Research On Strong-Field Ionization Dynamics Of Atoms And Molecules Based On Attosecond Angular Streaking

With the rapid advancement of laser technology and experimental detection techniques,the exploration of the interaction between strong lasers and atoms and molecules has seen rapid development over the past two decades.Exposure of atoms and molecules to strong laser fields triggers a series of higher-order non-linear phenomena.The study of these phenomena has led to the generation of the shortest light pulses to date,and has facilitated the emergence of attosecond science.Ionization,as one of the most fundamental processes among these phe-nomena,occupies a central position in the research of strong-field physics.It is crucial for understanding and exploring a series of ultrafast processes,including photoelectron interference and diffraction,the generation of higher-order harmon-ics,the synthesis of attosecond pulses,and the dissociation and Coulomb explosion of molecules.The ionization process occurs on an attosecond time scale,repre-senting the current limit of temporal resolution achievable by human detection technologies.The breakthrough progress in femtosecond strong laser technology has made it possible to study the dynamics of atoms and molecules on attosecond time and nanometer spatial scales.Among these,the attosecond angular streak-ing technique,as a method for detecting electron dynamics with attosecond-level resolution,has been highly favored by the scientific community since its intro-duction.This technique has been applied to the in-depth study of fundamental issues in strong-field atomic and molecular physics,encompassing non-adiabatic effects,temporal characteristics of tunneling ionization,multi-electron dynamics,molecular structural effects,and strong-field induced autoionization phenomena.This paper utilizes a cold target recoil ion momentum spectroscopy coin-cidence measurement system to measure the electron momentum in tunneling ionization processes.Combined with attosecond angular streaking techniques,it investigates the tunneling ionization processes related to the atomic and molecular structures,achieving ultrafast imaging of these structures.The specific research contents are as follows:（1）First,through experiments measuring attosecond angular streaking with resolved transverse momentum in atomic systems（Ar,Kr,Xe）,this study in-vestigates the three-dimensional tunneling ionization process of electrons within atomic systems,yielding a quantitative reconstruction formula capable of calcu-lating the initial transverse momentum distribution of electrons.This reveals the impact of initial transverse momentum and long-range Coulomb forces on pho-toelectron emission.High-precision temporal resolution measurements of pho-toelectron emission and quantitative reconstruction of the initial transverse mo-mentum distribution have been achieved.Previous studies utilizing attosecond angular streaking to investigate tunneling ionization often relied on simplified two-dimensional models to analyze the actual three-dimensional photoelectron emis-sion process,typically neglecting the dynamical information of electrons along the laser propagation direction.This work experimentally measures the three-dimensional photoelectron momentum distribution of three inert gas atoms（Ar,Kr,Xe）in elliptically polarized laser fields.By conducting transverse momen-tum slices of the three-dimensional momentum distribution,it was found that the angular deviation of photoelectron emission within the laser polarization plane decreases with increasing transverse momentum.Combined analysis with three-dimensional long-range Coulomb-corrected strong field approximation simulations reveals that the transverse momentum-dependent photoionization phenomenon is primarily related to the electron’s initial transverse momentum,the tunneling exit position,and the long-range Coulomb force exerted by the atomic nucleus on the electron.Based on this analysis,a quantitative reconstruction formula for the ini-tial transverse momentum distribution was developed.This research provides a new scheme for enhancing the precision of attosecond time measurements,assess-ing the impact of the tunneling exit position,and the three-dimensional long-range Coulomb forces on the electron dynamics along the laser propagation direction.It also holds promise for further application in the study of chiral molecules,using transverse momentum-dependent photoelectron emission to decode chiral struc-tures.（2）Secondly,research on the molecular attosecond angular streaking ion-ization dynamics of aligned molecules was conducted,extracting photoionization information related to symmetric molecular structures,and achieving molecular orbital structure imaging with attosecond temporal resolution and sub-angstrom spatial resolution.Unlike atoms,molecules exhibit anisotropy in their structure,making their tunneling ionization process more complex.Taking N₂molecules as an example,the tunneling ionization process of N₂molecules in an elliptically po-larized laser field,dependent on molecular alignment,was studied experimentally using molecular alignment techniques.Analysis with the strong field approxima-tion theoretical model revealed that the alignment-dependent characteristics of the tunneling ionization process in N₂molecules are determined by the orbital structure of the N₂molecule.The results indicate that the orbital structure ef-fects of molecules play a significant role in attosecond ionization dynamics and the ultrafast control of internal electronic motion within molecules.Taking H₂⁺as an example,the smallest photoelectron emission offset angle in the alignment depen-dency of H₂⁺was extracted,corresponding to a special molecular alignment angle,which was found to be related only to the internuclear distance of H₂⁺,indepen-dent of the laser field strength and the system’s ionization energy.Based on this discovery,a linear relationship between the special alignment angle and the inter-nuclear distance of H₂⁺was theoretically derived.Utilizing this linear relationship allows for the quantitative acquisition of the internuclear distance information of H₂⁺through an experimentally observable quantity（special alignment angle）,thereby achieving sub-angstrom resolution imaging of molecular structures.（3）Finally,research on orientation-dependent attosecond angular streaking ionization dynamics in polar molecular systems has been conducted,developing a self-referenced attosecond angular streaking technique for polar molecules.This research reveals the impact of the permanent dipole effect of polar molecules on tunneling ionization,extracting information on charge migration and tunneling time under the influence of a laser field.Previous studies on tunneling ionization processes were mainly focused on atomic systems,where the accuracy of exper-imental results relied heavily on the precise calibration of laser parameters.To overcome this challenge,experimental techniques employing ion-electron coinci-dence measurements were utilized,taking the polar molecule CO as an example.This leveraged the temporal resolution advantage of attosecond angular streaking techniques,extracting the electron momentum information emitted from both the C and O ends in a single experimental measurement,to study the asymmetrical tunneling ionization process induced by permanent dipole effects.Experimental results indicate that the deflection angle of electrons emitted from the C end is larger than that from the O end in the molecular coordinate system.Analysis combining numerical solutions of the time-dependent Schr（?）dinger equation and strong field approximation theoretical models,which include permanent dipole moments and Coulomb corrections,found that the presence of CO molecule’s per-manent dipole effect leads to different”dressing”of the polar molecule’s ground state energy by the laser field in the first and second halves of a laser cycle.This ultimately results in asymmetric charge migration and tunneling ionization phe-nomena.This study provides a new perspective for decoding additional tunneling ionization information related to molecular structures from the offset angles of emitted electrons.