The Paschen–Back effect is the splitting of atomic energy levels in the presence of a strong magnetic field. This occurs when an external magnetic field is sufficiently strong to disrupt the coupling between orbital () and spin () angular momenta. This effect is the strong-field limit of the Zeeman effect. When , the two effects are equivalent. The effect was named after the German physicists Friedrich Paschen and Ernst E. A. Back.
When the magnetic-field perturbation significantly exceeds the spin–orbit interaction, one can safely assume . This allows the expectation values of  and  to be easily evaluated for a state . The energies are simply
The above may be read as implying that the LS-coupling is completely broken by the external field. However  and  are still "good" quantum numbers. Together with the selection rules for an electric dipole transition, i.e.,  this allows to ignore the spin degree of freedom altogether. As a result, only three spectral lines will be visible, corresponding to the  selection rule. The splitting  is independent of the unperturbed energies and electronic configurations of the levels being considered.
More precisely, if , each of these three components is actually a group of several transitions due to the residual spin–orbit coupling and relativistic corrections (which are of the same order, known as fine structure). The first-order perturbation theory with these corrections yields the following formula for the hydrogen atom in the Paschen&ndash;Back limit: