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).
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Literature
F. Paschen, E. Back: Normale und anomale Zeemaneffekte. Annalen der Physik (4th ser.), 39, 897–932; 40, 960–970 (1912)
F. Paschen, E. Back: Liniengruppen magnetisch vervollständigt. Physica 1, 261–273 (1921)
G. Herzberg: Atomic Spectra and Atomic Structure (Dover, New York 1944, 112ff)
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Hentschel, K. (2009). Paschen—Back Effect. In: Greenberger, D., Hentschel, K., Weinert, F. (eds) Compendium of Quantum Physics. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-540-70626-7_140
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