Time-dependent Wave Packet Study Of Multiphoton Ionization For NH₃ Molecule In Strong Laser Field

Molecular reaction dynamics studies the microcosmic dynamics and principle of the chemical reaction, which is based on atomic and molecular level. It can elucidate not only structures, characters and functions of all kinds of transient species during reaction, but also intrinsic law of chemical reaction from the research on state-to-state reaction dynamics and the interaction of coherent states. The wave packet dynamics is an important branch of physical chemistry. Its theoretical framework has many applications in molecular physics and the field-matter interaction systems.The appearance and development of femtosecond laser technique, especially the application in physics and chemistry, promote the development of wave-packet dynamics objectively.The photoionization and photodissociation of molecules in strong laser field have attracted much interest in recent years. When an atom or molecule is subjected to an intense external field, some new phenomena will appear. Now the theoretical research of photoionization dynamical behavior is mainly focused on small molecules. The introduction of the appropriate computational approaches to the dynamics of wave-packets on this system is very important for understanding the treatment of complex systems. Besides, it has theoretical and practical instruction for chemical reaction controlling.Scince P. Johnson applied multiphoton ionization technique to the molecule system, molecular resonance enhanced multiphoton ionization(REMPI) has been achieved. This technique is an effective tool for studying electronic excited states including radiation states without fluorescence. The resenance enhanced multiphoton ionization photoelectron energy spectroscopy of the molecules can provide informations about intermediate excited states. It's best excellence is that all of the electronic excited states ionized by laser can be studied. In this thesis, we research the 2+2 multiphoton ionization photoelectron spectroscopy of NH₃ molecule in strong laser field. The main work is given as follows.(1) Firstly, we calculate two-dimension analytic potential surfaces involving (X|⁾ , (A|⁾ and (X|⁾⁺ state. We also compute the transition moment for the (X|⁾ -(A|⁾transition and (A|⁾-(X|⁾⁺transition. At the same time, we use the mass-weighted coordinate to simplify the expression of kinetic energy operator in the time-dependent Schr?dinger equation. Then we can write the analytic formula for potential surfaces and correlative transition dipole moment of NH₃ molecule in this system.(2) Using discrete variable representation scheme and split operator method to calculate the hamiltonian equation, and then obtain 2+2 multiphoton ionization photoelectron energy spectroscopy of NH₃ molecule. The simulated photoelectron spectrum fairly well represented the experimental result. We also research the potential energy surface of excited stateΑchanging with laser intensity.(3) The effect of laser fields on the NH₃ molecule interaction potentials is obtained through analyzing the photoelectron energy spectrum. It shows that different laser intensity can effect the position of NH₃ molecule potential energy surfaces and then induce the change of intensity for photonelectron energy spactrum. The conclusion may help to understand the dynamics behavior of the molecules in strong laser field, and the idea may be instructive for chemical reaction controlling.This submitted work is divided into five chapters. The first chapter is the introduction, which briefly discusses the basic theory of molecular reaction dynamics. Also the development of the wave-packet dynamics is presented. Chapter two introduces the process of photoionization and the development of resonance enhanced multiphoton ionization technique. In chapter three, theories, notions and computational methods used in the simulation process are presented. In chapter four, the 2+2 multiphoton ionization photoelectron energy spectroscopy of NH₃ molecule is given. The effect of laser fields on the NH₃ interaction potentials is obtained through analyzing the photoelectron spectrum. The last chapter is our conclusion.