Photochemistry

Lecture 8Photodissociation

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

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.

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

Antarctic ozone hole ClONO2 + HCl Cl2 + HNO3

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 becomes continuous at sufficiently short wavelength as h crosses a dissociation threshold

Absorption spectrum

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

Wavefunctions in the continuum

Vertical excitation favoured by Franck-Condon 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

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

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 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

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

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

Cage effect in Solution

h

Escape from cage

geminate recombination

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

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

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 photo-dissociation by ionization and imaging

Cl2 photolysis image – detect Cl atoms

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.

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)

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

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

Superposition of many waves of different frequency

Initially created wavepacket has same shape has ground state wavefunction

Wavepacket evolves with time like a classical particle

predissociation

Onset of dissociation

Vibrating bound molecules

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.

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