There is a substantial research effort in laser cooling and trapping of atoms, the field that garnered the 1997 Nobel Prize in Physics. These techniques take advantage of the fact that laser light tuned near an atomic resonance can exert significant forces on atoms. Such radiative forces are used to cool atoms to very low temperatures ( 1 mK) and confine them in a laser trap. Details of these forces and properties of the laser cooled samples are being investigated. In addition, collisions between these extremely cold atoms are being studied in order to shed light on atom-atom interactions in this novel regime. Since the atoms are moving so slowly ( 1 m/s), laser light can be used to control the collisions to a significant extent. These ultracold collisions are important to understand because they can be a density-limiting mechanism for laser cooled samples. Ultracold atoms are also an ideal starting point for photoassociation, a process in which two free atoms absorb a photon and form a long-range excited molecule. This has opened a whole new area in molecular spectroscopy since it allows very high-resolution probing of states which are otherwise inaccessible. Using these techniques, we are learning a great deal about exotic molecular states as well as the long-range interactions between atoms.

A significant new multi-investigator effort is now underway to extend laser cooling and trapping techniques, which have been applied so fruitfully to atoms and ions, to molecules. This will allow the investigation of collisions and chemical reactions of molecules in the ultracold regime. Concurrent with the experimental work, we are also surveying new theoretical concepts. For example, we are initiating both experimental and theoretical projects on potential uses of the STIRAP (Stimulated Raman Adiabatic Passage) process for the production of state-selected ultracold molecules.

Another new collaboration involves experimental and theoretical investigation of ultracold atoms excited near the ionization limit. This work includes both highly excited (Rydberg) atoms and dense plasmas formed at extremely low temperatures.

As theoretical support for our experiments in laser cooling and trapping, we have constructed the ``numerical atom''. This is a system of computer programs that may be used to numerically simulate laser cooling, and laser spectroscopy in general, for an arbitrary atom in an arbitrary light field. Extension of this machinery to the case of molecules is in progress.

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