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Photochemistry
Lecture 8Photodissociation
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Photodissociation ABCD + h AB + CD
Importance Atmospheric and astrophysical environment Primary step in photochemical processes – free
radical production Fundamental studies of dynamics of chemical
reactions
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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
Solar energy converted into thermal energy…heating…temperature inversion.
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Catalytic destruction of ozone 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
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Antarctic ozone hole ClONO2 + HCl Cl2 + HNO3
Hetergeneous catalysis on polar stratospheric clouds
Cl2 + h Cl + Cl Regeneration of ozone destruction mechanism
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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 ……..
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Direct dissociation – excitation into continuum of excited electronic state
Absorption spectrum becomes continuous at sufficiently short wavelength as h crosses a dissociation threshold
Absorption spectrum
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The excited state may correlate to different dissociation limit to ground state
e.g., for BrCl, the first excited state correlates with Br + Cl*
Cl* 2P1/2 state
Cl 2P3/2 state
(energy difference =E, spin-orbit splitting)
Br + Cl
Br + Cl*
E
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Wavefunctions in the continuum
Vertical excitation favoured by Franck-Condon factors
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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
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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.
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Lifetime broadening of predissociating levels
2/ tE 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
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Upper state predissociation evident in linewidths of P and R branch transitions of Se2
P branch
R branch
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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.
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Energy requirements 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
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Localization of excitation The weakest bond is most likely to break - but consider -bromochlorobenzoyl ester
The excitation in the S1 state is localized in the benzene ring, and therefore cannot effectively be transferred into the weakest C-Br bond.
Dissociation depends on suitable pathway on excited state PE surface
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Stabilization of radical products
Propensity to undergo dissociation in a series of compounds may depend on stabilization of radical
e.g., phenyl vs benzyl radical formation
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Cage effect in Solution
h
Escape from cage
geminate recombination
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Classic example – photodissociation of I2 in solution In gas phase, quantum yield
for photodissociation is unity for < 499 nm
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
I2
I + I
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Picosecond flash photolysis on I2 in CCl4
Photodissociate I2 using ps light pulse, detect I atoms with second delayed ps light pulse.
Rapid decay due to geminate recomb.
Longer timescale recombination outside cage
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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
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Measuring the velocities of the products of photo-dissociation by ionization and imaging
Cl2 photolysis image – detect Cl atoms
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Imaging the products of photo- dissociation
Cl2 photolysis image
Perpendicular distance travelled is determined by fragment (Cl) KE
Cl2 + h = Cl + Cl
h-D0 = 2KE(Cl)
Anisotropic image shows propensity for ejection in a specific direction relative to laser polarization.
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Images from the photodissociation of ClO2 – different predissociating levels of excited state populated.
O atom detection - Different rings correspond to vibrational states (v‘) of ClO product
ClO2 ClO2*(v) ClO(v') + O(3P2)
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Femtosecond studies of simple dissociation processes. 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
10 fs pulse has a bandwidth of 500 cm-1
cf typical vibrational frequencies Several vibrational levels excited
simultaneously
2/ tE
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Wavepacket formation 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
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)
......)()( 1100 vv tata
)/exp()( tiEcta iii
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Superposition of many waves of different frequency
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Initially created wavepacket has same shape has ground state wavefunction
Wavepacket evolves with time like a classical particle
predissociation
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Onset of dissociation
Vibrating bound molecules
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Controlling the outcome of dissociative processes in polyatomic molecules Can we use short pulses
(femtosecond) to create a wavepacket that evolves in time such as to cause a particular dissociation process?
We can create variable initial wavepackets by choosing the shape of the light wave pulse.
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Superimposing coherent waves of many different frequencies allows construction of arbitrary light wave forms
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University of Wurzburg
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Computer optimised laser pulse
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Shaped laser pulses for controlling photochemical processes
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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