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Alfred Landé organized the magnetic response of coupled atomic angular momenta. The Landé g-factor relates a magnetic energy shift to the projection of total angular momentum, accounting for both orbital angular momentum and spin. For LS coupling, $g_J=1+[J(J+1)+S(S+1)-L(L+1)]/[2J(J+1)]$ when $J\ne 0$, neglecting the small correction to the electron spin g-factor. For $L=0$ and $J=S=1/2$, it gives $g_J=2$. Different atomic terms therefore split differently in a weak magnetic field.^[ref]

A physicist.^[ref]

An experimental atomic physicist whose work concerns the Zeeman effect and the Paschen-Back effect. At sufficiently strong magnetic fields, magnetic coupling competes with the internal coupling of angular momenta, changing the pattern of spectral lines. His book with Alfred Landé joins this atomic interpretation to the practical apparatus and analysis needed to resolve the splittings.^[ref]

Friedrich Paschen investigated atomic spectra in strong magnetic fields. The Paschen-Back effect, associated with his work with Ernst Back, occurs when the external magnetic interaction competes with internal spin-orbit coupling. The weak-field description through total angular momentum then ceases to provide the simplest labels. Separate orbital angular momentum and spin projections become useful, changing the pattern of spectral lines. Comparing weak- and strong-field patterns tests the hierarchy of interactions within an atom.^[ref]

A physicist.^[ref]