Does the magnetic field decrease in a Zeeman slower as atoms slow down?

[throneoo]

Background: I have limited knowledge on QM and only know that magnetic fields split the energy levels of an atom

It seems counter-intuitive to me that as the atoms slow down along the tube the magnetic field strength decreases
http://es1.ph.man.ac.uk/AJM2/Atomtrapping/Zeeman-B-field.jpg

as far as I understand, the point is to increase the field so the less blue-shifted photons can fit into the energy gap between the excited and ground state , which is decreased by amplifying the effects of the splitting

Apparently it isn't the case, as shown by the figure. So where am I wrong?

 

[DrClaude]

blue-shifted photons

The laser is red-shifted from the atomic transition (including the magnetic field). The Doppler effect brings it closer to resonance, where scattering is strong. But as it slows down, the atom sees the laser light as being more and more red-detuned. Decreasing the magnetic field lowers the energy of the transition, such that the laser is again close to resonance.

 

[throneoo]

The laser is red-shifted from the atomic transition (including the magnetic field). The Doppler effect brings it closer to resonance, where scattering is strong. But as it slows down, the atom sees the laser light as being more and more red-detuned. Decreasing the magnetic field lowers the energy of the transition, such that the laser is again close to resonance.

My confusion is actually about is the field's effect on the energy levels. Surely if they split, the energy gap between the highest ground state and the lowest excited state will decrease and that gap is wider with a weaker magnetic field? So I don't see how that helps the transition.

After doing some reading, I think the selection rule has something to do with this, as it negates my assumption that the electron can jump between any two states.

 

[DrClaude]

You want the cooling transition to be a closed one, so yes, selection rules play a role. But what is more important for your question is what is the behavior of the two levels used for cooling with respect to a magnetic field. Experimenters use a decreasing magnetic field because the chosen excited state gets shifted more than the ground state, and hence the separation in energy grows with the magnetic field.