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1、Coherent ControlCoherent Control attempts to control a chemical reaction with light, usually a cleverly shaped ultrashort laser pulse.Why its hard: IVR.Using shaped pulses and an iterative approach.Wave- functionPotentialC = catalystConventional methods of chemical controlMuch can be done, but not e
2、verything wed like.Coherent ControlChemical reactions proceed in a manner determined by the molecular Hamiltonian. What if wed like to change this and make different products?Bring in a light frequency to excite a bond wed like to break. But its not so easy! Theres a lot more to it.A long-held dream
3、 of chemists. Its now coming true. Shaped ultrashort pulses are the key.Coherent control slides mostly thanks to Gustav Gerber, University of Wurzberg, Germany Margaret Murnane and Henry Kapteyn, JILA Robert Levis, Temple UniversityIVR occurs on a few-fs time scale, so long pulses excite entire mole
4、cule, and the weakest bond breaks, no matter which bond was excited.Excite one bondA few fs later, however, the entire molecule is vibrating.Intramolecular Vibrational RedistributionThe bond vibratesSingle-parameter controlSurprisingly, these simple methods often work, but they are not general.Coher
5、ent control: Using shaped ultrashort pulses to control the reactionCan an ultrashort pulse cause a molecule to vibrate in such a way as to break the bond of our choice?The physics of coherent controlMoleculeE-field of LaserPerturbed SystemWave- functionPotentialThe pulse electric field perturbs the
6、molecule and potentially dissociates it.The trick is to compute the required pulse electric field.Wave- functionPotentialTrying to do the theory for coherent controlH system = H molecule + H radiation + H interactionH radiationknown H moleculesmall molecules: approximatelarge molecules: unknownH int
7、eractionweak field: knownstrong field: unknownFirst, we need to know the complete Hamiltonian for the molecule and radiation:For all but the simplest molecules, its hopeless to solve the problem.We could try to solve the problem theoretically, but its easier to just do it iteratively in the lab.Puls
8、e-shaping is important for coherent control.Genetic algorithm for coherent controlThis algorithm was developed for computer optimization, but, for coherent control, it can be implemented as part of an experiment.A genetic algorithm can minimize the pulse length.Using a learning algorithm to perform
9、coherent controlCoherent control of a simple gas phase reactionShaped PulseCO2CCl4CCl2OReaction under studyMurnane and Kapteyn, University of ColoradoH3CCH3COH3CCH3CO+Coherent control with acetone (gas phase)Levis and coworkersAcetone can be broken into various pieces. A laser pulse could help.25H3C
10、-C-CH3+=20151050Normalized ion signal6050403020100 Mass, amu M-15+M-30+ H2O+CH3+C+ C+H+O2+(Acetone+)OMass spectrumH3CCH3COCH3CO+6040200Normalized Ion IntensityAcetone+CH3CO+generation2210 3 0Mass (amu)70605040302010Goal: Optimize CH3CO+ at 43 amuOptimizing one acetone photo-fragmentScience 2001, 292
11、, 709 Levis and coworkersMaximization of the relevant photo- fragment occurs rapidly.Science 2001, 292, 709 Levis and coworkers=10080604020CH3CO+ Signal2520151050Generation NumberH3C-C-CH3CH3CO + CH3OCOCH3COCH3+COCH3+Manipulating the dissociation yields in acetophenoneDifferent pulse shapes can opti
12、mize different photo-fragments.Levis and coworkers1.61.41.21.00.80.620151050 GenerationRatio: C7H5O/C6H5 Normalized ion intensity and ratioNormalized ion intensity and ratioCOCH3+COCH3+GenerationRatio: C6H5/C7H5O 2.22.01.81.61.41.21.020151050COCH3100 kcal/mole85 kcal/moleReversing the ratio: Increas
13、ing the phenyl yieldLevis and coworkersOptimizing the phenyl fragment yield also works.COCH3+COCH3+COCH3enhancesuppressWhat do these pulses look like?Levis and coworkersThe pulse that maximizes the ratio of the two fragments.Interestingly, a very simple pulse maximizes the phenyl radical (but not th
14、e ratio).Delay (fs)-150-75075150395420Wavelength (nm)SHG FROG traceto POLLIWOG apparatusinput pulseMolecules are not isotropic, so pulse polarization shaping is important.A complex polarization-shaped pulseCoherent polarization control of a complex molecule in the gas phaseGerber and coworkersCohere
15、nt control is here!It works, not only in the gas phase, where dephasing times are long, but also in the liquid phase. This is potentially very useful!Almost any wavelength will do, as high intensity broadens the energy levels significantly, making all processes effectively resonant.Rabitz has shown
16、that it is robust and should occur for essentially all systems.By determining the precise field that optimizes the desired product, we also learn about the molecule.Coherent control has applications far beyond chemistry.Successful closed-loop coherent control experiments in physics and chemistry(1) Fluorescence spectrum manipulation (Wilson, 1997)(2) Atomic excitation tailoring (Bucksbaum, 1999)(3) Vibrational excitation tailoring i
