Photochemistry Lecture 8 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  O 2 + h → O + O( < 242 nm)  O 3 + h → O 2 + O( < 1180 nm)  O + O 2 + M  O 3 + M  O + O 3  O 2 + O 2  Solar energy converted into thermal energy…heating…temperature inversion.

Catalytic destruction of ozone  e.g., CF 2 Cl 2 + h  CF 2 Cl + Cl Cl + O 3  ClO + O 2 ClO + O  Cl + O 2 Formation of reservoir species e.g., Cl + CH 4  CH 3 + HCl ClO + NO 2 + M  ClONO 2 + M

Antarctic ozone hole  ClONO 2 + HCl  Cl 2 + HNO 3 Hetergeneous catalysis on polar stratospheric clouds  Cl 2 + h  Cl + Cl Regeneration of ozone destruction mechanism

Smog formation  Production of OH radical in troposphere via sequence…  NO 2 + h  NO + O  O( 1 D) + H 2 O  OH + OH  Oxidation of hydrocarbons (with regeneration of OH and NO 2  OH + RCH 3  RCH 2 + H 2 O  ……+ O 2  RCH 2 O 2 ……..

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*  2 P 1/2 state Cl  2 P 3/2 state (energy difference =E, spin-orbit splitting) Br + Cl Br + Cl* E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 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 Se 2 P branch R branch

Photodissociation of polyatomic molecules  Potentially more than one product channel for sufficiently high photolysis energy  e.g., formaldehyde CH 2 =O + h  H + HCO  H 2 + 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 T 1 1-Br-Np only dissociates from S 1 1-Cl-Np does not dissociate D0D0

Localization of excitation  The weakest bond is most likely to break - but consider -bromochlorobenzoyl ester  The excitation in the S 1 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 I 2 in solution  In gas phase, quantum yield for photodissociation is unity for < 499 nm  In CCl 4,  = 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 I2I2 I + I

Picosecond flash photolysis on I 2 in CCl 4 Photodissociate I 2 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 = D 0 + E int (AB) + E int (CD) + KE(AB) + KE(CD) E int 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 (D 0 and E int ) Use multiphoton ionization and ion imaging to make these measurements

Measuring the velocities of the products of photo- dissociation by ionization and imaging Cl 2 photolysis image – detect Cl atoms

Imaging the products of photo- dissociation Cl 2 photolysis image Perpendicular distance travelled is determined by fragment (Cl) KE Cl 2 + h = Cl + Cl h-D 0 = 2KE(Cl) Anisotropic image shows propensity for ejection in a specific direction relative to laser polarization.

Images from the photodissociation of ClO 2 – different predissociating levels of excited state populated. O atom detection - Different rings correspond to vibrational states (v‘) of ClO product ClO 2  ClO 2 *(v)  ClO(v') + O( 3 P 2 )

Femtosecond studies of simple dissociation processes.  Pulses of light as short as a few fs ( s) 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

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)

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) 2 X fragmentation (X=Cl, Br,I)  CpFe(CO) 2 X  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