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Photoelectron Spectroscopy

Photoelectron Spectroscopy. Lecture 7 – instrumental details Photon sources Experimental resolution and sensitivity Electron kinetic energy and resolution Electron kinetic energy analyzers. He I  h  = 23.1eV. HV. He I  h  = 21.2 eV. Laboratory Photon Sources.

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Photoelectron Spectroscopy

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  1. Photoelectron Spectroscopy • Lecture 7 – instrumental details • Photon sources • Experimental resolution and sensitivity • Electron kinetic energy and resolution • Electron kinetic energy analyzers

  2. He I h = 23.1eV HV He I h = 21.2 eV 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. HV E = 19.8 eV 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 >1,000

  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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