Controlling Excited State Dynamics Of Some Molecules By Using Shaped Femtosecond Laser Pulses

Coherent control is the effective manipulation of molecular excited statesdynamics on the atomic and molecular level using the coherence property of laser.Pulse shaping technique is an important experimental method to achieve coherentcontrol. In this thesis, processes of molecular alignment,photoionization/photodissociation, etc. are studied by using pulse shaping technique.Change of alignment degree of methyl iodine molecule with sub-pulse separation inpulse train is observed. The mechanisms of excited state dynamics are explained bystudying the modulation of cyclopentanone ionization/dissociation processes underthe irradiation of laser pulse train. The control of ionization, dissociation, multipleionization processes and the identification of isomers are achieved. In detail, the studyof this thesis is divided into the following parts:（1） The angular distribution of CH3I has been studied experimentally by using aFourier Transform-Limited single laser pulse and pulse train. In this thesis, thepolarization angle is defined as the angle between the detection axis of thetime-of-flight （TOF） and the polarization vector of the laser light. Horizontalpolarization and vertical polarization denote respectively parallel and perpendicular tothe detection axis of the TOF. The ion intensity angular distribution is measured bychanging the polariztion angle. All ion distributions are similar. The maximum yieldoccurs at parallel polarization and the minimum yield occurs at perpendicularpolarization. In order to elucidate the alignment mechanism, we measure the ratio offragment ion yields obtained at parallel polarization with that obtained atperpendicular polarization, within a range of laser intensities. The intensity ratios ofboth CH3+and I+fragments increase with increasing laser intensity, in a range of1.45×1013W/cm2to3.15×1013W/cm2, which is a proof of the dynamic alignmentmechanism. In order to select a pulse train with proper pulse separation that can beused to optimize molecule alignment, we study the effect of pulse separation in the train on the ratio of ion intensity at the parallel polarization with that at theperpendicular polarization. The molecular alignment degree can be approximatelyexpressed by using the ratio of ion intensity at the parallel polarization with ionintensity at the perpendicular polarization. The ratio becomes larger with highermolecular alignment degree. For CH3+and I+, the ratio reaches maximum when thepulse separation is200fs. We choose the pulse train with pulse separation of200fs tooptimize molecular alignment. Alignment degree from200fs separation pulse train isalmost identical to that from single pulse. In order to interpret the mechanism ofoptimized alignment, we studied the effect of femtosecond laser pulse chirping on thealignment degree of CH3I molecular, the ratio of fragment ion yields at parallelpolarization to that at perpendicular polarization is also monitored. Except thefluctuation at duration values larger than1.5ps due to noise from small ion intensities,the ratio reduces when pulse duration increases and no sign of enhanced alignment isobserved near the duration value equaling the effective duration of a200fs pulseseparation pulse train. So it is the structure of pulse train rather than the effectiveduration of pulse that is crucial to molecular alignment of CH3I in our experiment.（2） The photoionization and photodissociation processes of cyclopentanone arestudied by using shaped femtosecond laser pulses. When the molecule is irradiated bythe pulse train, the shape of the mass spectrum changes with increasing pulseseparation. The integrated peak intensities of each peak are calculated. For smallerproducts （m/z equaling1²8）, the overall intensities show decreasing trend withincreasing pulse separation. Peak43’s intensity shows an increasing trend withincreasing pulse separation. The overall intensities of larger products （m/z equaling55,72and84）, increase first, reach maximum near0.8¹ps, then decrease at evenlonger pulse separation. We can see that some modulation curves’ peaks and dips don’talways fall at the same place. An overall trend is curves with large m/z values tend tohave opposite peak and dip positions to curves with small m/z values. Besides theoverall intensity trend, strong modulations for some of the peaks （e.g. m/z equaling1,15,27etc.） are observed. A maximum modulation depth of78.9%is observed for m/z equaling12and a minimum modulation depth of14.51%is observed for m/z equaling84. The ion yield ratio between different products can be changed dramatically bychanging pulse separation. At0.198ps, I₈₄/I₁₂=40.92. At0.279ps, this ratiobecomes3.60. The product yield ratio changes by11.37fold. Similarly, at0.198ps,I₅₅/I₁₂=4.225, the ratio becomes0.232at0.279ps. The product yield changes by18.23fold. The ratios of mass peak intensity under the irradiation of pulse train to thatunder the irradiation of Fourier Transform-Limited laser pulse with increasing pulseseparation are calculated. The largest enhancement, the ratio of7.47, is observed forthe mass peak m/z equaling42. Based on the experimental observations fromcontrolling experiments, the compositions and formation pathways for different peaksin mass spectra are assigned. Two possible mechanisms are proposed for thecontrolling of photoionization and photodissociation processes observed in currentpaper, which are based on multiphoton intrapulse interference and the oscillation ofwavepacket on potential energy surface.（3） C-C and C-O bond cleavage of ethanol molecules is studied by using shapingpulse, in closed loop experiment. The product ion yield ratio C2H5+/CH2OH+ofethanol molecules is optimized. The ratio reach maximum when pulse separation is180fs. The optimization control mechanism is discussed through experiment scanningthe sub-pulse separation in pulse train. We speculate that structure of pulse train playan important role in the optimization of the ratio.（4） The ratio of different product yield of cyclopentanone C2H3+/C5H8O+isoptimized in closed loop experiment. The ratio from pulse train is2.2times as big asthat from to that from single pulse. The optimized pulse train is composed of7sub-pulse and the pulse separation is117fs.（5） The isomers （para-xylene and o-xylene） are identified by using pulse train.Parent ion intensity of para-xylene decreases quickly with increasing pulse separation,while that of o-xylene appears oscillates with increasing pulse separation. Thebehaviors of parent ion intensity change of the two isomers with increasing pulse separation are different. Thus, identification of para-xylene and o-xylene is achieved.（6） The multiple ionization processes of CH3I and its fragments are optimized byusing pulse train. When the molecule is irradiated by the pulse train, the shape of thespectra are different at different pulse separation. When irradiated with a pulse trainwith150fs pulse separation, the integrated intensity of multiple ionization product I2+is2.1times as big as that under the irradiation of transform limited pulse, the intensityof I3+is5.4times as big as that under the irradiation of transform limited pulse.