Study Of Ultrafast Multi-Body Dissociation Dynamics Of Several Polar-Molecule Systems

After absorbing light,molecular systems induce subsequent electronic and nuclear dynamics,a process that is central to the interaction between light and molecular systems.From the energy domain,according to the shell of the electron,it can be from a few electron volts（e V）to hundreds or even thousands of electron volts,and from the time scale,the process of interaction with molecules is elec-tronic transition or molecular vibration and the rotation of the molecular system,and the time scale expands from attosecond（as）to femtosecond（fs）and even to picosecond（ps）.To accurately understand and calculate the multi-dimensional potential energy surfaces to improve the perspective of molecular physics,in the past few decades,people have theoretically carried out different approximation conditions to decouple the electronic-nuclear motion in the molecule,including the electron-electron correlation effect.Molecular manipulation techniques and different spectroscopic detection methods have also been systematically devel-oped.In this thesis,we used the femtosecond pump-probe scheme combined with ion-ion multi-body coincidence measurement technology to comprehensively study the ultrafast dissociation dynamics of polar molecular samples,i.e.,nitrous oxide（N₂O）,carbonyl sulfur（OCS）,and hydrogen chloride（HCl）dimer,under the interaction of strong laser field in the spatial-temporal dimensions,focusing on the ultrafast localization of electrons in the molecular dissociation.The non-adiabatic effect between different electronic states and the influence of ultrafast proton motion on the multibody fragmentation process has been systematically studied.Because of the high nonlinearity of high field tunneling ionization,it is strongly dependent on the orbital distribution of molecular valence electrons.Therefore,the molecular electron localization process can be characterized in combination with the coulomb explosion.Experimentally,we have tracked the two-body dissociation dynamics and electron localization induced by the ionization of the linear asymmetric molecule N₂O to B²Πin the strong laser field on the fem-tosecond time scale.At the same time,the two dissociation channels N+NO⁺and O+N₂⁺were observed,in which the molecular dissociate into molecular ion frag-ments and atoms,and we observed spatial-temporal Pzsum distribution evolution.Combining the semi-classical trajectory and the high field tunneling ionization the-ory,the electron localization mechanism is attributed to the orbital hybridization caused by the coupling of different electron states at short inter-nuclear distance,while the polarization interaction between dissociated ions and atoms is dominant at the large inter-nuclear distance.Secondly,taking the three-atom OCS molecule as an example,combined with pump-probe and multi-body coincidence ion momentum imaging technol-ogy,the time-resolved Coulomb explosion imaging study of the evolution of the molecular dissociation process was performed.By measuring the ultrafast nuclear motion on potential energy surfaces after molecules ionization in the B²Πstate under the pumping laser,the probe laser is used to track the ultrafast nuclear motion on the potential energy surface.The time-dependent kinetic energy re-lease（KER）spectrum and ion-ion energy correlation of the generated two-body and three-body Coulomb explosion channels were measured to identify different dissociation paths.Molecular dynamics simulation based on high-precision po-tential energy surface calculation and semi-classical Landau-Zener surface jump theory,the corresponding electronic states of different dissociation channels were identified.Through the calculation of potential energy surface and nuclear tra-jectory,the coupling between different electronic states is analyzed,and the rela-tionship between bond breakage and molecular structure evolution are studied.Finally,we use the simple hydrogen-containing molecule HCl dimer as the model system to study the influence of ultrafast proton motion on the many-body fragmentation process during the dissociation of a molecular system.Due to the low counting rate of multi-body Coulomb explosion in cluster system and the complex characteristics of multidimensional potential energy surfaces,we ex-perimentally established a stable measurement to analyze and calculate different stable potential energy surfaces from a theoretical perspective to study the multi-body dissociation mechanism.In the process of three-body fragmentation,we extracted and analyzed the data of ion-ion coincidence in different dimensions of momentum,and observed that the KER of Cl⁺and HCl⁺ions split from one peak to two peaks when the ion-ion emission angle changes in the ion-ion emission angle and ion energy correlation map.In theory,by scanning the nucleus spac-ing of two H atoms relative to different Cl atoms,two different dissociation paths were found on the potential energy surface,one of which corresponds to the direct three-body fragmentation process,while the other fragmentation process corre-sponds to the protons transfer between two HCl molecules.From the perspective of quantum wave packets,wave packets can travel back and forth between two potential Wells,From the perspective of classical description,it can be understood as the molecular fragmentation process in which protons are transferring between two molecules,and according to the different positions of the protons finally falling to the Cl atom,the energy spectrum generated by the Coulomb explosion sepa-rates into double peaks.The multibody coincidence measurement of molecular clusters has always been a major challenge,mainly due to the difficulty of obtaining sufficient counts for corresponding physical analysis.After long-term stable data acquisition,we studied the four fragments generated after the HCl dimer was ionized and real-ized the coincidence measurement of the four ions H⁺,H⁺,Cl⁺and Cl⁺.In the process of many-body fragmentation of this system,there are very active pro-tons and relatively slow-moving Cl⁺,therefore,by extracting and analyzing their different ion emission angles and energy correlation maps,energy-energy correla-tion diagrams,and angle-angle correlation maps,how the ultrafast proton motion affects the four-body fragmentation process of the molecular system is studied.Through molecular dynamics simulation,the influence of ultrafast dissociation of two different protons on the subsequent Cl⁺fragmentation process was deeply ex-plored,and the observed angle distribution of ion emission and changes of energy spectrum were qualitatively explained.By simulating the four-body Coulomb explosion trajectory with different initial structures,extracting the four ion mo-menta after the Coulomb explosion,and comparing it with the experimentally measured momentum,the initial structure of each Coulomb explosion event was reconstructed by genetic algorithm,and a relatively accurate value was obtained in the reconstruction of intermolecular distance data.