Download-DepartmentofChemistry：-化学系

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1、PhotochemistryLecture8PhotodissociationPhotodissociationABCD+hAB+CDImportanceAtmosphericandastrophysicalenvironmentPrimarystepinphotochemicalprocesses–freeradicalproductionFundamentalstudiesofdynamicsofchemicalreactionsAtmosphericChemistry–theozoneholeInthestratosphere,oz

2、oneprotectstheearthfromdamagingUVradiationviatheChapmancycleO2+h→O+O(<242nm)O3+h→O2+O(<1180nm)O+O2+MO3+MO+O3O2+O2Solarenergyconvertedintothermalenergy…heating…temperatureinversion.Catalyticdestructionofozonee.g.,CF2Cl2+hCF2Cl+ClCl+O3ClO+O2ClO+OCl+O2Formationofrese

3、rvoirspeciese.g.,Cl+CH4CH3+HClClO+NO2+MClONO2+MAntarcticozoneholeClONO2+HClCl2+HNO3HetergeneouscatalysisonpolarstratosphericcloudsCl2+hCl+ClRegenerationofozonedestructionmechanismSmogformationProductionofOHradicalintroposphereviasequence…NO2+hNO+OO(1D)+H2OOH+OHOxida

4、tionofhydrocarbons(withregenerationofOHandNO2OH+RCH3RCH2+H2O……+O2RCH2O2……..Directdissociation–excitationintocontinuumofexcitedelectronicstateAbsorptionspectrumbecomescontinuousatsufficientlyshortwavelengthashcrossesadissociationthresholdAbsorptionspectrumTheexcitedstate

5、maycorrelatetodifferentdissociationlimittogroundstatee.g.,forBrCl,thefirstexcitedstatecorrelateswithBr+Cl*Cl*2P1/2stateCl2P3/2state(energydifference=E,spin-orbitsplitting)Br+ClBr+Cl*EWavefunctionsinthecontinuumVerticalexcitationfavouredbyFranck-CondonfactorsSimplephotod

6、issociationwithinasingleelectronicstateisessentiallyforbiddenThiscouldbeconsideredastheextremelimitofvibrationalovertoneexcitation;vverylargePredissociationMoleculeexcitedtoboundstate–vibratesforperhapsafewperiodsthenundergoescurvecrossinganddissociatesonrepulsivePEcurveFr

7、anckCondonfactorforexcitationdeterminedbyoverlapwithboundstatewavefnasbefore.LifetimebroadeningofpredissociatinglevelsSometimesknownasthetime-energyuncertaintyrelationshipInthiscontext:tlifetimeofexcitedstateE“homogeneous”linewidthoftransition5ps1cm-1linewidthUpperstat

8、epredissociationevidentinlinewidthsofPandRbranchtransitionsofSe2PbranchRbranchPhot