Study On The Low-order Harmonic Generation Of Lithium Atom Induced By Laser Field

High order harmonic is the broadband light radiation released by the interaction between atoms(molecules)and strong laser field,it can not only obtain the extreme ultraviolet light source and X-ray source in the order of attosecond,but also synthesize the ultrashort attosecond.Through ultrashecond pulse can be detected and manipulated the internal structure of atoms(molecules)and the dynamic ultra-fast process of electrons.Since ionized atoms(molecules)are in an extreme environment,So there's no way to explain the physical phenomenon by perturbation theory.At present,the radiation characteristics of high order harmonics are mainly studied by numerically solving timedependent Schr?dinger equation.The strong field approximation ignores the effect of coulomb potential on the bound states and cannot be used to explain the physical process of low order harmonics.The semi-classical three-step model can perfectly explain the physical mechanism of harmonic emission above the threshold and near the cut-off region,but it cannot precisely explain the emission characteristics of harmonics near and below the threshold,This is due to the contribution of low order harmonics is closely related to both bound and continuous states,it involves multiple scattering of the parent ion,the emission process is more complex than the platform and cut-off region.So we need to study the low order harmonics more carefully and deeply,which is of great significance for the generation of high frequency ultra-violet frequency combs.The main content is as follows:In this paper,the time-dependent Schr?dinger equation of lithium atom driven by laser pulse is solved by using the precise model potential of lithium atom in the singleelectron approximation,and the low-order harmonic spectrum of lithium atom below the ionization threshold is studied by using the time-dependent generalized pseudo-spectral method.In addition to the usual odd harmonics,the low order harmonics of lithium atoms also have some satellite-peak structures,the mechanism of generating these satellite-peak structures is analyzed in detail by using the synchronous compression transformation technique(SST)with high calculation accuracy.The analysis results show that through SST time-frequency analysis technology,we can identify two types of low-order harmonic satellite-peak structures of lithium atoms: One is caused by the transition between the dressed states of the bound states of atoms in the laser field,this part of the harmonic peaks(satellite-peak)will ends with the end of the laser pulse,which includes the instantaneous Stark shift in the bound states.The other kind of satellite-peaks includes not only the contribution of the transition between these dressed states,also contains no field the contribution of the excited states to ground states transition of the atom,and and this part of the satellite-peaks even though the laser pulse is over,atoms still have large population Numbers in some excited states,so there's still the emission of harmonics,using the satellite-peaks of this feature on peaks,we can through calculating the energy difference between the main and satellite-peak to estimate the largest Stark shift of the atomic excited state in laser field.