COOLING AND TRAPPING:

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The small number of atoms created requires an efficient use of them. Laser traps permit the concentration of atoms in a small region of space moving with very small velocities. This is the ideal environment to perform spectroscopic studies.
  Fig. 1. The Energy of the reaction products must be decreased tremendously before they can be confined in the shallow magneto-optic trap.
In addition to working with an extremely small number of atoms, there is another problem. The nuclear reaction gives the francium atoms too much energy to be easily trapped with lasers. We found a way to remove energy from the atoms quickly and efficiently. The key element for the final stage in the cooling process was a special version of the magneto-optic trap pioneered by the group of Prof. Carl Wieman at the University of Colorado, which traps atoms from an atomic vapor. We used a special coating on the inside of a glass cell to keep the francium atoms from sticking to the walls. Atoms could bounce back and forth freely inside the container allowing many chances to be stopped by the laser beams.

Figure 2. Photograph of the glass bulb used to capture francium. Electric current in the copper tubes above and below the bulb create the magnetic field gradient necessary for trapping. The mirrors retro-reflect the laser beams. Neutral francium atoms enter from the left.
For any trap to be effective it needs to have a position dependent force (a spring) and a velocity dependent force (drag). When working with atoms and lasers, the force comes from the multiple absorption and emission of light. The absorption is from a laser beam that pushes the atoms in a particular direction. The emission is random and in no preferred direction. The resulting force imbalance is enough to slow an atom in a few centimeters. To make the force position dependent it is necessary to have a combination of inhomogenous magnetic fields and proper polarization. The velocity dependence of the force comes from the Doppler effect. The Magneto Optic Trap (MOT) combines these principles and requires a very well defined laser frequency.

To profit from these forces, the atom has to cycle constantly between two energy levels without going into any other level. The D2 line in alkali atoms is ideal for this task. In the case of alkali atoms it is necessary to use a second laser to repump any atom that has fallen into the wrong ground state.

The francium MOT captures atoms that travel at velocities of less than 10 m/s and keeps them trapped for about 30 seconds, in a volume of less than 1 mm in diameter. The residence time of an atom in the trap depends on the quality of the vacuum in the container. A constant stream of atoms replaces those lost, so the number of trapped atoms can remain constant for very long periods of time.

 

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