Investigations On Ionization Dynamics Of Aligned Diatomic Molecules In Strong Laser Fields

With the development of ultrafast laser technology,investigations on light-matter interaction enter a new research era,i.e.,strong laser field physics.In strong field physics,the interactions of intense laser fields with atoms and molecules lead to a series of interesting ultrafast nonlinear phenomena,such as above-threshold ionization(ATI),nonsequential double ionization(NSDI),high-order harmonic generation(HHG),and so on.These ultrafast phenomena allow one for imaging dynamic processes and structures of atoms and molecules with sub-angstrom spatial and attosecond temporal resolution.Strong field ionization of atoms and molecules is one of fundamental physical processes in intense laser-matter interaction.It serves as the basis for understanding of the complex ultrafast phenomena and plays an important role in ultrafast atomic and molecular imaging.In this thesis,we experimentally and theoretically investigate the ionization dynamics of atoms and molecules,especially for aligned diatomic molecules,with the help of theoretical models and experimental setups like time-of-flight spectrometer and cold target recoil ion momentum spectroscopy(COLTRIMS).The main works and achievements are listed as follows:1.Investigations on atomic ionization in an intense midinfrared elliptically polarized laser field.Investigations of the complex molecular ionization dynamics need a comprehensive understanding of the simple atomic ionization behavior in an intense laser field.Therefore,we first investigate the single ionization of noble gas atoms in an intense laser field.The ellipticity dependence of the monovalent ion yields has been measured at a series of intensities in midinfrared(2000 nm)elliptically polarized laser fields and compared to various theoretical model predictions,i.e.,the Ammosov-Delone-Krainov(ADK),strong-field approximation(SFA),and Coulomb-Volkov distorted-wave approximation(CVA)models.In contrast to the case at 800 nm,the SFA simulations at 2000 nm match with the ADK calculations,indicating the insignificant impact of nonadiabatic effect on ionization rate at midinfrared wavelength.Furthermore,compared to SFA calculations,the CVA simulations for both 2000 nm and 800 nm are in better quantitative agreement with the measurements,which implies the pivotal role of Coulomb potential in strong field atomic ionization rate.2.Investigations on high-order above-threshold ionization of aligned Nz molecules in an intense laser field.We experimentally investigate the two-dimensional photoclectron momentum spectra of aligned diatomic molecules in intense laser field.Our results reveal a novel prominent valley structure in the molecular alignment dependence of the high-energy photoelectron spectra along the laser polarization.Resorting to the molecular strong-field approximation and a simple semiclassical analysis,we show that this valley structure stems from the destructive two-centre interference of the laser-driven rescattered electrons in diatomic molecules.Based on this two-centre interference with aligned diatomic molecules,we demonstrate for the first time a tomographic method to extract the molecular internuclear separation.3.Investigations on low-energy fan-shaped pattern of aligned Na molecules in an intense laser field.We experimentally study the photoelectron angular distributions(PADs)using aligned N2 molecules and focus on a fan-shaped interference pattern near the ionization threshold.It is found that the amplitude of the radial stripes of the fan-shaped pattern is significantly changed with the molecular alignment.In order to understand the alignment effect of the PADs,we develop a molecular quantum-orbit strong-field approximation theory.Using our model,we well reproduce the observed fan-shaped interference patterns.Further analysis reveals that the alignment dependence of the fan-shaped interference pattern is closely related to the sp-mixing electron orbitals of N2 molecules and the relative phase between the electron orbitals.