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8/8/2019 PES Lecture1 History
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Photoelectron Spectroscopy
Lecture 1: Development of PhotoelectronSpectroscopy
Photoionization Koopmans Theorem
Brief Historical Overview
Current Topics
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Ionization occurs when matter interacts with light ofsufficient energy (Heinrich Hertz, 1886)(Einstein, A. Ann. Phys. Leipzig 1905, 17, 132-148.)
Ehn = electron kinetic energy + electron binding energy
hn
e- e-e-
Photoelectric Effect
Photoelectron spectroscopy uses this phenomenon
to learn about the electronic structure of matter
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General Overview of Spectroscopy
Spectroscopy uses interaction of electromagneticradiation with matter to learn something about thematter.
If electromagnetic radiation present is in resonance withthe energy spacing between different states (electronic,vibrational, rotational, etc) of matter, radiation will beabsorbed and transitions will occur.
The radiation that is transmitted through the sample ismeasured, and spectrum can be reported as eithertransmittance or absorbance of radiation.
Photoelectron spectroscopy is entirely different!
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Photoelectron vs Other
SpectroscopiesOthers Photon must be in
resonance with transition
energy Measure absorbance or
transmittance of photons
Scan photon energies
Photoelectron Photon just needs enough
energy to eject electron
Measure kinetic energy ofejected electrons
Monochromatic photonsource
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Why would a chemist care about
ionizations anyway? Models for description of electronic structure are
typically based on an orbital approximation. Tjalling C. Koopmans, "Ordering ofWave Functions and
Eigenvalues to theI
ndividual Electrons of an Atom."Physica 1933, 1, 104 Koopmans Theorem: The negative of the energy of an
occupied orbital from a theoretical calculation is equal tothe vertical ionization energy due to the removal of anelectron from that orbital.
!=N
i
iJ )(.).(
iiEI JI!
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Ionization is still a transition
between states Initial State: Neutral (or anion)
Final State: Atom/Molecule/Anion after anelectron is removed, plus the ejected electron
M M+ + e-
More on this next time
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Historical Timeline
First spectrophotometer: 1850s
First IR:1880s
First crystallography: 1912 First NMR: 1938
First EPR: 1944
First PES: 1957
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What took so long?
Development of electron kinetic energyanalyzers with sufficient resolution to be useful.
Development of suitable sources of ionizingradiation vacuum UV, soft X-ray
Development of electron detectors
Development of UHV technology
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First Ionization Energies: cesium 3.89 eV (319 nm)ferrocene 7.90 eV (157 nm)
water 12.61 eV (98 nm)
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Kai Seigbahn: Development of
X-ray Photoelectron Spectroscopy
Nobel Prize in Physics 1981(His father, Manne Siegbahn, won the Nobel Prize in Physics in 1924for the development of X-ray spectroscopy)
C. Nordling E. Sokolowski and K. Siegbahn, Phys. Rev. 1957, 105, 1676.
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Electron Spectroscopy for
Chemical Analysis (ESCA)S. Hagstrm, C. Nordling and K. Siegbahn, Phys. Lett. 1964, 9, 235.
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David Turner: Development ofUltraviolet Photoelectron Spectroscopy
D.W. Turner and M.I. Al Jobory, J. Chem. Phys. 1962, 37, 3007
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Current Topics ofInterest:
high resolution
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Current Topics ofInterest:
angular dependence
800 nm
400 nm
CS2 photoelectron images(Abel inverted)
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Current Topics ofInterest:
variable photon studies
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Current Topics ofInterest:
applications to chemical problems
NN
9 8 7 6 5Ionization Energy (eV)
NNMabrMb
Q-
Adiabatic States
=- = (1/2)(=a - =b)
=+ = (1/2)(=a + =b)
Hab =
IE1
IE2
2Hab
IE2
IE2IE1
IE1
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Summary
PES is a fairly new technique, continuingto develop
PES has unique features compared toother spectroscopies
Valence spectroscopy: information onbonding
Core spectroscopy: qualitative andquantitative analysis, chemical shift