Instrumentation for

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Instrumentation for Photoelectron Spectroscopy
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Overall Experimental Resolution
1 eV 8066 cm-1 23.06 kcal/mol 96.48 kJ/mol
0.037 au
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Samples

• Gas-Phase Photoelectron Spectroscopy
• Atoms, Neutral Molecules, Anions, Clusters,
  etc.
• Molecular beam allows cooling of sample by
  super-sonic jet expansion
• For neutral molecules, need a vapor pressure of
  10-4 torr in high vacuum at temperatures lt 500
  C
• Condensed-Phase Photoelectron Spectroscopy
• Film on conductive surface
• For valence spectroscopy, need uniform film
• (vapor deposition, SAMs, spin coating)

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Photon Sources
Laboratory gas discharge sources, 0.005 eV
resolution (40 cm-1) He I - 21.2 eV energy
covers important valence structure most common
for UPS He II 40.8 eV energy covers deeper
valence region valuable in comparison to He I
for changing probabilities (cross sections) of
ionizations. Ne I 16.7 eV energy good for low
valence region Laboratory X-ray sources, 1 eV
resolution Mg K? 1253.6 eV energy allows
ionization of core electrons Al K? 1486.6 eV
monochromator increases resolution other
sources from 100 8000 eV available Laser
sources, 8 eV max energy, very high resolution
and intensity photoelectron spectroscopy of
negative ions multiphoton ionization
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Synchrotron Photon Sources
range of resolutions with monochromators continuou
s range of photon energies additional cross
section, resonance, polarization information
The Advanced Photon Source, Argonne National Lab
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Electron Kinetic Energy Analyzers
Throughput What of photoelectrons produced are
detected Resolution How close in kinetic
energy can two electrons be, and still be
separated by the analyzer Resolving Power
E/?E higher kinetic energy, lower
resolution Time of Flight Analyzers Resolving
power 100 Deflection Analyzers Resolving
power gt1,000
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Deflection Analyzer with Electron Optics
Rather than scanning through electron kinetic
energies with a deflection analyzer Use an
electron-optics lens to slow electrons to a pass
energy
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Analyzer Throughput
Analyzer Entrance
steradian solid angle subtended by a circular
surface A sphere subtends 4? steradians
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TOF Magnetic Bottle Spectrometer
Magnetic field in ionization region allows a
large solid angle of photoelectrons to be
collected, increasing spectrometer
sensitivity. In principle, 2? steradians of
photoelectrons can be collected.
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Electron Detectors
Channel Electron Multiplier
Microchannel Plate
CCD detectors can also be used, typically with a
phosphor screen to convert electrons to photons
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Vacuum Pumps

• Low pressure is required for operation of
  electron detectors
• Pressure must be low enough to allow
  mean-free-path of electrons through the analyzer
• Pressure must be low enough that gas-phase
  samples are volatile
• Ultra-high vacuum is required to lower surface
  contamination for condensed-phase spectroscopy

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How Low Must Pressure be for a Surface to be
Clean?
If sticking coefficient S 1 And pressure
2.5 x 10-6 Torr A monolayer will form in 1 second
Lower pressure to 10-9 Torr A monolayer forms
in 1,000 seconds S is usually ltlt1
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Gas-Phase Photoelectron Instrument
h?

Sample Chamber
Photon Source
Photon Induces the Photoelectric Effect.
Electrons are separated according to kinetic
energy
Output is plot of Ionization Energy vs. Counts

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Surface Photoelectron Instrument
Sample Prep and Storage
Fast Entry
Main Analysis
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Mass-Selected Gas-Phase Photoelectron Instrument
MCP (mass detector)
Time-of-flight mass spectrometer
magnet
2.5 m
5x10-11 Torr
Time-of-flight magnetic botter photoelectron spect
rometer
4.0 m
Electrospray Ionization source
Z-stack MCP (photoelectron detector)