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.