Numerical Simulation For The Ionization Of Atoms And Molecules In Ultrashort Laser Pulses

Ultra-intense and ultra-short laser pulse technology is an indispensable tool for modern physics research,with the continuous development of laser pulse technology,it makes it possible to study the ultrafast dynamics in atoms and molecules,meanwhile,it also has a wide range of applications on chemistry,biology,material science,pharmacy and other disciplines.As one of the most fundamental and important phenomena in strong-field physics,the laser-induced ionization of atoms and molecules has been attracting everyone's interest and the study of its mechanism is a hot issue,such as multi-photon ionization?MPI?,above threshold ionization?ATI?,tunneling ionization?TI?and over barrier ionization?OBI?.In addition,the finding of the following new phenomena,including high-order harmonic generation,non-sequential double ionization,auger decay and so on,constantly put forward new challenges for everyone,so that the research on ultra-fast physics is more comprehensive and in-depth.Recent years,our research on strong field ionization tends to the situations of small molecules and even complex molecules,due to the more complex structural features,we will certainly find more interesting physical phenomena.As one of the most well-known theory model on the research of strong field ionization,the strong-field approximation?SFA?provides us a convenient way to study the interaction between the laser and the materials,which describes the essence of strong field ionization intuitively.Un-der different conditions the extended SFA models have their own modifications,which makes SFA have higher accuracy and wider applicability.In this thesis we study the ionization dy-namics of a variety of atoms and molecules in intense laser pulses based on the SFA theory and its derivative theory,while optimizing the calculation model.With the ionized photoelec-tron momentum spectrum,we explain the time effect of electron absorbing energy and measure the carrier-envelope phase of attosecond pulse trains,besides,we image the molecular orbital structures.The main works are shown as the following:1.Within the strong-field approximation,we study the time-resolved ionization process of a hydrogen atom in strong laser fields.By analyzing the time-dependent photoelectron mo-mentum distribution,we succeed in explaining how electrons gradually identify the laser fre-quency component and how corresponding above threshold ionization spectrum peak structures are built.After taking use of different laser fields to interact with H atoms,we find that at any time during the interaction between an atom and a laser field,the electron may instantaneously absorb all possible energies and build a wide momentum spectrum.As the interaction evolves in time,the coherent superposition of all previous instantaneous ionization events gradually filters out all other energy components except the part with an energy n?-I_p,which is cor-responding to the equation of Einstein photoemission,where I_pis the ionization potential,?is the photon energy,and n is a positive integer.Meanwhile,by tracking the time-dependent momentum spectra,we verify the Heisenberg uncertainty principle and show the different me-mentum between the direct-ionized and the rescattering photoelectrons.Finally,by changing the pulse width of XUV pulses,the establishment of”streaking”or”side-band”structures can all be directly viewed.2.Ionization of a hydrogen atom exposed to an attosecond pulse train and a few-cycle mid-dle infrared?MIR?pulse is calculated with the strong field approximation.The ionization events initiated by two attosecond pulses in the train are streaked in the presence of a weak MIR pulse,making the two ionization events overlap or separate in momentum representation.By changing the weak MIR pulse intensity,the interference structure in the photoelectron momentum distri-bution can be precisely tailored.When the MIR field is too weak to generate significant ion-ization signs,the centers of these interference structures are determined by the”Volkov”phase.When the MIR field is strong enough to produce substantial ionization,because of the highly nonlinear characteristics of tunneling ionization,the small change in the laser field produced by the weak attosecond pulses will be exponentially enlarged onto the ionization rate.In other words,the overlapped attosecond pulse train and MIR field trigger the XUV-phase-dependent photoelectron angular distribution.Either the interference pattern or the angular distribution can be used to extract the carrier envelope phase of attosecond pulses,which makes it possible to visualize the sub-XUV-cycle dynamics.3.Combined with the quantum chemistry software MOLPRO,we extend the strong field approximation to the calculations of the single ionization of complex macromolecules.At first,we replace the ground state in the strong field approximation theory by the molecular orbital wave function calculated by MOLPRO;Then the space integral part of the transition matrix element is solved by numerical integration method,because for complex molecules we have no analytic expressions.Such improvement makes the strong field approximation theory more applicable.Using this method,we calculate the single ionization of the HOMO and HOMO-1 orbitals of aligned and antialigned C₂H₂molecules.These two molecular orbital structures can be successfully reconstructed by differential normalizing the lateral momenta of the photo-electrons.Furthermore,we extend the derived model to the Coulomb-Volkov approximation for diatomic molecule.With such method,the simulation result of H₂⁺molecular ion is modified,as well as the photoelectron momentum distribution of the single ionization of an oxygen molecule is also calculated.