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Formation of Ultracold NaLi Molecules
The power of Feshbach resonances
Feshbach resonances occur in a cold atomic gas when there is a possible bound molecular state with magnetic moment which differs from the vector sum of the magnetic moments of its constituent atoms. This allows the energy of the molecular state to be adjusted relative to the energy of the free atoms. Things become interesting when the energies are nearly equal at magnetic fields which can be practically generated in the presence of ultracold atoms. Under such circumstances, coupling between the free and bound states can raise or lower the energies of the free atoms (effectively causing repulsion or attraction as the atoms try to move to lower their energy), or can allow the free atoms to bind into a molecule while releasing very little energy. The repulsion and attraction have led to many interesting studies of strongly interacting many-body systems (see the ferromagnetism project on this page, or one of many projects on atomic BCS superfluids), but the ability to form molecules without releasing much energy allows ultracold chemistry to take place.
Feshbach association of Ultracold molecules
While molecules can be formed by allowing thermal atoms to mingle until they crash together and stick (as in traditional chemistry or 3-body collisions in clouds of atoms), or by blasting them with appropriately tuned photons (as in photoassociation), these processes tend to relase a lot of energy because the final molecular state is tightly bound and has a much lower energy than the inital atomic state. This energy must dissipate somehow, and it generally leads to heating of the sample of atoms. Such heating defeats the efforts taken to bring the atoms to ultracold temperatures in the first place.
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We are working to form stable molecules comprised of one lithium and one sodium atom. These two species have proven to cooperate well together, as their favorable collision properties allow them to collide many times without unwanted inelastic reactions. Furthermore, because we can generate large samples of bosonic Sodium-23 and fermionic Lithium-6 (several million atoms of each), we may be able to produce large clouds of fermionic NaLi molecules. Such a large fermionic dipolar gas has interesting applications in the study of complicated many-body systems, as well as potential applications to quantum information.
Practical challenges for forming ultracold NaLi molecules
One major concern when creating such strongly interacting clouds of atoms and encouraging them to form molecules is the potential for 3-body losses. Because the free and bound states are nearly resonant (very low energy difference), two atoms will spend a long time in close vicinity, greatly encouraging the chance that a third atom will approach and enable an inelastic 3-body collions. This is the uncontrolled formation of a deeply bound molecule which dumps its energy into the third particle, and all 3 atoms will be lost from the trap. Such 3-body losses effectively give a time limit to association molecules, isolate them from other atoms which could inelastically collide, and observe the presence of molecules.
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