One hundred years ago Alfred Landé unriddled the Anomalous Zeeman Effect and presaged electron spin

Abstract

In order to commemorate Alfred Landé’s unriddling of the anomalous Zeeman Effect a century ago, we reconstruct his seminal contribution to atomic physics in light of the atomic models available at the time. Landé recognized that the coupling of quantized electronic angular momenta via their vector addition within an atom was the origin of all the apparent mysteries of atomic structure as manifested by the anomalous Zeeman effect. We show to which extent Landé’s ideas influenced the development of quantum physics, particularly Wolfgang Pauli’s path to the exclusion principle. We conclude with Landé’s brief biography.

A century ago the Stern-Gerlach experiment ruled unequivocally in favor of Quantum Mechanics

“…In modern notation, the ground state of each atom is characterized by the term symbol 2S+1 LJ . Note that the maximum expected deflection is proportional to the magnetic dipole moment µZ = −JgµB, where g is the g-factor [36,44,45] Perhaps the most striking result Gerlach obtained with his advanced SGE-type apparatus was the deflection pattern for a beam of nickel atoms [25], see FIG. 13.…”

“…Not everyone was unfazed by the outcome of the SGE. Stern's and Gerlach's Frankfurt colleague, Alfred Landé, who had worked on unriddling the anomalous Zeeman effect since 1919, provided a prescient interpretation of the SGE based on his understanding of the electronic structure of atoms [44,45]. Upon his entry into the fray, Landé gradually modified the sets of quantum numbers introduced by Bohr, Sommerfeld, and Debye to characterize atomic states.…”

A Century Ago the Stern–Gerlach Experiment Ruled Unequivocally in Favor of Quantum Mechanics

“…Stern's and Gerlach's Frankfurt colleague, Alfred Landé, who had worked on unriddling the anomalous Zeeman effect since 1919, provided a prescient interpretation of the SGE based on his understanding of the electronic structure of atoms. [43,44] Upon his entry into the fray, Landé gradually modified the sets of quantum numbers introduced by Bohr, Sommerfeld, and Debye to characterize atomic states. By adapting the concept of vector addition of angular momenta to the case of quantized electronic angular momenta of atoms, Landé came up with an organizing principle that made it possible to capture both the patterns and the subtleties of atomic Zeeman spectra amassed on the eve of the discovery of electron spin in 1925.…”

“…[25] In modern notation, the ground state of each atom is characterized by the term symbol 2S + 1 L J . Note that the maximum expected deflection is proportional to the magnetic dipole moment μ Z = À Jgμ B , where g is the g-factor, [36,44,45] J the total angular momentum quantum number, and μ B the Bohr magneton. See text.…”