A New Spin on Photoemission Spectroscopy Page: 24 of 259
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1.2 Spin - the ups and downs of being an electron
It is probably not possible to overestimate the breadth of consequences the electron
spin, or intrinsic angular momentum, has on our daily lives. This is surprising as there
are only ever two possibilities: an electron must be either "spin-up" or "spin-down", with
no in-betweens. With only two possible states, electron spin is the simplest degree of
freedom possible, and yet, along with certain rules which Nature follows with respect to
spin, the electron's spin makes possible the structure of the Periodic Table. It is also a key
component of magnetism in matter which allows a impressive array of daily technologies
including electric motors, generators, the hard-drives which house all the backup copies of
this thesis, and even the reliable refrigerator magnet. Therefore it is interesting and well
worthwhile to review the basics of electron spin physics.
1.2.1 Almost discovery - the Stern-Gerlach experiment
The binary nature of electron spin is rooted in the basics of quantum mechanics, so it is
not surprising that the history of its discovery is thoroughly entangled with the turbulent
evolution of early quantum theory. One of the pillars of quantum physics is the quantization
of a particle's possible angular momentum into discreet units, nowadays expressed as:
I L = _ l(l + ) (1.1)
where l is constrained to be a non-negative integer. An additional rule referred to as "space
quantization" requires the component of the angular momentum vector along any particular
axis to fall in discreet units of the magnitude of Planck's constant, h = 1.055 x 10-34J .
Lz = mL (1.2)
where mL can be an integer from 1 to -l. This holds for any axis chosen; the z-axis is not
particularly special, and equation 1.2 is just as valid written for L, or Ly. Often times
the system under consideration provides a natural axis to reference direction to, such as
an applied magnetic field vector, where the physical interaction is given by the component
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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/m1/24/: accessed April 25, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.