A New Spin on Photoemission Spectroscopy Page: 57 of 259
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spin-orbit split 4f 5/2 and 7/2 core levels. Remarkably, Siegbahn's work was also able to
resolve the core-levels well enough to observe their shifts in binding energy depending on
the chemical environment of of the originating atom.40 These so-called "chemical shifts"
are are nicely illustrated in Figure 2.5(b), one example of the many works to be based on
this technique. The upper panel shows the C is peak split into two peaks corresponding to
carbon atoms with the different bonding geometries of the carboxylic acid molecule being
measured. Note that the peak ratios match the numerical ratios of the different carbon
configurations in the molecule. This same concept is illustrated again in the lower panel
as well for sodium benzoate. Siegbahn's introduction of vastly improved electron energy
analyzers and his group's pioneering work led to the use of XPS as a quantitative chem-
ical analysis technique which they dubbed Electron Spectroscopy for Chemical Analysis
(ESCA). A vast amount of the group's related research is summarized in two books used by
many as standard references.41,42 His work revolutionizing the field of PES was rewarded
with the Nobel prize in 1981.
2.3 Angular resolution
The photoemission-based techniques used by Siegbahn to investigate materials' core
levels can also be used to study the valence band, as suggested in Figure 2.4. This was
first shown by Berglund and Spicer in 1964 when they identified the 3d DOS in Ag and
Cu. 43,44,45 The electronic states in dispersive valence bands are well defined in both energy
and momentum. Because momentum is conserved in optical excitation during the pho-
toemission process, one can consider attempting to measure both the binding energy and
momentum, or k vector, of the electronic states in the valence bands. This is exactly what is
done in an Angle-Resolved Photoemission Spectroscopy (ARPES) experiment, which allows
studying an electronic structure's energy and momentum dependencies.
Although simple in concept, the actual photoemission process is extremely complicated,
and simplifications and assumptions must be made to interpret the data for the experiment
to be worthwhile. The good news is that in a vast number of situations, these simplifications
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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/m1/57/: accessed April 19, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.