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  • Photoassociation Makes Cold Cs2 Molecules

Photonics Spectra
Nov 1998
Robert C. Pini

Physicists at the Centre National de la Recherche Scientifique south of Paris have taken Bose-Einstein condensate research another step forward by using a new method of associating pairs of supercold atoms to form a supercold molecule. Instead of trying to supercool whole molecules, this procedure involves colliding pairs of extremely cold atoms into a photon, which is then absorbed as the molecule forms in an electronically excited state.
Through this process of photoassociation, the research team led by physicist Pierre Pillet at the center's Laboratoire Aimé Cotton has been able to combine cold cesium atoms into a cloud of quasi-immobile molecules in a vapor-cell magneto-optical trap at a temperature of 300 µK, which corresponds to an average speed of 13 cm/s.
In the past, researchers have been able to achieve supercold atoms by using the cycles of absorption and spontaneous emission created by the force of radiant laser pressure. Although the method has been successful at cooling atomic samples down to a millionth of a degree above absolute zero, it has not been possible for researchers to achieve the same success at forming entire supercold molecules, which are complex clusters of varying levels of energy. Molecules formed under this procedure never become stable and dissociate into free atoms after approximately 10 ns.
In this case, the spontaneous emission by the photoassociated molecules leads not to dissociation, but to the stabilization of the cold molecules in ground state.
According to Pillet, the cooling, pumping and photoassociation relied on three diode lasers, each operating at approximately 852 nm, which corresponds to the transition of cesium. Although cesium atoms are particularly well-adapted for formulating cold molecules, Pillet expects that the method could one day be extended to other kinds of atoms. Using a Ti:sapphire laser for photoassociation, the researchers already have begun to understand more of the mechanisms of the formation process.

Multilaser instrumentation
Sensitive detection instruments are a linchpin in a research environment characterized by physical extremes. In this case, the researchers used a dye laser at 716 nm pumped by the second harmonic of a pulsed Nd:YAG to photoionize the Cs2 molecules, selectively detected through a time-of-flight mass spectrometer. Analysis of the cesium ions from the latest results showed the formation of translationally cold dimer Cs2 triplet ground state molecules. "We have also gotten cold molecules in singlet ground state," Pillet said. "They have long life, and we see them fall due to gravity."
The scientists cannot determine the efficiency of photoionization, but they can detect up to 1000 ions per burst, which would give a rate of at least 1000 cold molecules simultaneously within the magneto-optical zone. "We have a rate of formation of [at least] one molecule per microsecond," Pillet said.
Now Pillet and the research team at Laboratoire Aimé Cotton will turn their attention to the formation of other cold molecules using this method. They hope to be able to obtain molecular clouds of utmost thickness and density.
One objective would be to examine what properties of cold atoms are transferred into a cold molecule. Pillet predicts applications in metrology and laser-controlled cold systems chemistry. "The next important step will be in the trapping of cold molecules; to keep and to accumulate them."

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