distances over which chemical reactions typically occur.
When the wavelengths of two potassium-rubidium molecules overlapped under the right conditions, they broke their molecular bonds and reformed as one molecule of two rubidium atoms and one of two potassium atoms.
“The two molecules are highly reactive when they are close together, with nearly 100 percent probability of reaction when they are within a nanometer of one another,” said Paul Julienne, a theorist at NIST who was also on the team. “The very long quantum wavelengths of the molecules, more than 100 nm, means that they can only get within 1 nm of each other by specifically quantum ways that depend strongly on temperature and whether the two fermions are in the same or different spin states.”
The research team found also that the reactions were highly influenced by the nuclear spins of the atoms in the molecules. According to the Pauli Exclusion Principle, two identical fermions cannot occupy the same state at the same time. As a result, molecules that have the same spins are less likely to be near each other than those that have different spins, slowing down the reaction rate by up to factor of 100.
Physicists began using lasers to cool atoms to near-zero temperatures more than 20 years ago; however cooling entire molecules is a much more recent development. Factors including combined nuclear spins and rotational and vibrational states add a huge extra level of complexity to the techniques needed to trap and cool atoms, the first cooling of entire molecules was only achieved in 2008. The simple but reactive potassium-rubidium molecules were ideal for the experiment. In addition each molecule is polar, positively charged on the rubidium side and negatively charged on the potassium side, allowing the physicists to easily manipulate the molecules with an electric field.
“There has been work on ultracold molecules made from ultracold atom gases before, but those are molecules in very high vibrationally excited states, so the molecules are barely bound,” said Jun Ye, the other team leader at JILA. “In our experiment, we have ground-state molecules–in their lowest energy states–at nanokelvin temperatures.”
The researchers who conducted the experiment say that this technique could yield new insight into intermolecular forces and could have applications in quantum computing and high resolution spectroscopy.
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