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1、Photochemistry,Lecture 8 Photodissociation,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 stratos
2、phere, 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 energyheatingtemperature inversion.,Catalytic destruction of ozone,e.g., CF2Cl2 + h CF2Cl + Cl Cl + O3
3、 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 rad
4、ical 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 wave
5、length 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 + C
6、l*,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
7、 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 uncerta
8、inty 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 pr
9、oduct 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 predissociationenergy transfer from initially excited
10、 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
11、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
12、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 p
13、hotodissociation 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
14、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 (electron
15、ic) 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
16、 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 +
17、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 state
18、s (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-
19、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 vibrati
20、onal 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 differe
21、nt 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 pul
22、ses (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 ligh
23、t 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,