Photoelectron Spectroscopy

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Photoelectron Spectroscopy

• Lecture 8 probability of photoionization
• Cross-sections
• Gelius model
• Asymmetry parameters

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Ionization is still a transition between states

• Initial State Neutral (or anion)
• Final State Atom/Molecule/Anion after an
  electron is removed, plus the ejected electron
• M ? M e-
• ?init M ?final M e-
• Transition Probability ? ?init m ?final d?
• For direct photoionization, transition
  probability is always gt 0
• Photoionization probability is typically
  described in terms of a cross-section (much more
  on this later)

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Photoionization Cross-section

• The probability per unit area, per unit time that
  a photon of a given energy can be absorbed by an
  atom to excite the photoelectrons.
• Fictitious area representing the fraction of
  incoming photons that will be absorbed in the
  photoionization process.
• Unit barn (10-24 cm2)
• or megabarn (10-18 cm2)

photons absorbed per unit time
cross-section
incident photon flux
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Partial Photoionization Cross-sections

• If more than one orbital level is excited, then
  the cross-section becomes the summation of
  partial photoionization cross-sections (PPCS)
• PPCSs are a function of the photoelectrons
  kinetic energies, and therefore are a function of
  the incident photon energies.
• When PPCS is measured at a specific angle, it is
  called a differential cross-section, ds/d?, which
  is related to snl by an asymmetry parameter,
  ?(h?)
• This specific equation is for a randomly-oriented
  ensemble of atoms in an unpolarized field.

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Calculated atomic orbital ionization
cross-sections
J.J. Yeh and I. Lindau, At. Data Nucl. Data
Tables 1985, 21, 1.
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So far weve only talked about atoms what about
molecules?

• Gelius model cross-section behavior of
  molecular orbitals is dependent on the atomic
  orbital character from which the MO is comprised.
• (Gelius and Siegbahn, Faraday Discuss. Chem.
  Soc., 1972, 257.)

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Variable photon synchrotron studies Green has
collected data on ferrocene at 25 different
photon energies from 20-120 eV.
Branching Ratios Ratio of band intensities as
a function of photon energy.
Photoionization Cross-Sections A Guide to
Electronic Structure, J. C. Green and P.
Decleva, Coord. Chem. Rev. 2005, 249, 209-228.
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Electronic Structure of Ferrocene
What is the point group?
Looking down the Cp-Fe-Cp vector Cps can lie in
two extreme conformations
staggered
eclipsed
D5h
D5d
d orbitals transform as a1, e1, e2
d orbitals transform as a1g, e1g, e2g
Were going to use these labels
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Metallocene Ligand ? Group Orbitals
(Cp)22- (D5d)
Cp- (D5h)
e "
e2g
2
e2u
e "
1
e1g
e1u
a1g
a2u
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(Cp)22-
M2
(Cp)2M
Cp-
e1u
a2u
4p
e2g
(a2ue1u)
e2u
a1g
e
"
4s
2
e2u
e2g
a1g
e1g
?
3d
e2g
e1g
(a1ge1g e2g)
a1g
e
"
1
e1u
e1u
e1g
a2u
a
"
a2u
2
a1g
a1g
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Asymmetry Parameters
? 90

2
? 54.73

1

0

-1
? 0
? 180
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Photoelectron Imaging