A New Spin on Photoemission Spectroscopy Page: 82 of 259
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2.7.2 Ultra-high vacuum system
One of the major requirements of a high-quality photoemission experiment is vacuum.
There are three reasons that photoemission experiments must, in general, be performed in
a vacuum chamber. The first reason is because of the macroscopic distances (~ 1m) that
photoelectrons must travel from the sample to the ultimate detector. The mean free path of
free electrons in atmosphere is only about 150 nm, so they will be scattered by gas molecules
long before reaching the detector. At pressures around 1 x 10-6 torr, the mean free path
of the photoelectrons increases to over 100 m, so the vast majority of the emitted electrons
will reach the detector without any trouble. The second reason involves modern electron
detection devices which are typically "channeltron" or "multichannel plate" (MCP) based
detectors. Coincidentally, these detectors often can only be operated in vacuum of around
1 x 10-6 torr or better, as well. The third vacuum requirement is by far the most stringent
and involves the sample surface.
We saw in section 2.3 that the low inelastic mean free paths of the photoelectrons within
most materials is so low that ARPES has a very shallow probing depth. This translates
to requiring the sample surface to be high quality with minimal foreign overlayers. If the
sample is held in a vacuum of 1 x 10-6 torr of gas molecules which have a sticking coefficient
of 1 on the sample surface (every molecule which strikes the surface adsorbs, without any
subsequent desorption), a full monolayer of the gas will form in only 1 second. After this
1 second, the majority of the ARPES signal will already be from the overlayer rather than
the sample itself. Therefore, it is best to have the sample exposed to as low a residual gas
pressure as possible, typically well into the ultra-high vacuum range (UHV) with pressures
in the low 10-11 torr region. This typically requires the entire experiment (sample, sample
manipulator, analyzer, excitation source, etc) to be contained within a vacuum chamber
system capable of these pressures. In fact, meeting the strict vacuum requirements can form
the bulk of the practical work of an ARPES experiment.
Although it is easy to write, UHV is not always so easy to achieve. A pressure of 10-11
torr is almost 14 orders of magnitude lower pressure than atmosphere, bringing the density
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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/m1/82/: accessed April 24, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.