Paschen—Back Effect

In 1921, two experimental physicists in Tübingen, Friedrich Paschen (1865–1947) and Ernst Back (1881–1959), observed that with strongly increasing magnetic field strength, the complicated multiplets of the anomalous ►Zeeman effect change into the simpler patterns typical of the normal Zeeman effect (see Fig. 1). Initially, this observation remained inexplicable. With the discovery of ►spin in late 1925, however, and the realization that the anomalous Zeeman effect is characteristic of systems with spin S >0, whereas the normal Zeeman effect governs atoms with a total S = 0, the Paschen–Back effect could be understood as a decoupling of S and orbital angular momentum L, since the influence of the total spin becomes neglectable for diminishing spin-orbit coupling. (See also ►Russell–Saunders coupling, ►jj-coupling, Stern–Gerlach experiment and ►vector model).