Ionization Of Atoms In A Strong Laser Field

With the rapid development of ultrashort and ultrastrong laser technology, the research on the interaction between the laser pulses and atoms has attracted extensive attention. Some strong-field physical phenomena have been observed, including high-order harmonic generation(HHG), above-threshold ionization(ATI), non-sequential double ionization(NSDI) and so on. Because the ionization of atoms in a strong laser field is the basis of all subsequent physical processes, the research on ionization of atoms is of great significance. Therefore, we theoretically investigate the ATI process and double ionization process of atoms in a strong laser field. The main contents of this thesis are as follows:Firstly, we investigate the ATI process of an atom by numerically solving the one-dimensional(1D) time-dependent Schr?dinger equation(TDSE). We investigate the influence of the long-range and short-range potentials on the plateau structure of the atomic ionization spectra. It is found that, under the same incident laser intensity, the ionization spectra of the atoms always exhibit a clear double-plateau structure in the long-range potential, and the intensity difference between the two plateaus in the short-range potential case is less than that in the long-range potential case. In addition, with the decrease of incident laser intensity, the ionization spectra of the atoms change from double plateaus to single plateau gradually in the case of the short-range potential. We find that the reason causing the difference of plateau structure between the short-range and long-range potential cases is as followed: the ionization probability of electron in a short-range potential is less than that in a long-range potential under the same incident laser intensity; while, the rescattering cross-sections of an electron in a short-range potential is greater than that in a long-range potential under the same ionization probability.Secondly, we investigate ATI process using the time-dependent pseudo-spectral scheme by numerically solving the three-dimensional(3D) TDSE of a single-electron atom in the momentum space. First of all, we investigate the photoionization processes of excited atoms under high-frequency laser pulses. By calculating photoelectron energy spectrum and two-dimensional momentum angular distribution spectrum near the ionization threshold, it is found that the principal quantum number of the initial state wave function can be determined by the position of the first peak in photoelectron energy spectrum and the angular quantum number of the initial state wave function can be determined by the angular distribution of the two-dimensional momentum of the photoelectron. By changing the laser parameters, we found that this rule does not change with the intensity and duration of the incident laser pulse. Therefore, we can utilize these ionization signals of high-frequency laser pulse to identify initial state wave function of an atom. This scheme may provide a new method to solve the imaging problems of the atomic wave function. Next, we discuss the photoelectron spectrum in different incident laser intensity. We find that, with the increase of the incident laser intensity, each ATI peak on the energy spectrum changes from a smooth single peak into a multi-peak structure, and if we further increase the intensity of the incident laser, each ATI peak with multi-peak structure changes back to a single peak again. These calculation results agree well with the experimental results. By analyzing the population of atomic bound state, we find that it is the multi-photon excitation of bound state that leads to the occurrence of this phenomenon. Finally, we discuss the dependence of the photoelectron energy spectrum and the angular distribution of the photoelectron energy on the laser intensity and the laser’s carrier envelope phase(CEP) under the Freeman resonant condition. We find that, if the electron is ionized from the ground state, the ATI peak position moves one pU(pU is the pondermotive energy) with the increase of incident laser intensity; and if the electron is ionized from a higher excited state, the ATI peak position does not move with the increase of incident laser intensity. The reason is as followed: for the ionization process of an electron from the ground state, the energy level of the ground state moves little due to the larger ionization potential, while, the energy level of the final continuum state moves one pU. Therefore, the ATI peak position moves with the increase of the incident laser intensity. For the ionization process of an electron from a higher excited state, both energy levels of the initial state and the final state interacting with the laser field moves one pU. Therefore, the ATI peak position does not move with the increase of incident laser intensity. Furthermore, we investigate the influence of the CEP of the laser pulse on the photoelectron energy spectrum. The results show that the ionization probability of photoelectron, which is near the Freeman resonant position on the photoelectron energy spectrum, changes with the CEP of the laser pulse. By analyzing the contribution to each partial wave of the photoelectron spectrum, we can find the partial wave which obviously affects the photoelectron spectral intensity. By utilizing the information of angular distribution of photoelectron, we may provide a new method to detect the CEP value of the multi-cycle laser pulses.Thirdly, based on the B-spline theory, we solve the two-electron TDSE and investigate two-photon double ionization of helium in an XUV laser pulse. First of all, we check the validity of the calculation method in an intense XUV laser pulse. Next, we investigate the influence of laser pulse duration on the electron energy spectrum. We find that, with the increase of the laser pulse duration, energy spectrum changes from single peak into double peaks. By analyzing the double-ionization dynamics process in a long laser pulse, we find that the interaction between the two electrons leads to the formation of the double peaks in the ionization process. Finally, we discuss the ionization processes of the helium with the excited state 1S2 S and 1S2 P as the initial states.