photoelectron spectroscopy
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
![Page 1: Photoelectron Spectroscopy](https://reader031.vdocument.in/reader031/viewer/2022032313/56812bc8550346895d901971/html5/thumbnails/1.jpg)
Photoelectron Spectroscopy
• Lecture 7 – instrumental details– Photon sources– Experimental resolution and sensitivity– Electron kinetic energy and resolution– Electron kinetic energy analyzers
![Page 2: Photoelectron Spectroscopy](https://reader031.vdocument.in/reader031/viewer/2022032313/56812bc8550346895d901971/html5/thumbnails/2.jpg)
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
1s
2s
2p
3s
3p
HV
He Ih = 21.2 eV
He Ih = 23.1eV
![Page 3: Photoelectron Spectroscopy](https://reader031.vdocument.in/reader031/viewer/2022032313/56812bc8550346895d901971/html5/thumbnails/3.jpg)
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)
1s
2s
2p
HV
E = 19.8 eV
(C5H5)2Fe
He I PES
He* (23S) PIES
e2g
a1g
e1u
e1g
e2ga1g
e1u
e1g
10 11 12 13
78910IP/eV
Ek/eV
![Page 4: Photoelectron Spectroscopy](https://reader031.vdocument.in/reader031/viewer/2022032313/56812bc8550346895d901971/html5/thumbnails/4.jpg)
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
![Page 5: Photoelectron Spectroscopy](https://reader031.vdocument.in/reader031/viewer/2022032313/56812bc8550346895d901971/html5/thumbnails/5.jpg)
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.
![Page 6: Photoelectron Spectroscopy](https://reader031.vdocument.in/reader031/viewer/2022032313/56812bc8550346895d901971/html5/thumbnails/6.jpg)
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
![Page 7: Photoelectron Spectroscopy](https://reader031.vdocument.in/reader031/viewer/2022032313/56812bc8550346895d901971/html5/thumbnails/7.jpg)
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.
![Page 8: Photoelectron Spectroscopy](https://reader031.vdocument.in/reader031/viewer/2022032313/56812bc8550346895d901971/html5/thumbnails/8.jpg)
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
![Page 9: Photoelectron Spectroscopy](https://reader031.vdocument.in/reader031/viewer/2022032313/56812bc8550346895d901971/html5/thumbnails/9.jpg)
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
![Page 10: Photoelectron Spectroscopy](https://reader031.vdocument.in/reader031/viewer/2022032313/56812bc8550346895d901971/html5/thumbnails/10.jpg)
Throughput of Deflection Analyzers
Analyzer Entrance
steradian: solid angle subtendedby a circular surface
A sphere subtends 4 steradians
![Page 11: Photoelectron Spectroscopy](https://reader031.vdocument.in/reader031/viewer/2022032313/56812bc8550346895d901971/html5/thumbnails/11.jpg)
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
![Page 12: Photoelectron Spectroscopy](https://reader031.vdocument.in/reader031/viewer/2022032313/56812bc8550346895d901971/html5/thumbnails/12.jpg)
• 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
Hemispherical Analyzer with Electron Optics
![Page 13: Photoelectron Spectroscopy](https://reader031.vdocument.in/reader031/viewer/2022032313/56812bc8550346895d901971/html5/thumbnails/13.jpg)
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.