A New Spin on Photoemission Spectroscopy Page: 87 of 259
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experiment. In addition to the 0 rotary seal, the cryostat is mounted to the main chamber
on a X-Y-Z large bore single-bellows manipulator (see Figure 2.15) enabling large sample
translations (to travel the long height differences between the analyzer, LEED, and sample
storage locations) in addition to fine sample positioning and alignment during measurement.
2.7.4 Light source
Critical to any photoemission experiment is a high quality light source which must
meet several requirements. First, the light source must provide photons of energy greater
than the sample's work function, or else no electrons can escape into vacuum. Secondly,
the bandwidth of the light source is a contributing factor to the total energy resolution
of the experiment, so a highly monochromatized source is desired. Third, the experiment
count rate will be proportional to the source intensity, and so ARPES greatly benefits from
intense sources. Finally, the beamspot should be as small or smaller than the size of the
sample or domain of good sample surface to ensure that the measured signal is primarily
from the actual sample of interest. Also, the angular resolution of typical analyzers can
be adversely affected by large beamspots. The last three requirements taken all together
can be described as requiring a source of high "brightness". Due to matrix element effects
(section 2.6), it can be very useful to have a tunable source which can have its energy varied
over a wide range of energy.
A helium discharge UV lamp (the SPECS UVS300) is installed on the lab system (see
Figure 2.15) which can provide both He I and He II lines (21.22 eV and 40.81 eV) with
good bandwidths of only - 1 meV. It is also a fairly intense source and is easy to operate.
Unfortunately, however, there are no focusing optics currently. The beam is coupled from
the He plasma to the main chamber via a quartz capillary which acts to guide the photons
to the sample as well as limit the amount of helium getting into the main vacuum. The
beam is divergent out of the capillary, so with the capillary only a couple cm away from the
sample the beamspot is already a few mm in diameter. This limits its usefulness for small
samples, as well as may adversely effect the angular resolution of the electron analyzer.
Besides choosing from the two He lines, the source is not additionally tunable (although the
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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/m1/87/: accessed April 18, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.