Structure-dependent Strong-field Nonlinear Effects

Tunneling ionization has always been a very important and concerned topic in the field of basic research in ultrafast optical physics.The electronic tunneling ionization image in an atom is that electrons are directly released from the parent ion to a continuous state.Compared with the simple case of atoms,the tunneling ionization phenomenon in molecules is more interesting.So far,the influence of neighboring ion nuclei in asymmetric molecules on the electron tunneling process is still unclear.After electron tunneling and ionization reach the continuous state,it may be re-trapped by the parent ion,which will cause the nonlinear effect of high harmonic generation.The research on the generation of high-order harmonics also continuously involves various structural systems,from the atomic to the molecular and even the solid structure system.For the above content,the main research work in this paper includes:1.As compared to the intuitive process that the electron emits straight to the continuum from its parent ion,there is an alternative route that the electron may transfer to and be trapped by a neighboring ionic core before the eventual releasing.We used an Ar-Kr~+ion as a prototype system to track the electron tunnelling dynamics from the Ar atom towards the neighboring Kr~+,which is temporally captured into the resonant excited states of the Ar-Kr~+before its eventual releasing.We analyzed the role of adjacent ion nuclear coulomb potential in the process of tunneling,founding that the different capture mechanism of electronic distribution can be reflected in different angles and different transverse momentum distributions within the polarization plane.The influence of Coulomb potential of neighboring ionic cores provides new insights into the understanding and controlling of tunnelling dynamics in the complex system.2.The high harmonic spectrum,generally,only contains the odd orders or both even and odd,a spectrum containing pure even harmonics is extremely rare.Using TDDFT(time-dependent density functional theory)method and quantum chemistry software Octopus,we studied the high order harmonics generated from CO molecule and NO molecule.We found that when the polarization of the laser is perpendicular to the molecular axis,the harmonic intensity produced from NO molecule in the polarization direction of laser field is lower than that produced from CO molecule,in the direction perpendicular to the laser polarization,the pure even harmonic intensity of NO molecule is slightly higher than that of CO molecule.It is found that these phenomena are caused by the influence of the molecular orbital wave function and the permanent dipole moment.This discovery is of great significance for further understanding the phenomena of high order harmonic generation in molecules and studying the extreme nonlinear effects.