Study on the low-order harmonics generation for below ionization threshold of lithium atom in laser field

By using the precise model potential of lithium atom with the single-active-electron approximation, the properties of low-order harmonics below ionization threshold emitted by lithium atoms in the laser field are studied by numerically solving the three-dimensional time-dependent Schroedinger equation. Our results show that for the low-order harmonics below the ionization threshold of lithium atom, in addition to the usual odd harmonics, there are also side-peak structures. The radiation characteristics of these low-order harmonics are analyzed by using the synchronous squeezing transform (SST) techniques. We found that these side-peaks are composed of the transition between the dressed states of atoms in the laser field and the transition from the excited state to the ground state in field-free case. If there is still a radiation for the side-peak after the laser pulse ends, the energy difference between the side-peak position and the main peak can be used to estimate the maximum Stark shift for the excited state of atom in laser field. (authors)