| With the development of the intense field, especially the emergence of the ul-trafast femtosecond or attosccond laser pulses in recent year, the researches of pho-toionization and photodissociation via the intense field arc subject to more and more extensive attention. Compared with the study of weak field, the dynami-cal behaviors of atom and molecule excited by the intense field show many new features, such as multiphoton ionization, tunnel ionization, above-threshold ioniza-tion, above-threshold dissociation and molecular-bond softening and stabilization etc.. The time-dependent wave packet method as an important branch of the phys-ical chemistry, has been successfully applied to the theoretical research of the gas phase dynamics, such as the quantum scattering calculation of photodecomposition, elementary reaction and collision preach can, the numerical simulation of the inter-action between laser and material, etc.. The time-dependent wave packet method has many advantages. In addition to the efficient numerical calculation, this method provides definite physical meanings and intuitive image for the dynamic. And it has the intuition of classical mechanics and no lack of accuracy of quantum mechanics. Besides, the time-dependent wave packet method is especially suitable for the study of molecular evolution after exciting by femosccond laser pulse, thus it becomes a powerful tool for the research of molecular photoionization problems.Recently, the time-dependent wave packet method has important application in the fields of chemical reaction, gas-solid surface interaction, photodissociation, photoionization, low temperature physics, semiconductor physics etc.. On the other hand, the emergence of femtosecond laser technology and its application in physics and chemistry objectively have promoted the development of time-dependent wave packet method. Nowadays, people can make use of ultra-short pulse to produce a wave packet on the excited state and detect its evolution process in real-time. This kind of femtosecond experimental requires the simulation by the time-dependent wave packet method. If the theory of wave packet doesn't exist, only the cxperi- mental data can not reveal much problem. The theoretical and experimental de-velopment has formed a new field, wave packets dynamics theory and experiment supplement each other, promote each other and impel the vigorous development of this field.Using the time-dependent method to calculate the interactions between the fem-tosecond laser field and molecular interactions, people realize its advantages to deal with problems of quantum mechanics. Besides, the emergence of FFT and DVR technology makes the calculation of basis functions to be more convenient and pre-cise, time-dependent wave packet method is widely applied to the area of molecular dynamics. The exemplification of handling molecular dynamics problems is simulat-ing molecular reactions scattering process by the application of modern quantum computing method, calculating eigenvalue problems and simulating the dynamic process of wave packets excited by ultrafast femtosecond laser pulse.Nowadays, the theoretical research of photoionization dynamics behavior of molecule excited by intense field mainly focuses on small molecules system. Since Zcwail at al. first applied photoionization technology to Nal molecular system, photoionization technology is becoming effective tools to discuss the dynamics be-haviors on the excited states. The time-resolved photoclcctron spectra can provide with the relevant information on the excited state. Its great advantage is that any excited states can use this technology when the molecule is ionized by laser field. In this thesis, we employ the time-dependent wave packet method and photoionization technology to study the non-adiabatic effects of Nal molecule, Autlcr-Towncs split-ting in photoelcctron spectra of K2 molecule and quantum control of a molecular system in an intense field via the selective population of dressed states.This thesis includes six chapters and our main work is the third chapter, the fourth chapter and the fifth chapter.Chapter I:Introduction. We give a brief introduction to development of fem-tosecond laser technology and its application in recent years, and briefly describe the scope of strong field and physical phenomena caused by strong field. Then, We introduce an important concept in the molecular dynamics field:wave packet. The basic properties of wave packet and the method of wave packet preparation, detection and control arc also briefly introduced.Chapterâ…¡:Time-dependent quantum wave packet method. We first write the timc-dependent Schrodinger equation coupled by the external field. Then, we briefly introduce two common method of numerical calculating the initial wave packet in molecular system. Employing several timc-dcpcndcnt quantum wave packet evo-lution methods, we can calculate the wave packet at any moment and obtain the interested physical quantity.Chapterâ…¢:Non-adiabatic effects of Nal molecule. Nal molecule is a hot topic in the research of strong photoionization, but most theoretical calculation arc based on the adiabatic potential surfaces. Because the dissociative covalcnt state crosses with the ionic state at an internuclear distance of about 7A forming an avoided crossing at this region, there will emerge some phenomenon which could not ob-serve under the adiabatic potential surfaces. Also, we find that the dynamics of Nal molecule calculated by adiabatic potential surfaces can not give a good expla-nation experimental results. According to above some problems, we use accurate timc-dcpcndcnt quantum wave packet to calculate the propagation and splitting of the wave packet after excited by femtosecond pulse. Then, we calculate the photo-clcctron spectra of Nal molecule both on the adiabatic and diabatic potentials. The non-adiabatic effects are expected to play an import role in the photodissociation dynamics of Nal molecule. On the basis of the study of photoionization of Nal molecule, we further explain the source of non-adiabatic effects and briefly discuss the dependance of non-adiabatic effects on the laser wavelength and evolutive time.Chapter IV:Autler-Townes splitting in photoclcctron spectra of K2 molecule. At present, the main research systems of Autlcr-Towncs splitting are atoms, single molecules and quantum dot, only a few experimental studies have addressed the phenomenon in molecular system. Also the research results of Autlcr-Towncs split-ting in molecular system are mainly based on resonant excitation. We theoretically investigate Autlcr-Townes splitting in the photoclcctron spectra of K2 molecule driven by pump-probe pulses via employing the time-dependent wave packet ap-proach. We discuss the influences of the pump laser intensity and/or wavelength on Autlcr-Townes splitting for three case:a resonant laser pulse, near resonant laser pulse and far-off resonant laser pulse. The corresponding features are exhibited differently at varied wavelengths of pump pulses.Chapter V:Quantum control of molecular system in an intense field via the selective population of dressed states. We present a novel strong field multi-photon quantum control scheme in molecular system based on the selective population of dressed states. The four states of K2 molecule arc involved, and we theoretically in-vestigate physical mechanism of quantum control on K2 molecule with an ultrafast strong laser pulse by solving the time-dependent Schrodinger equation exactly using the wave packet approach. The structures of the triple splitting in the 3-photon ionization spectra of K2 molecule arc presented to analyze the information of se-lective population of dressed states. It is found that the tunability of the dressed states energies is achieved by regulating laser intensity and high selectivity of the dressed state population is attained by altering the envelope and wavelength of the intense laser pulse.Chapter VI:Conclusions and outlook. We give a brief conclusions and an outlook also given in this chapter.
|
PDF Full Text Request