Structural Imaging And Ultrafast Dissociation Dynamics Of Molecular Clusters

The ultra-short intense femtosecond laser has been widely used to study the ionization and dissociation dynamics of atoms,molecules,and clusters in recent years.The charged clusters will break up after removing several electrons from clusters by the femtosecond laser fields.The momenta of the fragments contain rich information about the interaction between laser and clusters,including the conformation of the clusters.Thus,the structures of the clusters can be retrieved and their fragmentation dynamics can be revealed by coincidently measuring the momenta of the fragments.Moreover,the ultrafast dynamics of the dissociation and its structural evolution can also be tracked in real-time by combining with a pump-probe technique.In this dissertation,we describe the reconstruction of the stereo configuration of several molecular clusters such as OCS dimer,benzene（Bz）dimer,and Bz₂Ar.The related multi-body fragmentation dynamics from laser-induced Coulomb explosion（CE）are studied by performing the coincident measurement in a cold target recoil ion momentum imaging spectrometer（COLTRIMS）.Furthermore,we have also tracked the femtosecond time-resolved three-body fragmentation dynamics of（N₂O）₂,and studied the influence of neutral or charged neighboring molecules on the single molecular ions in bound and dissociated states.In the first work,we have conducted a four-body coincident measurement from CE of OCS dimers and the stereo configuration of the dimer is reconstructed from the measured fragments momenta.The sequential and concerted fragmentation processes in the four-body dissociation of the molecular dimer are discussed and distinguished according to the correlations between the measured kinetic energy of fragments.By comparing the simulated momenta of the four-body CE with the measured ones,the initial structure of each CE event is retrieved from the measured coincident momenta of the fragment ions generated in the concerted process.The simulation is done by a classical trajectory calculation and the reconstruction is performed with the genetic algorithm optimization scheme.Finally,four isomers,non-polar S-in,non-polar O-in,polar,and cross-shaped isomers,are resolved and their structural parameters are obtained based on the experimental observations.We have also extended the laser-induced Coulomb explosion imaging method to structural imaging of Bz₂Ar clusters containing benzene molecules.By measuring the corresponding three-body Coulomb explosion channel and combining it with the configuration of eight isomers calculated by quantum chemistry,the structure of isomers is determined to be close to an equilateral triangle.The spacing between the Ar atom and the Bz molecule is also retrieved from the momentum of the fragment ions.Next,we tracked the three-body dissociation kinetics of（N₂O）₂ on the femtosecond time scale and studied the effects of the neighborhood effects of neutral and molecular ions on the bound and dissociated molecular ions on the femtosecond scale based on these processes.The results show that the existence of adjacent particles can induce the rotational excitation of molecular ions.By changing the pump-probe delay,we track the different rotational excitations of ions by different neighbors on the fs scale,showing that the rotation is faster when neighboring with the ionic molecule than neighboring with a neutral one.For the dissociation of N₂O⁺,it is demonstrated that the adjacent molecular ion can change the parity of the molecular states through the spin-exchange effect,and then opens a forbidden transition channel to allow a new dissociation path.We also track the ion-molecular collisions between the dissociation products from the cluster dissociation,i.e.N₂O and NO⁺,and N₂O⁺and NO,on the fs timescale.We have also proposed a novel in-situ method to measure the charge distribution of Bz cations experimentally using an Ar atom as a probe.The measured charge distribution is successfully applied to imaging the structure of Bz₂.According to the known Bz Ar structures,by measuring the kinetic energy of the CE of the Bz Ar cation,the charge distributions of the Bz cations(Bz⁺and Bz²⁺)are retrieved.The results agree well with the charge distribution calculated from the quantum chemistry method.Furthermore,we apply the measured charge distributions to the structural reconstruction of Bz₂,the structures of T-shaped and PD-shaped Bz dimer isomers are resolved and determined.