Studies On The Ultrafast Molecular Ionization Dynamics With Cold Target Recoil Ion Momentum Spectroscopy

Investigation of the interaction between matter and laser field is an important method to study the physics of the microscopic world.For atoms and molecules subject to strong femtosecond laser fields,a fundamental process to occur is ionization,which may further induce a diverse range of highly nonlinear phenomena,such as above-threshold ionization,nonsequential double ionization,and high-order harmonic generation etc.Through the study of these phenomena,we can deepen the understanding of the atomic and molecular structure and its ultrafast dynamics.Recently,the ionization suppression of a diatomic molecule,which means that the ionization rate of the molecule is found to be lower than an atom with a comparable ionization potential(called a companion atom),has attracted a lot of attention.Several physical effects are suggested to explain this phenomenon,such as diatomic molecular two-center interference effect,charge-screening effect,nuclear vibration effect,and different asymptotic behaviors of the ground-state wave functions etc.Experimental investigations have been performed to justify the effects mentioned above and great attention has been paid to the one based on the S matrix,i.e.,the two-center interference effect.However,consensus has been reached so far.To shed more light on the physics behind the molecular ionization suppression,in this thesis,we investigate the photoelectron momentum distributions of diatomic molecules subject to close-to-circularly polarized laser fields with a home-made cold target recoil ion momentum spectroscopy(COLTRIMS).The significant role played by the symmetry of the molecular ground-state wave function has been revealed.The main contributions are described as follows:Experimentally and theoretically,we explore the photoelectron momentum distributions of N2 and O2 and their companion atoms Ar and Xe,which possess nearly identical electron binding energies to the corresponding molecules,subject to close-to-circularly polarized laser fields.Compared to Xe,significant deviations of the results for O2 have been observed experimentally.In the meantime,the measured photoelectron momentum distributions of N2 match those of Ar.With the theoretical analysis based on strong field approximation,the results that N2 behaves just like its companion atom Ar and O2 behaves significantly different from Xe have been attributed to the two-center interference effect of diatomic molecules,which is disparate for N2 and O2 due to the parity difference of their molecular ground-state wave functions.