Low-field seeking states

Fig. 1. Hyperfine diagram for the ground state of atomic hydrogen. The high field seeking states a and b can be stabilized in a high magnetic field, while the low field seeking states c and d are trapped in the minimum of a magnetic field...

Atoms are provided from an rf discharge source, at cryogenic temperatures. After thermalization on the cold cell walls, typically 250 mK, the low field seeking states, c and d (Fig. 1) are attracted to the center of a Ioffe-Pritchard trap, a linear quadrupole trap with a coil at each end to confine the atoms axially. The trapping field is initially about 0.9 T, sufficient to capture atoms with a temperature of about 0.5 K. Once the temperature of the walls is reduced, the temperature of the trapped gas rapidly falls by evaporation, the escaping atoms being trapped on the helium surface. At about 60 mK, the gas becomes isolated from the wall and evaporation ceases. 

The temperature for Bose-Einstein condensation varies with density as n20. Because density is limited by three-body recombination, the search for the transition leads naturally to lower temperatures. Unfortunately, at temperatures below 0.1 K, adsorption rapidly becomes prohibitive. To avoid this problem, Hess [4] suggested confinirig the atoms in a magnetic trap without any surfaces. The states confined are the "low-field seeking" states, (HT, electron spin "up"). These are the hyperfine states (F-l,m-l) and (F=l,m=0). 

In the remainder of this Chapter, Stark deceleration of a molecular beam is presented in more detail, followed by a description of the process of trapping neutral polar molecules. An overview of the applications of the slow beams and trapped samples of molecules that can be produced is given, before the Chapter is concluded. The experiments described in this Chapter to exemplify the operational characteristics of the various components, have been performed in different molecular beam machines and using different molecules. The deceleration and 3D trapping of molecules in low-field seeking states is explained using the OH radical as a model... 

Fig. 9.5. The potential energy W(z) of n polar molecule as a function of position z along the molecular beam axis. The molecule is in a low-field seeking state and the electric field is created by two high voltage electrodes at opposite polarity.

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