High-Precision Measurement Of Photoemission Time And Position In Molecular Strong-Field Tunneling Ionization

Tunneling ionization is one of the most important processes in strong-field optical physics.The studying of this process is an important basis for understanding the mechanism of interaction between strong laser fields and matter.Using tunneling ionization of atoms and molecules in strong laser pulse,one can not only generate ultrashort attosecond laser pulses,but also achieve high-resolution imaging of electron and nuclear motions inside atoms and molecules.Compared to atoms,the anisotropic potential of molecules leads to more complex molecular tunneling ionization than atoms.People lack a deep understanding of molecular tunneling ionization,and the accurate description and characterization of the electron wavepacket distribution of molecular tunneling ionization has not yet been achieved.In this paper,we present an investigation on tunneling ionization of laser-aligned molecules using the non-adiabatic molecular alignment technique,revealing the important role of the highest occupied molecular orbitals on the distribution of molecular tunneling electron wavepacket.By combining the molecular alignment technique with the attoclock and the attosecond photoelectron holography technique,respectively,the high-precision measurement of photoelectron emission time and emission position in molecular tunneling ionization is achieved,which is important for the high-resolution imaging of atomic and molecular structures and their dynamic processes.The research contents of this paper are as follows.(1)The high-precision resolution of molecular orbital shapes is achieved using photoelectron momentum distribution generated by elliptically polarized lasers.In previous studies,the identification of different molecular orbitals by linearly polarized laser was often used,which required large differences in the shapes of these molecular orbitals.It is difficult to distinguish molecular orbitals with similar shapes from the photoelectron momentum distribution obtained by linearly polarized lasers.In this paper,we measure the photoelectron momentum distribution of two molecules with similar molecular orbitals,i.e.,O2 and CO2 molecules,in the elliptically and linearly polarized laser.We find that the two molecules can be clearly identified from the photoelectron momentum distribution in the elliptically polarized laser field,while it is difficult to distinguish in the linearly polarized laser field.Through classical trajectory analysis,it is found that the difference between the two molecular orbitals is masked by the strong Coulomb focusing effect in the linearly polarized laser field.The Coulomb focusing effect on photoelectrons can be reduced in the elliptically polarized laser field,thus the accuracy of molecular orbital imaging can be improved.By analyzing the momentum distribution of photoelectrons,it is found that the momentum distribution in the elliptically polarized laser field carries the information of the photoelectron emission time,which provides the basis for the accurate measurement of the electron emission time.(2)Combining molecular alignment with attoclock techniques,the attosecond precision measurement of the photoelectron emission time of N2 and CO2 molecules in tunneling ionization was achieved.Accurate information of photoelectron emission time is very important for understanding many phenomena in strong field physics.In this paper,the alignment directions of N2 and CO2 molecular axes are continuously scanned by changing the polarization directions of the aligned lasers.Based on the attoclock principle,the measurement method of molecular attoclock is developed,and the attosecond precision measurement of photoelectron emission time under different molecular arrangement directions is achieved.We find that the photoelectron emission time has a temporal offset of tens of attosecond with respect to the peak of the laser field,and this offset depends on the molecular alignment direction and the molecular orbital structure.The extracted photoelectron ionization time shows a significant disagreement with the prediction of the molecular Ammosov-Delone-Krainov model.(3)Combining molecular alignment and strong-field photoelectron holography techniques,the picometer precision measurement of the photoelectron emission position of N2 molecule in tunneling ionization was achieved.Previous studies on tunneling ionization mainly focused on the distribution of electron wavepackets at the tunneling exits,but did not pay attention to a more basic problem,that is,where the tunneling photoelectrons are emitted from inside atoms and molecules.This position is of great significance to the study of tunneling ionization non-adiabatic effect and photoelectron interference.In this paper,we designed an experimental scheme of photoelectron holography in orthogonal two-color laser field.The change of photoelectron holographic structure of N2 molecule with the relative phase of orthogonal two-color field is experimentally measured for two alignment cases with molecular axes parallel and perpendicular to the tunneling direction.Using the quantitative correspondence between the photoelectron emission position and the change of holographic interference fringe,the high-precision measurement of the photoelectron emission position is achieved in N2 molecule.The experimental result shows that the photoelectron emission position of N2 molecule depends on the molecular alignment direction in tunnelling ionization.When the molecular axis is aligned along the tunneling direction,the photoelectron emission position is about 95 picometer away from the geometric center of the molecule.By means of attosecond photoelectron holography,the spatial information of molecular interior at the scale of picometer is extracted experimentally.We for the first time demonstrate that picometer-resolved spatial information can be experimentally extracted using the strongfield photoelectron holography technique.