Zeeman patterns in the e-a system of FeH, showing (a) the P₁ (4.5) line (λ_vac = 533.76 nm) at zero field and at 0.53 T, with unresolved ∆ M_J = + 1 and −1 components equidistant from the zero-field line centre, and (b) the Q₂(3.5) line (λ_vac = 532.54 nm), recorded at 0.56 T, with better-resolved structure. Note the extended wavenumber scale. Simulations (calc) use effective ɡ_J Landé factors optimised from the experimental profiles.