Photoelectron Spectroscopy

1
Photoelectron Spectroscopy

• Lecture 7 instrumental details
• Photon sources
• Experimental resolution and sensitivity
• Electron kinetic energy and resolution
• Electron kinetic energy analyzers

2
Laboratory Photon Sources

• Gas discharge VUV sources 0.005 eV resolution
  (40 cm-1)
• He I 21.2 eV (most common for UPS)
• He II 40.8 eV
• Ne I 16.7 eV

3p
3s
2p
2s
1s
3
Related (sort of) Metastable Atoms

• Rare gas in high voltage can also form a
  metastable state
• He 23S 19.8 eV, lifetime 10 sec
• M He ? M He e-
• Transition probability depends on spatial overlap
• Penning Ionization Electron Spectroscopy (PIES)
• or Metastable Atom Electron Spectroscopy (MAES)

2p
2s
1s
4
Laboratory Photon Sources

• X-ray guns, 1 eV resolution
• Most used are Mg K? (1253.6 eV) Al K? (1486.6
  eV)
• other sources from 100 8000 eV available

5
Laboratory Photon Sources

• Laser sources, 8 eV max, very high resolution
  and intensity
• pulsed source not continuous flux of photons
• photoelectron spectroscopy of negative ions
• Two or more photon ionization
• Using powerful laser source, even these very low
  probability events can be observed.
• Complete separate field of study is multi-photon
  ionization (MPI) spectroscopy.
• Advantage extremely high resolution.
• We will discuss these in last lecture if we have
  time.

6
Synchrotron Radiation Source

• range of resolutions with various monochromators
• continuous range of photon energies
• additional cross section, resonance, polarization
  information

The Advanced Photon Source, Argonne National Lab
7
Why does the photon source chosen matter?

• We know that we need to select a photon source
  with sufficient energy to cause ionizations of
  interest to occur.
• Choice of photon source sets the kinetic energy
  of the photoelectrons of interest.
• Now we need to consider how to measure the
  kinetic energy of these electrons.

8
Electron Kinetic Energy Analyzers

• A few important concepts
• 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
• electrons with higher kinetic energy are faster
  than electrons with lower kinetic energy

9
Deflection (Electrostatic) Analyzers

• Electrons can be separated, focused by kinetic
  energy using an electric field
• Most common is the hemispherical analyzer
• Resolving power E/?E gt1,000

10
Throughput of Deflection Analyzers
Analyzer Entrance
steradian solid angle subtended by a circular
surface A sphere subtends 4? steradians
11
More about kinetic energy and deflection
analyzers

• Resolving power E/?E
• This means resolution is dependent upon kinetic
  energy
• Scanning through kinetic energy range to collect
  spectrum different working resolutions for
  different portions of the spectrum
• Measured photoelectron count rate (intensity)
• Also dependent upon kinetic energy
• How do get around these difficulties?
• Slow down electrons before they get to analyzer

12
Hemispherical 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
• Gain better resolution, but lose sensitivity

13
Time-of-Flight Analyzers

• Resolving power 100
• Need to have packets of electrons
• Hence useful with lasers low photon energy
  (therefore low kinetic energy), pulsed source

• Magnetic Bottle 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.