Zeeman to Paschen-Back Crossover

What you are seeing: the 2p levels of hydrogen as a function of magnetic field $B$ . At weak fields the splitting is $g_J m_J \mu_B B$ (Zeeman). At strong field $\mathbf L$ and $\mathbf S$ decouple and the splitting becomes $(m_L + 2 m_S) \mu_B B$ (Paschen-Back). Crossover occurs when Zeeman energy equals fine-structure energy.

Figure 1. Energy splitting from weak to strong

B

B (T)2.00

WHAT TO TRY

Sweep B up from zero: at weak field the 2p sublevels fan out linearly as g_J m_J mu_B B (the anomalous Zeeman effect), each level keeping its total-J character.
Push past the crossover field marked on the plot: L and S decouple and the levels regroup into the Paschen-Back pattern, splitting as (m_L + 2 m_S) mu_B B. Some sublevels cross and swap order.
Watch the emission comb at the bottom: the spectral lines split and shift as you tune B, which is how a measured line pattern reads back the field strength of a star or lab magnet.