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Photochemistry
Lecture 8
Photodissociation
Photodissociation

ABCD + h  AB + CD

Importance
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Atmospheric and astrophysical environment
Primary step in photochemical processes – free
radical production
Fundamental studies of dynamics of chemical
reactions
Atmospheric Chemistry – the ozone
hole
In the stratosphere, ozone protects the
earth from damaging UV radiation via the
Chapman cycle
 O2 + h → O + O ( < 242 nm)
 O3 + h → O2 + O ( < 1180 nm)
 O + O2 + M  O3 + M
 O + O3 O2 + O2

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Solar energy converted into thermal
energy…heating…temperature inversion.
Catalytic destruction of ozone
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e.g., CF2Cl2 + h  CF2Cl + Cl
Cl + O3  ClO + O2
ClO + O  Cl + O2
Formation of reservoir species
e.g., Cl + CH4  CH3 + HCl
ClO + NO2 + M  ClONO2 + M
Antarctic ozone hole
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ClONO2 + HCl  Cl2 + HNO3
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Hetergeneous catalysis on polar stratospheric clouds
Cl2 + h  Cl + Cl

Regeneration of ozone destruction mechanism
Smog formation
Production of OH radical in troposphere
via sequence…
 NO2 + h  NO + O
 O(1D) + H2O  OH + OH

Oxidation of hydrocarbons (with
regeneration of OH and NO2
 OH + RCH3  RCH2 + H2O
 ……+ O2  RCH2O2 ……..

Direct dissociation – excitation into continuum
of excited electronic state

Absorption
spectrum
Absorption spectrum
becomes continuous at
sufficiently short
wavelength as h
crosses a dissociation
threshold
The excited state may correlate to different
dissociation limit to ground state
Br + Cl*
E
e.g., for BrCl, the first
excited state correlates
with Br + Cl*
Br + Cl
Cl*  2P1/2 state
Cl  2P3/2 state
(energy difference =E,
spin-orbit splitting)
Wavefunctions in the continuum
Vertical excitation
favoured by FranckCondon factors
Simple photodissociation within a single
electronic state is essentially forbidden
This could be considered as the
extreme limit of vibrational
overtone excitation; v very
large
Predissociation
Molecule excited to bound
state – vibrates for
perhaps a few periods
then undergoes curve
crossing and dissociates
on repulsive PE curve
Franck Condon factor for
excitation determined by
overlap with bound state
wavefn as before.
Lifetime broadening of predissociating
levels
Et   / 2
Sometimes known as the
time-energy uncertainty
relationship
In this context:
t  lifetime of excited state
E  “homogeneous”
linewidth of transition
5 ps  1 cm-1 linewidth
Upper state predissociation evident in linewidths
of P and R branch transitions of Se2
P branch
R branch
Photodissociation of polyatomic
molecules

Potentially more than one product channel for
sufficiently high photolysis energy
e.g., formaldehyde CH2=O + h
 H + HCO
 H2 + CO
Latter requires rearrangement via 3-membered ring
transition state

Should generally consider dissociation in
polyatomics as occurring via a form of
predissociation…..energy transfer from initially
excited state to a dissociative state.
Energy requirements
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State in which excited
molecule resides must be
higher than dissociation
energy
For the halonaphthalenes
X-Np
1-I-Np can dissociate from T1
1-Br-Np only dissociates from S1
1-Cl-Np does not dissociate
D0
Localization of excitation
The weakest bond is most likely to break
- but consider -bromochlorobenzoyl ester
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The excitation in the S1 state is localized in the benzene
ring, and therefore cannot effectively be transferred into
the weakest C-Br bond.
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Dissociation depends on suitable pathway on excited
state PE surface
Stabilization of radical products
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Propensity to undergo dissociation in a series
of compounds may depend on stabilization of
radical
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e.g., phenyl vs benzyl radical formation
Cage effect in Solution
h
geminate
recombination
Escape from
cage
Classic example – photodissociation of
I2 in solution
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In gas phase, quantum
yield for photodissociation
is unity for  < 499 nm
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In CCl4,
 = 0.66 at 435.8nm
 = 0.83 at 404.7nm

As excess kinetic energy
of I fragments increases,
becomes easier to break
out of the solvent case
I+I
I2
Picosecond flash photolysis on I2 in
CCl4
Rapid decay due to
geminate recomb.
Longer
timescale
recombination
outside cage
Photodissociate I2 using ps light pulse, detect I atoms
with second delayed ps light pulse.
Conservation of energy in gas-phase
photodissociation (cf photoelectron spectroscopy)
ABCD  AB + CD
E(ABCD) + h = D0 + Eint(AB) + Eint(CD) + KE(AB)
+ KE(CD)
Eint is the vibration-rotation (electronic) energy of
fragments – in solution this would be rapidly
degraded by collisional vibrational relaxation
KE(AB) related to KE(CD) by momentum
conservation
Measure kinetic energy and internal energy of one
product AB or CD – can figure out other
unknowns (D0 and Eint)
Use multiphoton ionization and ion imaging to make
these measurements
Measuring the velocities of the products of photodissociation by ionization and imaging
Cl2 photolysis
image – detect Cl
atoms
Imaging the products of photodissociation
Cl2 + h = Cl + Cl
Perpendicular distance
travelled is determined
by fragment (Cl) KE
h-D0 = 2KE(Cl)
Anisotropic image
shows propensity for
ejection in a specific
direction relative to
laser polarization.
Cl2 photolysis image
Images from the photodissociation of ClO2 – different
predissociating levels of excited state populated.
ClO2  ClO2*(v)  ClO(v') + O(3P2)
O atom detection - Different rings correspond to
vibrational states (v‘) of ClO product
Femtosecond studies of simple
dissociation processes.
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Pulses of light as short as a few fs (10-15s)
routinely created with certain types of laser
Frequency bandwidth of pulse broadens as
pulse duration shortens
Et   / 2
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10 fs pulse has a bandwidth of  500 cm-1
cf typical vibrational frequencies
Several vibrational levels excited
simultaneously
Wavepacket formation
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Excite molecule with femtosecond laser pulse- frequency
bandwidth overlaps transitions to several vibrational states
Produce a vibrational wavefunction which is a
superposition of many vibrational states
  a0 (t ) v0  a1 (t ) v1  ......
Can form a localised wavepacket through interference
between these waves
Not an eigenstate thus coefficients evolve with time; this
becomes equivalent to the wavepacket moving like a
classical particle (but also spreading in a non classical
fashion)
ai (t )  ci exp(iEit / )
Superposition of
many waves of
different frequency
Wavepacket evolves
with time like a
classical particle
predissociation
Initially created
wavepacket has same
shape has ground
state wavefunction
Onset of
dissociation
Vibrating
bound
molecules
Controlling the outcome of dissociative
processes in polyatomic molecules
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Can we use short pulses
(femtosecond) to create a
wavepacket that evolves in
time such as to cause a
particular dissociation
process?
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We can create variable
initial wavepackets by
choosing the shape of the
light wave pulse.
Superimposing coherent waves of many different
frequencies allows construction of arbitrary light wave
forms
University of Wurzburg
Computer optimised laser pulse
Shaped laser pulses for controlling
photochemical processes
Adaptive control of CpFe(CO)2X
fragmentation (X=Cl, Br,I)

CpFe(CO)2X  CpFe(CO)X + CO
 CpFeX + 2CO
 FeX + 2CO +Cp
Cp = cyclopentadienyl
Optimise laser pulse shape to maximise
yield of e.g., CpFe(CO)X; factor of 2
improvement in CpFe(CO)X to FeX ratio