A New Spin on Photoemission Spectroscopy Page: 83 of 259
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of gas molecules from 3 x 1019 per cm3 down to only 4 x 105. These pressures require the use
of high quality stainless steel chambers with all-metal seals/flanges (usually copper gaskets
and Conflat flanges). Materials for instrumentation internal to the chamber, or anything
else exposed to the vacuum, must be carefully selected to be stable and have minimal vapor
pressures both at room temperature and at standard UHV bakeout temperatures (100-
2000C). Adequate pumping must also be included and often makes up a large part of the
size and expense of an ARPES system. As it can take up to a week to bring a particular
chamber from atmospheric pressure down to UHV, it is also extremely beneficial to have an
"air-lock" for putting samples into and out of the main vacuum without having to "vent"
the entire system. Reliable techniques for moving samples around inside the vacuum system
to the analysis position is also a critical dimension to any ARPES experiment.
Just achieving UHV pressures around the sample to be studied is not enough. As the
sample was likely exposed to atmosphere prior to insertion into the vacuum system, a clean
surface must be somehow freshly prepared in-situ in order to give ARPES data. Different
samples can be cleaned with a wide array of recipes. Many surfaces can be prepared with
high temperature annealing, or a combination of ion-sputtering and annealing. Other types
of crystals can give better surfaces by cleaving the sample in vacuum to expose an entirely
fresh surface. Naturally layered materials, such as the HTC cuprates and CMR manganites,
have natural cleavage planes which allow this technique to work excellently. Still other types
of samples can be "grown" via evaporative techniques, such as molecular-beam epitaxy
(MBE), in-situ to give a clean surface for study. In any case, it is often beneficial to have
dedicated chambers which can be valved off from the main vacuum for surface cleaning and
preparation. Surface characterization tools such as low-energy electron diffraction (LEED)
and Auger electron spectroscopy (AES) can often be helpful (in addition to ARPES itself)
for determining sample quality and inspecting preparation techniques.
The UHV vacuum system for the lab ARPES station was carefully designed and con-
structed in order to meet these requirements. Schematics of the system from the 3D mod-
eling software used for design (CoCreate Onespace) are shown in Figure 2.15. The ARPES
experiment is performed in the large central chamber. Pumping is provided from the large
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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/m1/83/: accessed April 25, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.