Photoassociation in a Bose-Einstein condensate. (General Developments).

Two colliding atoms can absorb a photon and be photo-associated into an excited, diatomic molecule. NIST researchers investigated the fundamental rate at which this process can take place in a Bose-Einstein condensate (BEC) and demonstrated that simple classical saturation arguments do not work when applied to this situation.

While the photoassociation reaction has been well-studied in a thermal gas, one can argue that the same process will run into limitations under the conditions present in a BEC. A simple classical argument says that a pair of atoms has a narrow range of internuclear separations at which the photoassociation process can be driven to take place. Once atoms with this range of separations are depleted, the population must be replenished by the movement of atoms, and that this is limited by their thermal velocity (about 0.5 mm/s in this case). On the other hand, a quantum mechanical description of the BEC has a single wavefunction for all the atoms; each atom is extended across the entire region where the experiment is performed. The measured rate coefficient for the reaction is in good agreement with results from a quantum mechanical two-body scattering theory for all the intensities that were achievable experimentally, and can exceed the classical limit by more than four orders of magnitude.

CONTACT: Paul Lett, [phone]; [e-mail].