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Time-dependent Wave Packet Study of Multi-photon Ionization for Small Molecules in Strong Laser Field

Author: LiuChunHua
Tutor: ZhangQingGang；MengQingTian
School: Shandong Normal University
Course: Atomic and Molecular Physics
Keywords: Time-dependent wave packet method photoelectron spectrum multi-photon ionization auto-ionization strong field effect
CLC: O437
Type: Master's thesis
Year: 2006
Downloads: 58
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Abstract


The photoionization and photodissociation of molecules in strong laser field haveattracted much interest in recent years. When an atom or molecule is subjected to anintense external field, some new phenomena will appear. The wave packet dynamics is animportant branch of physical chemistry. In the molecular collision study, it has beenproved to be very effective. Its theoretical framework has many applications in molecularphysics and the field-matter interaction systems. The time-dependent wave-packet methodis very attractive. Besides the numerical efficiency, time-dependent wave-packet methodis conceptually simple. It provides a classical-like interpretation and also the quantumprecision. And besides, the wave-packet method is very applicable in dealing with theevolution of the system.Recently, in the theoretical research fields of photoionization dynamical behavior,much attention is focused on small molecules. In the theoretical treatment of singlemolecule dynamics, one spatial dimension and two-state system is the simplest one. Theintroduction of the appropriate computational approaches to the dynamics ofwave-packets on this system is very important for understanding the treatment of complexsystems. Besides, it has theoretical and practical instruction for chemical reactioncontrolling.Resonance Enhanced Multi-photon Ionization Photoelectron Spectroscopy (REMPIPES) is an excellent method in the studies of the excited states of small molecules. In thisthesis, we study the multi-photon ionization time-resolved photoelectron spectroscopy ofRbI molecule in strong laser field. The “split operator-Fourier transform” method ofquantum mechanics time-dependent wave packet propagation is employed in thesimulation process. The main work is given as below.(1) Firstly, we introduce the theoretical model of the 1+3 multi-photon of RbImolecule, and give the time-dependent Schr?dinger equation of the simulation process.Because two ion states are involved in this ionization process, both the normalthree-photon ionization process and auto-ionization process are considered in thesimulation.(2) The wave-packet is propagated using the “split operator-Fourier transform”method. The time-resolved photoelectron spectrum of RbI molecule 1+3 multi-photonionization is simulated using the time-dependent wave-packet method. Because twoionization process are involved in the simulation, the photoelectron spectrum is composedof two signals.(3) The effect of laser fields on the RbI interaction potentials is obtained throughanalyzing the time-resolved photoelectron spectrum. The calculated results show that thechange of delay time will influence both the position and the intensity of thephotoelectron spectrum. This influence is substantially due to the existence of the crossingbetween excited states and the strong laser field which will change the position of relevantcurves. The change of delay time may influence the ionization ratio to some extent, andthe idea may be instructive for chemical reaction controlling.This submitted work is divided into five chapters. The first chapter is theintroduction, which briefly discusses the basic theory of molecular reaction dynamics.Also presented is the development of the time-dependent wave-packet method. Chaptertwo introduces the resonance-enhanced multi-photon ionization photoelectronspectroscopy technique and the photoelectron spectrum conception. In chapter three, thetheoretic and computational methods used in the simulation process are presented. Inchapter four, the theoretical model of RbI 1+3 multi-photon ionization is given. The“split operator-Fourier transform” method of time-dependent wave packet propagationis employed to reproduce the experimental obtained MPI PES of RbI molecular indifferent pump-probe delay time. The simulated photoelectron spectra fairly wellreproduced the experimental result. The effect of laser fields on the RbI interactionpotentials is obtained through analyzing the time-resolved photoelectron spectrum. Thelast chapter is our conclusion.

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