A New Spin on Photoemission Spectroscopy Page: 30 of 259
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rotation which could take two values, giving the electron an intrinsic amount of angular
momentum as well as a corresponding magnetic moment, separate from the angular mo-
mentum and magnetic moment due to its orbital motion. As the popular story goes6, after
they gave their 1925 manuscript about the spinning electron to a very supportive Ehrenfest
for publication in Naturwissenschaften,7 Uhlenbeck mentioned the idea of the spinning elec-
tron to Lorentz. Lorentz pointed out that it could not be true, as an electron spinning fast
enough to give the necessary magnetic moment would require the surface of the "classical
radius" of the electron to be moving faster than the speed of light. Uhlenbeck then asked
for Ehrenfest to stop the submission, however he told them it was too late. Apparently,
Kronig had made a similar proposal to Pauli a year earlier, but pushed it no further after
the idea was ridiculed by Pauli.
Uhlenbeck and Goudsmit published a subsequent two page "Letter to the Editor" 8 in
which they used the electron spin to explain the Zeeman splittings, the forbidden lines and
the doublet fine structure of X-ray spectra including their width dependence on nuclear
charge (a Z4) and given quantum numbers. They also managed to include mention that
their spinning electron with two possible values allows the Pauli exclusion principle to
function without assuming "an unmechanical 'duality' in the binding of electrons." 8 The
article appeared in Nature followed by a seeming endorsement by Bohr himself: the electron
spin was officially discovered.
Modern quantum mechanics now describes particles as having, in addition to their
orbital angular momentum (given by equations 1.1 and 1.2), an intrinsic angular momentum,
called spin, governed by similar quantization rules:
ISI = h s(s + 1) and (1.8)
Sz = mh. (1.9)
Similar to the rule for the z-component of angular momentum, m8 can take on values from
-s to s in integer sized steps. However, the rule for possible s values is different from those
for l values. The angular momentum commutation relations in quantum mechanics only
asked: 'Who is that?' He never heard of them, strange. And when he said: 'That means a fourth degree of
freedom.' Then I asked him: 'What is a degree of freedom?' "6
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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/m1/30/: accessed April 20, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.