Numerical Simulation For The Ionization And Dissociation Of Atoms And Molecules Driven By Ultrashort Laser Pulses

With the rapid development of ultrafast laser technology,the femtosecond and attosecond laser pulses interaction with matter has become an important means to explore the microstructure and movement laws of matter.Using the ultrafast characteristics of these laser pulses,we can directly observe and manipulate ultrafast motions on the atomic and molecular scales.The interaction of ultrashort intense laser pulses with atoms and molecules has entered a completely new nonlinear region.Many novel physical phenomena have been found experimentally,such as above-threshold ionization,nonsequential double ionization,highorder harmonic generation,and above-threshold dissociation of molecules and so on.The continuous exploration of these novel phenomena has promoted the vigorous development of strong field physics,and has also directly promoted the development of some new subject areas,such as attosecond science.In theory,in order to explain these novel phenomena discovered experimentally,different theoretical models have emerged.The phenomena currently found in experiments poses a challenge to the perturbation theory.Due to the rapid increase in computing power,the numerical solution of the time-dependent Schr?dinger equation has become a powerful tool for studying strong field physics.Recently,semiclassical theory based on electronic trajectories has been widely used in the field of strong field ionization due to its clear physical process and simple calculation.In this dissertation,we use the numerical solution of the time-dependent Schr?dinger equation,the semiclassical method,and the classical method to study the ionization process of atoms and the ultrafast dynamics of the molecular dissociation ionization.The main research results are summarized as follows:1.We studied the tunneling ionization of the model neon atom with different angular momenta in the initial state.We studied the ionization of the model neon atom initially having different magnetic number m=±1 using the time-dependent Schr?dinger equation,classical trajectory Monte Carlo,quantum trajectory Monte Carlo and backward propagation methods.The tunneling exit,time-dependent tunneling rate,and photoelectron momentum distribution at tunneling are explored by cross comparing the photoelectron momentum distributions calculated with these methods.When the electron and the laser electric field rotate along the same direction,the tunneling exit is far from the nucleus and thus the time-dependent ionization rate is small.At the same time,different transverse momenta at tunneling exits induce different ultimate photoelectron momentum distributions.2.We studied the ionization process in subcycle in multicycle laser pulses.We cooperated with the experimental team of Prof.Wu Jian of East China Normal University and proposed an effective scheme to resolve the subcycle ionization process in multicycle laser pulses.We construct an optical waveform whose polarization axis rotates slowly for consecutive optical cycles,thus the ionization events triggered by different cycles will be steered into different directions in the polarization plane.Classical trajectory Monto Carlo simulations support the resulting angleresolved photoelectron momentum distribution.We further investigate characteristics of electrons triggered by different quarters of an optical cycle and explore the Coulomb focusing effect on electrons traveling along different trajectories.Our experiment verifies the numerical simulations and demonstrates the feasibility of retrieving subcycle dynamics with multicycle laser pulses.3.We studied the remote control of the dissociative ionization of H₂.Using the quantum entanglement in molecular systems,we pursued a fundamentally different route to remote control of the dissociative ionization of H₂in strong laser fields.An attosecond pulse train comprising two attosecond pulses is used to trigger the dissociative ionization of H₂,i.e.,producing a free electron and H₂⁺.Then a timedelayed midinfrared pulse is used to steer the freed electron wave packets and it was found that the nuclear energy spectra changed significantly.Similarly,the photoelectron energy spectra can be controlled via laserH₂⁺coupling but without the direct interaction between the laser and the free electron.This method demonstrates the quantum entanglement in molecular systems and offers a new route to control chemical reactions.4.We studied the highorder above-threshold dissociation of molecules.This work was also in collaboration with the experimental team of Prof.Wu Jian.They experimentally measured the electron-nuclear joint energy spectrum of the dissociative ionization of H₂in a strong laser field by coincidently measuring electron and ion ejected from the same molecule.By integrating the energy of electrons in a certain energy range,multiple peaks are observed in the obtained nuclear energy spectrum,and these peaks correspond to highorder above-threshold dissociation processes.In order to reveal the underlying mechanism of the highorder above-threshold dissociation,we numerically simulated the time-dependent Schr?dinger equations.The calculated electron-nuclear joint energy spectra share very similar structures with the experimental results.Numerical simulations allow us to trace the periodical emission of the correlated electron–nuclear wave packets in each optical cycle,leading to the coexistence of the discrete above-threshold ionization and above-threshold dissociation spectra in the dissociative ionization of molecules.