Photochemistry

1
Photochemistry

• Lecture 7
• Photoionization and photoelectron spectroscopy

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Hierarchy of molecular electronic states
Ionic excited states
Ionic ground state (ionization limit)
Neutral Rydberg states
Excited states (S1 etc)
Neutral Ground state
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Photoionization processes

• Photoionization
• AB h? ? AB e-
• Dissociative photoionization
• AB h? ? A B e-
• Autoionization
• AB h? ? AB (E gt I) ? AB e-
• Field ionization
• AB h? ? AB (E lt I) ?apply field ? AB e-
• Double ionization
• AB h? ? AB2 2e- ? A B
• AB h? ? (AB) e-(1) ? AB2 e-(2)
• ? A B
• Rule of thumb 2nd IP ? 2.6 x 1st IP
• Vacuum ultraviolet ? lt 190 nm or E gt 6 eV

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Importance of molecular ion gas phase chemistry

• In Upper atmosphere and astrophysical
  environment, molecules subject to short
  wavelength radiation from sun, gamma rays etc.
• No protection from e.g., ozone layer
• Most species exist in the ionized state
  (ionosphere)
• e.g., in atmosphere
• N2 h? ? N2 e-
• N2 O ? N NO .
• NO e- ? N O (dissociative recombination)
• In interstellar gas clouds
• H2 H2 ? H3 H
• H3 C ? CH H2
• CH H2 ? CH2 H

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Ion density in the ionosphere (E,F regions)
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Selection rules (or propensity rules) for single
photoionization

• Any electronic state of the cation can be
  produced in principle if it can be accessed by
  removal of one electron from the neutral without
  further electron rearrangement
• - at least, there is a strong propensity in
  favour of such transitions
• e.g., for N2
• N2(?u2?u4?g2) ? N2(?u2?u4?g1) e- 2?g
• N2(?u2?u4?g2) ? N2(?u2?u3?g2) e-
  2?u
• N2(?u2?u4?g2) ? N2(?u1?u4?g2) e- 2?u
• There is no resonant condition for h? because the
  energy of the outgoing electron is not quantised
  (free electron)

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Conservation of energy in photoionization

• AB h? ? AB e-
• h? I Eion KE(e-) KE(AB)
• I adiabatic ionization energy (energy required
  to produce ion with no internal energy and an
  electron with zero kinetic energy)
• Eion is the internal energy of the cation
  (electronic, vibrational, rotational..)
• KE(e-) is the kinetic energy of the free electron
  
• KE(AB) is the kinetic energy of the ion (usually
  assumed to be negligible)
• Thus KE(e-) ? h? - I - Eion

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• AB h? ? AB e-
• KE(e-) ? h? - I - Eion
• The greater the internal energy of the ion that
  is formed, the lower the kinetic energy of the
  photoelectron.
• This simple law forms the basis of photoelectron
  spectroscopy

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Photoelectron spectroscopy

• Ionization of a sample of molecules with h? I
  will produce ions with a distribution of
  internal energies (no resonant condition)
• Thus the electrons ejected will have a range of
  kinetic energies such that
• KE(e-) ? h? - I Eion
• Typically use h? 21.22 eV (He I line
  discharge lamp)
• or h? 40.81 eV (He II)
• For most molecules I ? 10 eV (1 eV 8065 cm-1)

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Photoelectron spectroscopy

• KE(e-) ? h? - I - Eion

KE(e-)
Eion
h?
Measuring the spectrum of photoelectron
energies provides a map of the quantised energy
states of the molecular ion
I
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PES - experimental
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PES of H2 molecule

• H2 has only one accessible electronic state
  H2(?g2) h? ? H2(?g) e- 2?g
• But for h? 21.2 eV, and I 15.4 eV the ions
  could be produced with up to 5.8 eV of internal
  energy in this case vibrational energy
• Peaks map out the vibrational energy levels of
  H2 up to its dissociation limit

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PES of H2
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Franck Condon Principle

• Large change of bond length on reducing bond
  order from 1 to 0.5.
• Franck Condon overlap favours production of ions
  in excited vibrational levels.

