Study On The Theories Of Strong-field Ionization Of Molecules

Strong-field ionization has been studied intensively in recent years. In this paper, we focus on the tunnel ionization and multi-photon ionization of molecules, and some researches and calculations on the theoretical models are presented.The atomic tunnel ionization theory (ADK) and its molecular version (MO-ADK) are introduced briefly, which is then applied to the linear molecule CO₂ and non-linear molecule SF6, subjected to a strong, linearly polarized laser pulse. For molecular orbital, we define bonding nodal planes and anti-bonding nodal planes. The ionization in the bonding nodal plane has a larger dependence on the laser intensity. As the field strength gets stronger, the information about the bonding nodal planes gradually disappears. Furthermore, the contributions from the partial waves with higher angular momentum quantum numbers grow as the laser intensity strengthens, which affect the total ionization yields.We present a detailed derivation of the tunnel ionization theory in complex systems (CS-ADK). A few misprints in the previous works have been found and corrected. The theory is then applied to CO₂, showing that CS-ADK yields better agreement with experiment than the MO-ADK theory, on both the total ionization amplitude and the angular distribution. The shortcoming of the CS-ADK theory is that it needs more input parameters, which involves more uncertainties. The discussions on the long-range interaction potential and the angular momentum barrier show that the former has a smaller effect on the angular ionization distribution than the latter. As far as the tensor nature of the polarizability is considered, we introduce the effective polarizability into the CS-ADK theory, which proves to improve the total ionization rates.Next, we investigate the ionization of N2 and CO₂ using the strong-field approximation (SFA) in both the length gauge (LG) and the velocity gauge (VG). Comparisons have been made between the different versions of SFA, the MO-ADK theory and recent experiments. The study shows that the VG SFA tends to underestimate the ionization yields by orders. The VG SFA also displays a larger laser parameter (both intensity and wavelength) dependence than the LG SFA, which is not observed in experiments and first-principle calculations. The LG SFA is more preferable in the description of the strong field ionization of N2 and CO₂. The accordance between the LG SFA and the improved MO-ADK results illustrate the fact that apart from the correctness of the asymptotic behavior, the characteristics of the wave function near the core region plays an important role. In order to explain the ionization of inner-shells of O2, we modify the dipole-moment in the LG SFA formulas. The result is improved, although still displays a derivation from the experiment.At last, outlooks of our future work are discussed briefly.