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Photoelectron Spectroscopy

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Photoelectron Spectroscopy Lecture 7 instrumental details Photon sources Experimental resolution and sensitivity Electron kinetic energy and resolution – PowerPoint PPT presentation

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Title: 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.
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