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Physics News Update
Number 817, March 29, 2007 by Phil Schewe and Ben Stein

Slow Salt

Through laser cooling it's relatively easy to cool atoms to microkelvin temperatures. This method is not useful for molecules, which possess a variety of internal vibrational and rotational motions. By indirect methods, however, stationary samples of molecular vapors have been chilled to mK temperatures by cooling molecules in cold helium or by decelerating polar molecules, or to microkelvin temperatures by welding together pairs of already cooled atoms.

Another cooling technique employs a spinning beam source whose speed cancels the velocity of the molecules emerging from the source. Molecular speeds down to around 60 m/s have been obtained. Now, two physicists at the Universitat Bielefeld (Germany) have produced a beam of potassium-bromine molecules (essentially a kind of salt) with an average molecular speed of 42 m/s; an estimated 7% of the beam travels even slower than 14 m/s (below 1.4K). At this speed, some of the molecules could be loaded into a trap.

The cold KBr molecules are made by sending a beam of K atoms into a counter-propagating beam of HBr molecules where the velocity of both species have to be tuned properly. Within the intersection zone the slow KBr molecules are formed by chemical reaction. There the density of trappable molecules is about two million molecules per cubic centimeter, but the researchers believe this can be increased by a thousandfold.

Besides KBr, beams of other heavy salt molecules can be produced (such as CsI) as well as beams of radicals (reactive molecules with unpaired electrons) such as CaBr and BaI. According to Hansjuergen Loesch (loesch@physik.uni-bielefeld.de), slow molecules are a prerequisite for performing cold chemistry, which would simulate conditions in cold planetary atmospheres or in cold interstellar clouds. If the chemistry is cold enough, new quantum effects might emerge. (Liu and Loesch, Physical Review Letters, 9 March 2007

The Ever-Shifting Face of Plutonium

A new theory explains some of the unusual properties of plutonium, the radioactive metal best known for its proclivity to undergo nuclear fission chain reactions, making it a potent fuel for nuclear weapons and power plants. Plutonium is one of the most unusual metals--it's not magnetic and it does not conduct electricity well. The material also changes its size dramatically with even the slightest changes in its temperature and pressure. The atom's unusual set of properties distinguishes it from even its closest neighbors on the periodic table, such as americium.

What makes plutonium unique? In the new theory, developed by condensed-matter theorists at Rutgers University in New Jersey, plutonium's eight outermost or "valence"; electrons can circulate among different orbitals, or regions around the atom. In plutonium's 5f orbital, the one with the greatest influence on its atomic properties, the number of valence electrons it contains is most often five (approximately 80% of the time), but can also be six (about 20% of the time) or four (less than 1% of the time), according to the theory. These electrons shuttle in and out of the 5f orbital very quickly--on the order of femtoseconds, or quadrillionths of a second, the researchers say.

Plutonium is an example of a strongly correlated material, in which the valence electrons interact with each other to a great degree, and cannot be treated as independent agents. Taking these interactions into account, the researchers combined two theoretical approaches to solid materials, called the local density approximation and dynamical mean field theory, to come up with their sophisticated analysis.

As their analysis shows, the 5f orbital dictates many of plutonium's key properties, such as its lack of conductivity and net magnetism. With their theory, the researchers have also explained the magnetic and electrical properties of americium and curium. They hope their approach will also elucidate the properties of rare-earth elements on the periodic table (Shim et al., Nature, 28 March 2007.)

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