A New Spin on Photoemission Spectroscopy Page: 55 of 259
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reference to set the energy scale to. The width of the edge can also be used as a measure
of the total energy resolution achieved in the experiment.
Second, the photoexcited electrons tend to have a short mean free path while still in
the sample, and can suffer inelastic collisions before leaving through the sample surface.
This can happen multiple times, with a random amount of energy lost each time, before
an electron escapes into vacuum to be measured. Each collision can generate other excited
electrons in the process, which themselves may suffer inelastic collisions before exiting the
sample. This effect creates a cascade of "secondary" electrons, building in intensity towards
lower kinetic energy in the spectrum. This indeed occurs, and causes the exponentially
increasing secondary background in the measured spectrum as shown in the diagram. Note
that a high enough photon energy is used so that the parts of the spectrum of interest are
not clouded over by this background (although a common data analysis procedure can also
be used to remove the "Shirley Background" from angle-integrated experiments36,37) In
particular, the valence band which is of most interest to many in the solid-state community,
is relatively free of this background as there are few electrons with higher energy.
Finally, the kinetic energy is measured with respect to some sort of analyzer, and so
it is the work function of the analyzer, (DA, which appears in the correct application of
equation 2.2, not the sample work function (FS. The Fermi levels are forced to match when
the sample and analyzer are electrically connected, and so the vacuum levels need not be
the same. However, it is DS, which determines the "low-energy cutoff" where electrons can
no longer escape the sample, coinciding with the vacuum level of the sample, Vs. Because
(is > (FA in the particular instance of this diagram, the analyzer can directly measure this
low-energy cutoff at a finite kinetic energy with respect to VA. Note that the final "peak"
in the measured spectrum is simply the low energy cutoff of secondary electrons.
Useful information can only be extracted from the PES experiment depicted in Fig-
ure 2.4 if it is done with a total energy resolution which allows observing the critical fea-
tures, and it must be done with a high enough count rate such that the data can be recorded
in a reasonable amount of time with good signal-to-noise ratios. This is where the work of
Kai Siegbahn played an enormous role. Siegbahn began work in nuclear spectroscopy and
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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/m1/55/: accessed April 19, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.