| Coherent control has been increasingly used to control the dynamic processes ofpolyatomic molecular systems. Pulse shaping is the most important means to performthe coherent control of molecular reactions. A dual-mass-spectrometer scheme isproposed and used in the isomeric identification and control of ionization anddissociation of acetone and its isomers; we investigate the dissociative ionization andcoulomb explosion of CH3I using the velocity map imaging, under the irradiation ofthe transform limited pulse and shaped pulse; we build the polarization shapingoptical system and generate the shaped laser pulse of which the polarization varieswith the time in experiment. The research works mainly includes the followingaspects:(1) A dual-mass-spectrometer scheme is proposed, in which two massspectrometers are used to obtain the time of flight mass spectra of two differentmolecules at the same time, under the irradiation of same shaped femtosecond laserpulse. In order to better characterize the degree of difference between two massspectra on the identification of acetone and its isomers, we introduce a parameter γ。The difference degree γ of the mass spectra of propylene oxide and acetone increasedfrom0.41using TL laser pulse to0.62using optimal laser pulse. The experimentalresults show that the differences between the mass spectra of acetone and its isomerare increased and the identification is achieved. We also simultaneously control thedissociative ionization processes of acetone and its isomers using thedual-mass-spectrometer scheme. We point out the advantages of thedual-mass-spectrometer scheme compared to traditional single mass spectrometrymethod. Firstly, using the dual-spectrometer scheme proposed in the current study, thelaser beam is divided into two beams and same laser field is irradiating on twoisomers simultaneously. Thus, a GA optimization can be performed, the fitness ofwhich is taken from the two mass spectrometers, without the interference of onespectrum to the other. The optimal pulses are obtained from a much larger searching space. It will have the ability to distinguish isomers when it is irradiating on each puremolecule sample. After calibration, it will also be able to tell the relativeconcentration of each molecule sample, when it is irradiating on the moleculesmixture. Secondly, the dual-mass-spectrometer scheme can also be used in theselective control of molecule reaction dynamics of two molecules, simultaneously. Aslong as the reaction products we are interested in involve common peaks in two massspectra, the dual-spectrometer scheme will be useful since GA optimization can’t beperformed in a single spectrometer. This selective control of ionization and/ordissociation products has the potential application in purification of moleculemixtures, since one of the molecules may be removed after dissociation or ionization.It can also be applied in obtaining products from one of the molecules in a mixture, inthe circumstances that other molecules also give unwanted products under theirradiation of transform limited pulse.(2) We investigate the dissociative ionization and coulomb explosion of CH3Iusing the velocity map imaging, under the irradiation of transform limited pulse andshaped pulse, and identify the channels of dissociative ionization. Through theanalysis of the kinetic energy distribution of I+under the irradiation of the transformlimited pulse and shaped pulse, we identify the dissociative ionization channel G1(0,1)，G1(0,1’) and the coulomb explosion channel G1(1,1)，G1(1,1’). Then, we controlthe fragmentation of CH3I using three sub-pulse sequence with the variation of thespacing and find that the intensity of I+varies with the variation of the sub-pulsespacing. Under the irradiation of the optimal pulses with the spacing208fs, theintensity of I+reaches the maximum value. We also analyze the changes of intensityof I+from the dissociative ionization channels and coulomb explosion channels. Theexperimental results show that both of the intensities of I+from dissociative ionizationchannels and coulomb explosion channels significantly increase under the irradiationof optimal pulses. We suggest the increase of intensity of I+is due to the increase ofthe ionization processes of CH3I under the irradiation of the optimal pulses andpropose a model of three pulse control mechanism to explain the reason that theintensity of I+increase. In addition, we also compared the angle distribution of I+from dissociativeionization channels and coulomb explosion channels, under the irradiation of thetransform limited pulse and shaped pulse. The experimental results show that: theangle distribution of I+from different channels under the irradiation of the optimalpulse is the same as that under the irradiation of the transform-limited pulse. But theangle distributions under the different sub-pulse spacing are different, which illustratethat the shaped pulse can control the degree of alignment of I+. For the study of themechanism of control, we will complete the work in the future.(3) We generate the time-dependent polarized shaped laser pulse using the dualliquid crystal panel polarization shaping method based on the4f pulse shaping system.By removing the polarizing plate on both sides of the liquid crystal spatial modulator,we generate the elliptical polarization shaped laser pulse of which the ellipsometryrate is time-dependent. In addition, we perform a/4wave plate behind the4f pulseshaping system and generate the linearly polarized shaped laser pulse of which thepolarization varies with time. We load the Sine phase and quadratic phase onto theliquid crystal spatial modulator respectively and analyze the characteristics of shapedlaser pulse generated by the two phases using theoretical simulation. Theexperimental results show that the polarization direction of linearly polarized shapedlaser pulse rotates over time. We also built a cross correlation optic system, so as todetect and analyze the polarized shaped laser pulse. The results show that thepolarization direction of chirped pulse and the sub-pulse in pulse sequence rotate overtime. The experimental and theoretical results coincide with each other, which provethat we can generate the time-dependent polarized shaped laser pulse in experiment. |
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