Ultrafast Electron Dynamics Of Molecules In Strong Laser Fields Based On Coincidence Measurement

The microcosmic material world has been evolving dynamically.The interaction between light and matter provides important means for exploring the microcosmic dynamic evolution process and understanding the structure of matter.As the most fundamental particle of matter,the behavior of electrons has an important effect on the ultrafast dynamic process of atoms and molecules.Ultrafast electron dynamics in atoms and molecules is the basis of studying the interaction between laser and matter,and has important scientific significance for understanding the nature of physics,chemistry and biology.Compared with nuclear wavepacket vibration,molecular chemical bond breaking on femtosecond time scale,electron motion takes place in extremely short time scale,requiring attosecond time-resolved precision measurement methods.In recent years,with the development of ultrashort laser pulses and advanced particle detection technology,it provides a powerful tool to reveal the laws of ultrafast motion inside atoms and molecules and to track and detect the ultrafast dynamics of electrons.This dissertation focuses on the study of molecular ionization process driven by ultrashort strong laser field based on the electron-ion three-dimensional momentum coincidence measurement,with emphasis on the measurement and manipulation of the ultrafast dynamic behavior of electrons in molecules.The principal contents and innovations are summarized as follows:1.Probing resonant photoionization time delay by self-referenced molecular attoclockBy using femtosecond laser pulse pump-probe technique and angular streaking effect of elliptically polarized probe pulse,a self-referenced molecular attoclock scheme is proposed.Experiments and theories are combined to reveal semi-classical images of transient resonance ionization of ArKr⁺.By selecting the electron momentum distribution corresponding to a specific molecular axis orientation,it is found that there are two different deflection angles corresponding to direct ionization and resonance ionization paths respectively.The electron tunneling dynamics of Aratoms under the influence of the Coulomb potential of adjacent Kr⁺are studied by selecting the electron momentum distribution under the condition of molecular axial orientation and combining with the improved semi-classical Coulomb-corrected strong-field approximation method.By utilizing the direct tunneling ionization as a self-referenced arm of the attoclock,the time delay of the electron trapped in the resonant state is revealed.2.Identifying photoelectron releasing order in strong-field dissociative ionization of H₂We demonstrate an approach to resolve photoelectron releasing order in the dissociative and non-dissociative channels of a singly ionized H₂ molecule driven by an orthogonally polarized two-color femtosecond laser pulse.The photoelectron kinetic energy releases and the regular nodes in the photoelectron angular distributions due to the participation of different continuum partial waves allow us to deduce the field-dressed ionization potential of various channels.It returns the ponderomotive energy experienced by the outgoing electron and reveals the corresponding photoionization instants within the laser pulse.The non-dissociative single ionization channel mainly occurs at the pulse peak,while the dissociative single ionization channel mainly occurs at the rising edge of the laser pulse.3.Resolving electron partial waves in multiphoton ionization of H₂moleculesWe show a general solution of the crossword puzzle using an orthogonally two-color scheme thanks to different photon energies and polarizations of the laser fields which lead to distinct photoelectron angular distributions.Each continuum partial wave of the releasing electron,with its amplitude and phase,is resolved in dissociative single ionization channel and non-dissociative single ionization channel by analyzing the laser-phase-dependent angular phase spectra.By comparing the relative phases between different ionization paths and different orders,the phase of external field interaction accumulated by photon absorption in orthogonally polarized two-color field is obtained.