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Photoelectron Spectroscopy • Lecture 7 – instrumental details – Photon sources – Experimental resolution and sensitivity – Electron kinetic energy and resolution – Electron kinetic energy analyzers 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 He I h = 23.1eV 3p 3s HV 2p 2s He I h = 21.2 eV 1s 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) (C5H5)2Fe e1u He* (23S) PIES HV e1g a1g 10 11 13 12 e1u e1g e2g a1g 2p 2s Ek/eV E = 19.8 eV He I PES e2g 1s 10 9 8 IP/eV 7 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 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. 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 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. 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 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 Throughput of Deflection Analyzers Analyzer Entrance steradian: solid angle subtended by a circular surface A sphere subtends 4 steradians 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 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 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.