A New Spin on Photoemission Spectroscopy Page: 29 of 259
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While the initial interpretation of the results was correct in that it showed space quanti-
zation, it was incorrect about the source of the angular momentum. In fact, the splitting
was due to an angular momentum (and magnetic moment) intrinsic to the electron itself,
distinct from that due to its orbital dynamics, called spin. The beam splitting into exactly
two parts clearly illustrates the binary nature of the electron spin. As one of the canonical
experiments in quantum physics, the SGE is frequently cited for its fundamental signifi-
cance as evidence of the electron spin as well as its simplicity and elegance. In fact, it is
usually used as an introduction to quantum mechanics and a conceptual vehicle for assorted
concepts in most quantum mechanics textbooks. In light of this, it is interesting to note
that the SGE was originally interpreted without spin; spin was not "discovered" until 4
1.2.2 Discovery - Uhlenbeck and Goudsmit
Despite the seeming agreement between the Sommerfeld-Bohr model of angular mo-
mentum and the SGE, there were some inconsistencies with carefully scrutinized atomic
emission/absorption spectra, including doublets, lines due to transitions forbidden in the
Bohr model, and complex multiplet splittings with the application of a magnetic field (the
"anomalous" Zeeman effect). A student of Paul Ehrenfest, Samuel Abraham Goudsmit
spent much time working with Paschen learning the details of spectral analysis and the
observed discrepancies. Intrigued by the forbidden lines, he recalled asking theoreticians
about the observations, "But you know how theoreticians are ... then they say: 'Poor ex-
periments.' "6 He and a new student to the group, George Uhlenbeck, continued studying
the spectrum, and worked with formula using half-integer quantum numbers to calculate
the line locations (Lande's interval rule). When Goudsmit showed Uhlenbeck that he could
give a simple interpretation of the recently published Pauli exclusion principle by introduc-
ing a new binary quantum number, Uhlenbeck recognized that it implied an extra degree
of freedom intrinsic to the electron.t They interpreted the extra degree of freedom as a
tIn an amusing quote meant to capture the differences between the two physicists (differences which
likely helped them make such an important breakthrough), Goudsmit describes, "In those days, all through
the summer when I told Uhlenbeck about Land6 and Heisenberg, for instance, or about Paschen, then he
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Jozwiak, Chris. A New Spin on Photoemission Spectroscopy, thesis or dissertation, December 1, 2008; United States. (https://digital.library.unt.edu/ark:/67531/metadc1014237/m1/29/: accessed April 25, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.