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PES of nitrogen

• I 15.6 eV, h? 21.2 eV
• Three main features represent different
  electronic states of ion that are formed
• Sub structure of each band represents the
  vibrational energy levels of each electronic
  state of the ion

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N2(?u2?u4?g2) ? N2(?u2?u4?g1) e- 2?g
N2(?u2?u4?g2) ? N2(?u2?u3?g2) e- 2?u
N2(?u2?u4?g2) ? N2(?u1?u4?g2) e- 2?u
2?g
2?u
2?u
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Koopmans Theorem

• Recognise that each major feature in PES of N2
  results from removal of electron from a different
  orbital.
• More energy required to remove electron from
  lower lying orbital (because this results in a
  higher energy molecular ion)
• If the orbitals and their energies do not relax
  on photoionization then
• I Eion -? (orbital energy)
• But in practise remaining electrons reorganise to
  lower the energy of the molecular ion that is
  produced hence this relationship is approximate

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PES of oxygen

• Removal of electron from ?u orbital of ?u4?g2
  configuration leads to two possible electronic
  states
• ?u3?g2 three unpaired electrons give either 2?u
  or 4?u states
• Breakdown of Koopmans theorem (no one-to-one
  correspondence between orbitals and PES bands)

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PES of O2 (First band not shown)
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PES of HBr reveals spin-orbit coupling splitting
as well as vibrational structure
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PES of polyatomic molecules

• Vibrational structure depends on change of
  geometry between neutral and ion
• e.g., ammonia neutral is pyramidal, ion is
  planar
• Long progression in umbrella bending mode

If many modes can be excited than spectrum may be
too congested to resolve vibrational structure
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High resolution photoelectron spectroscopy ZEKE
spectroscopy

• KE(e-) ? h? - I - Eion
• Instead of using fixed h? and measuring variable
  KE(e-), use tuneable h? and measure electrons
  with fixed (zero) kinetic energy
• Each time h? I Eion the ZEKE (zero kinetic
  energy) electrons are produced this only occurs
  at certain resonant frequencies.

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ZEKE Photoelectron spectroscopy

• KE(e-) ? h? - I - Eion

KE(e-)
Zero KE electron
Eion
h?
Measuring the production of zero KE electrons
(only) versus photon wavelength h? IEion
I
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Resolved rotational structure in ZEKE PES of N2
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ZEKE spectrum of N2 predominant ?J2

• Note that the outgoing electron can have angular
  momentum even though it is a free electron
• Thus change of rotational angular momentum of
  molecule on ionization may be greater than ? 1,
  providing
• Note the above formula ignores electron spin

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ZEKE spectroscopy

• The best resolution for this method is far
  superior to conventional PES (world record ? 0.01
  meV versus typical 10 meV for conventional PES)
• Thus resolution of rotational structure, or of
  congested vibrational structure in larger
  polyatomic molecules, is possible.
• Gives rotational constants of cations hence
  structural information e.g., CH4, O3 CH2,
  C6H6, NH4 (direct spectroscopy on ions
  difficult)
• In practise can only be applied in gas phase
  (unlike conventional PES- solids, liquids and
  surfaces).

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Vibrational structure in H bonded complex of
phenol and methanol
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Time resolved photoelectron spectroscopy
Photoelectron spectrum of excited states Use
two lasers one to excite molecule to e.g., S1
state, and one to induce ionization from that
state.
The photoelectron spectrum thus recorded reflects
orbital configuration of S1 state.
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Time resolved photoelectron spectroscopy
Dark state
S1
If ISC takes place from intermediate then
photoelectron spectrum may show excitation from
both initially excited (bright) S1 and T1
(dark) state.
Pump-probe photoelectron experiment (cf flash
photolysis) on fluorene delay ionizing light
pulse with respect to excitation
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Preparing molecular ions in known energy states
photoelectron-ion coincidence

• KE(e-) ? h? - I - Eion
• If the ionization events happen one at a time, we
  can determine internal energy of each ion that is
  produced by measuring the kinetic energy of the
  corresponding electron. If the ion subsequently
  fragments, we can investigate how fragmentation
  depends on initial state of the ion populated.

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PEPICO (photoelectron-photoion coincidence
apparatus)
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PEPICO spectrum of HNCOphyschem.ox.ac.uk/jhde
IE
MASS