Theoretical Study On The Ionization Mechanism Of Diatomic Molecules Under The Action Of Strong Laser Fields

Strong laser field induced ionization of atoms and molecules is one of the research hotspots of strong laser field physics.With the development of laser technology,people have been able to perform more complex ma-nipulations,such as the alignment and orientation of molecules.Advances in computing also allow for higher-dimensional simulations.Due to the structural characteristics of molecules,a series of phenomena which are different from atoms appear under the driving of strong laser field.These phenomena have been paid much attention by researchers.Some relevant results have shown that compared with symmetric molecules,the ionization dynamics of asymmetric molecules driven by strong laser field presents many novel phenomena.This thesis focuses on the theoretical study of double ionization of symmetric diatomic molecules,and the single or dou-ble ionization process of asymmetric diatomic molecules exposed to the strong laser field.Firstly,we study the double ionization mechanism of aligned symmet-ric molecules in linearly polarized laser field,which is simulated by solving the time-dependent schrodinger equation(TDSE)of two electronic system in which there are two-dimensional degrees of freedom for one electron.In this thesis,by studying the H2 molecular system,we find that the ori-entation angle(the angle between the molecular axis and the direction of polarization of the laser field)and the intensity of the laser field can both affect the double ionization dynamics.Because of the structure charac-teristics of diatomic molecules,the single ionization rate is always higher in parallel orientation.In the case of low intensity,the double ionization rate of parallel orientation is always smaller,and more non-sequential dou-ble ionization(NSDI)processes are presented in the momentum distribu-tions.As the laser intensity increasing,the structure characteristics of di-atomic molecules with different orientation angles will be weakened by the external laser field,and the momentum distributions trend to be consis-tent for parallel orientation and perpendicular orientation.Our results also show that the transverse motion of electrons is one of the important rea-sons for the occurrence of NSDI in the case of parallel orientation,the two-center interference effect also plays an important role in the orientation-dependent double ionization process,and these results provide theoretical support for the detection of ultrafast double ionization dynamics of orien-tation molecules.Secondly,we extend our study target from symmetric diatomic molec-ular system to asymmetric heteronuclear diatomic molecular system,and study the single ionization dynamics of asymmetric molecules HeH2+in a intense elliptically polarized laser field.We find that by comparing the photonelectron momentum distribution of atoms driven by elliptically po-larized laser field,the momentum distributions of asymmetric molecules present a novel ring structure.The results reveal that there are two differ-ent ionization channels in asymmetric molecular systems,i.e.the ground-state ionization channel and the excited-state ionization channel.The time delay between the two channels will map in the photoelectron momentum distribution driven by elliptically polarized laser field,which leading to the half-ring structure.One can distinguish these two ionization channels by analyzing the half-ring structure of photonelectron momentum distri-butions.We also extend our target from the HeH2+asymmetric molec-ular system calculated by the Born-Oppenheimer(BO)approximation to the non-BO approximation system even other asymmetric molecular sys-tems commonly used in experiments.The relevant theoretical results of these systems show that the time delay between the ground-state ionization channel and excited-state ionization channel of the asymmetric diatomic systems driven by elliptically polarized laser field can be distinguished by observing the photoelectron momentum distribution.Finally,we combine the asymmetric system with the double ionization process to simulate the single and double ionization process of the asym-metric HeH+and its isotope HeT+system exposed to a strong linearly po-larized laser field.The results reveal that the momentum distributions for single ionization always have more amplitude at positive momentum,but the momentum distributions for double ionization have the reverse trend.For both targets,these two electrons in double ionization prefer to release together along the lighter nuclei(H or T)side.At the same time,how-ever,the first electron in single ionization prefers to escape along the heav-ier nuclei(He)side.This potential mechanism is attributed to the inter-play of the rescattering of the first electron and the Coulomb induced large ionization time lag.By comparing the simulation results of different iso-topic molecules,we find that nuclear motion increases the contributions of these two electrons releasing together along the He side.Lighter isotopes are more likely to spread to larger internuclear distance,leading to more probability of sequential ionization(SDI),which eventually maps to the momentum distribution,we can find less asymmetry of momentum distri-bution for lighter isotope molecules.The relevant results also provide a theoretical basis for the study of molecular nuclear dynamics.