Investigated The Molecular Ionization And High Harmonic Generation By The Time-dependent Density Functional Theory

The high-order harmonic generate?HHG?from the molecules irradiated by strong laser field.The harmonic emission can be taken as a light source of the extreme ultraviolet and soft X-ray.The other application of the harmonic is a tool to generate ultrashort pulse whose duration is in attosecond scale.The research on the high-order harmonic has the important scientific significance and practical application value.In the study of the HHG,the multi-electron effect play important role.Due to the limitation of computational resources,the theoretical study on the interaction between the complex molecules and intense laser face great challenge.The time-dependent density functional theory?TDDFT?approach provides a way out of this dilemma.The TDDFT approach needs fewer sources and has higher computational efficiency.This theoretical scheme has been applied on the research of the atoms and molecules in strong laser field.By using the TDDFT approach,we systematically investigate the ionization and HHG of the one dimensional H₂ molecule.It is found that with the increase of the laser intensity,the molecular ionization is gradually increased and the cut-off energy of the harmonic spectrum becomes larger.The cut-off energy can be predicted by the semi-classical three step model.By comparing the results calculated from the numerical solution of the time-dependent Schr?dinger equation?TDSE?,the ionization probability from the TDDFT calculation is greater.The reason for this difference is the ionization energy calculation of two methods.For the same potential parameters,the calculated ionization energy from the TDDFT is higher than the TDSE.Thus the cut-off energy of harmonic spectrum calculated from the TDDFT is lower.The time-frequency profile of the HHG and the attosecond generation process are investigated.The instant profile of the HHG is agree well with the result from the TDSE.Basis on the results from above calculation,the TDDFT is shown to be effective on the study the interaction between the molecule and strong laser field.On the above basis,we investigate the HHG and molecular ionization of O₂ and N₂ molecules by using TDDFT scheme.The calculation results show that the molecular ionization is affected by the ionization energy of molecular orbital,the electronic distribution of the molecular orbital and the angular between the molecular axis and the polarization direction of the laser electric field.For the same symmetry of the molecular orbital,the ionization probability is smaller for the higher ionization energy.For the electronic orbital which have the same ionization energy,the ionization probability is large for the electronic orbital whose electron distribution along with the polarization of laser field.The effect of ionized electron motion on the high-order harmonic is investigated.It is found that the intensity of N₂ molecule HHG is higher than the O₂ molecule.The emission intensity difference between molecules can be attributed the dynamic of ionized electron in the strong laser field.