Experiments with Supersonic Beams as a Source of Cold Atoms

operated. More detail on the magnetic and rotational states of molecular oxygen can
be found in [86].
Since high-field-seeking states are incompatible with the coilgun in its current
method of operation, the avoided crossing sets a limit on the maximal value of the
magnetic fields that can be used to slow oxygen. The measured peak value of the
magnetic fields created on the axis of the coilgun solenoids is 5.2T, and finite element
calculations indicate that the maximal magnetic field adjacent to the coil windings
is just below 6 T. This maximal field which an atom can encounter in the coilgun is
well below the limit of 8 T. Thus, the avoided crossing is not expected to play a large
role in the operation of the coilgun. In order to achieve the highest slowing efficiency,
coil timing is tuned for molecules in the ground rotational J =2,MJ = 2 sub-level.
4.6.2 Oxygen Beam Creation and Detection
Since the ground state of oxygen is magnetic, a discharge is not required
to excite the molecule to a metastable state to enable its slowing by the coilgun.
However, because the boiling point of oxygen is ≈ 90K,thenozzlecannotbecooled
to 77 K by liquid nitrogen. Additionally, the mass of molecular oxygen is greater
than that of metastable neon, and so oxygen has a greater kinetic energy at the same
velocity. The magnetic moment of oxygen is less than that of metastable neon as
well, which means that less energy will be removed per coil. These two issues make it
highly advantageous to start with as slow an initial beam as possible. For this reason,
rather than continue to use neon as a carrier gas, krypton is used instead as its higher
mass leads to a slower initial beam. Oxygen is mixed with krypton at a 15:75 ratio,
and the valve is run with a backing pressure of 34 psi.
Oxygen is a liquid at 77 K (krypton is too, its boiling point is ≈ 120 K),
so cooling the nozzle with liquid nitrogen is not a possibility. However, it is still
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