Studies On CO₂ Dissociation Dynamics In Strong Laser Fields

With the continuous increase of laser intensity,molecular ionization and dissociation experiments induced by strong laser fields have become one of the most important research tools for understanding new phenomena in atomic-molecular physics.Molecules have complex dynamics processes in strong laser fields,and a large number of laser-induced experiments have been done with corresponding mechanisms of response proposed for different dynamics processes.Such as the mechanisms of higher harmonic generation,multiphoton ionization,tunneling ionization,over-barrier ionization,sequential double ionization,recollision and non-sequential double ionization,molecular dissociation,and cluster Coulomb explosion.In particular,the main challenge in the Coulomb explosion is to understand the selectivity of molecular bond breakage and to clarify which parameters control the bond fission.Removal of electrons from the molecule will result in transitions to excited states of the transient molecular ion,which,in most cases,will eventually lead to fragmentation and the occurrence of the well-known Coulomb explosion process.Two-body dissociation channels are identified by Photoion-Photoion Coincidence（PIPICO）,and three-body dissociation channels are identified by Photoion-Photoion-Photoion Coincidence（PIPIPICO）channels,whose Coulomb explosion-specific features are expressed as a sharp parabolic structure in the PIPICO and PIPIPICO spectra,and to investigate the fragmentation mechanism of dissociated channels based on dynamics information of the ion fragments from each dissociated channel.In this thesis,we studied the dynamics of CO₂ dissociation processes induced by strong laser fields at the Shanghai Soft X-ray Free Electron Laser Molecular Dynamic Imaging Experiment Station:cold target recoil ion momentum spectrometer（COLTRIMS）.With the standard COLTRIMS detection method,the time of flight and the impact position of the ion fragments arriving at the detector were obtained,and the complete initial three-dimensional momentum of each ion fragment was reconstructed.The 4 two-body fragmentation and 15 three-body fragmentation channels of CO₂ were clearly identified by using the newly proposed method in this thesis,and the dynamic processes of all dissociation channels were carefully studied.In addition,the thesis research work includes a COLTRIMS device developed with colleagues in the research group,which is suitable for versatile instruments to investigate the most fundamental reactions in atoms or molecules triggered upon absorption of photons,impact of electrons or bombardment with ions.The results and highlights of this thesis are the following:1.We have successfully developed a COLTRIMS setup and the acceptance of all experimental indicators has been completed.The experimental realization of electron-ion,ion-ion,and ion-ion-ion coincidence measurements indicates that the COLTRIMS setup for soft X-ray free electron laser applications in Shanghai can formally enter the experimental research stage.2.In the study of three-body fragmentation channels of CO₂,it was found that three-body dissociation channels with ion fragments of three different charge states are not visible and show a fuzzy"island"structure in the traditional PIPIPICO spectra.Considering the charge states of individual ion fragments,a charge-encoded multi-photoion coincidence（c MUPICO）method is proposed to identify the multi-body dissociation channels.The c MUPICO spectrum well presents these three-body dissociation channels with a sharp parabolic structure and successfully identifies four two-body fragmentation and 15 three-body fragmentation channels of CO₂,of which eight three-body dissociation channels are identified for the first time.Present results exhibit that c MUPICO is a powerful and practical tool to distinguish various dissociation channels with multiply charged multi-photoion.3.We studied the mechanism of two-body fragmentation for CO₂,and the kinetic energy release（KER）of the dissociation channel was calculated based on the reconstructed momentum of the ion fragments.The important role of the dynamics collimation mechanism in the two-body fragmentation process was revealed experimentally,and the bond lengths before molecular dissociation were inverse by the KER of the dissociation channel,which confirmed that the molecular bond lengths were stretched under the action of the laser field.The results show that the width of the laser pulse is a key factor in regulating the molecular bond length,which provides an important idea for manipulating the molecular structure.4.Dissociative ionization dynamics were studied experimentally for CO₂^q+（q≤4）induced by femtosecond laser pulses.The Newton diagram demonstrated that the three-body fragmentation of CO₂^q+（q≤4）occurred by sequential and non-sequential fragmentation processes,and the momentum correlation of the two oxygen ion fragments was used to distinguish between the two processes,and analyze these fragmentation processes of CO₂^q+（q≤4）in detail.It is revealed that the rotation of the intermediate CO²⁺ion plays an important role in the sequential fragmentation process.5.The three-body dissociation of the highly charged state CO₂^q+（q>4）was studied,and more dissociation channels were identified by using the c MUPICO method,and all dissociation channels were systematically analyzed.It is confirmed that all these dissociation channels are non-sequential fragmentation processes.It is confirmed that all these dissociation channels are non-sequential fragmentation processes,and the distribution laws of O ion KER distribution and the momentum angle between the first O ion and C ion in different dissociation channels are revealed.There are five chapters in this thesis research,the first chapter is about the domestic and foreign research,the background and significance of this thesis research.In the second chapter,the experimental setup and experimental method of COLTRIMS are described.Chapter 3 introduced the charge-encoded multi-ion coincidence method and its application.Chapter 4 introduced experimental study about the dissociation of CO₂molecules induced by strong laser fields.Chapter 5 summarizes the research work of the thesis.