Experiments to Control Atom Number and Phase-Space Density in ...

7.5.1.2 Absorption Scattering Cross Section at Large Magnetic Fields
In the presence of a magnetic field, the effective scattering cross section differs
from the scattering cross section at zero magnetic field. The energy levels at high fields
are separated by more than the transition linewidths and thus individual transitions can
be resolved. The energy level splitting as a fuction of an external magnetic field is shown
in figure 7.42.
Energy ShiftMHz
2000.
1000.
0.
1000.
2000.
3000.
2000.
1000.
1000.
100. 200. 300. 400. 500. 600. 700. 800. BGauss
Figure 7.42: Energy level splitting as a function of an external magnetic field. At high
magnetic fields the energy levels split to more than the transition linewidth.
Prior to optical evaporation, the atoms are pumped into the |F = 1/2〉 state. At
large magnetic fields they will therefore occupy the two lowest states. In this regime
a closed cycling transition exists. This means that a repump beam is not necessary.
Because the two states are energetically separated (∆f ≈ 76 MHz), atoms can be imaged
separately in each state. The cycling transition drives the atoms from the |(L = 0,S =
1/2)J = 1/2,mJ = −1/2〉 to the |(L = 1,S = 1/2)J = 3/2,mJ = −3/2〉 state, as shown
in figure 7.43.
Unfortunately, the transition frequency is slightly dependent on the magnitude
of the magnetic fields. It tunes with approximately 1.4 MHz/G. However, knowing the
magnetic field, the frequency of the imaging laser can be adjusted using the frequency-
offset lock.
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