A New Spin on Photoemission Spectroscopy Page: 114 of 259
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this part of the spectrum accordingly. We see that due to equations 4.5 and 4.6, it is often
advantageous to work with low kinetic energy photoelectrons, either by using low photon
energy for excitation or by retarding the photoelectrons electrostatically, for optimal energy
resolution.
An important caveat relates to the repetition period of the light source. If the width
of the collected time spectrum is greater that the period between excitation pulses, compli-
cations due to spectrum overlap will occur. In other words, the slowest electrons resulting
from one light pulse will arrive at the detector after the fastest electrons from the next
pulse, which may result in severely distorted spectra. From the arguments above, retarding
a spectra will 'stretch' it out in time, increasing the effective energy resolution. However,
this cannot be done without limit, as it can be stretched to widths greater than the available
time window between light pulses and cause overlapping. We will return to this issue later
when discussing specifics of our TOF design in section 4.4.5.
Now let us return to equation 4.3 to focus on the first term on the right hand side.
This term is due to the fact that perfectly monochromatic photoelectrons will arrive at the
detector at different times if the total length of their flight paths are different. Path length
differences can have several contributions. One is due to finite detector size. In Figure 4.1 a
cone of electrons is eventually detected; paths along the outside of the cone are slightly longer
than those along the center, introducing a path length difference. This can be minimized
by decreasing the size of the detector or otherwise minimizing the acceptance angle (0 in
the figure). This will also improve the effective angular resolution of the experiment, but
will come at the price of decreased count rate. Due to equation 4.3, total experimental
energy resolution is not greatly improved by attempting to decrease the magnitude of the
'path length' term much below the 'timing resolution' term, and so an optimal acceptance
angle for good resolution and count rate can be determined for a set timing resolution. Of
course, desired angular resolution may require further restriction of the acceptance angle.
Photon beam spot size on the sample also contributes to the total AL, and its contri-
bution greatly depends on the experimental geometry. For instance, at normal emission,
the finite size of the photon beam spot does not cause much path length variation to the93
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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/m1/114/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.