【作者】 姚洪斌；

【导师】 郑雨军；

【作者基本信息】 山东大学 ， 凝聚态物理， 2011， 博士

【副题名】含时量子波包法

【摘要】 随着强场技术的发展,特别是近年来超快飞秒和阿秒脉冲激光的出现,利用强激光脉冲场研究分子的动力学特性,尤其是分子在强场下的光电离和解离,受到愈来愈广泛的重视。相对弱场而言,强场下原子和分子的动力学行为表现出许多新的特征。例如,多光子电离、隧道电离、阈上电离和解离以及键的软化和硬化,等等。含时量子波包法作为物理化学的一个重要理论研究方法,已经成功应用于分子动力学诸多领域的理论研究中,如光解反应、基元反应和碰撞传能等的量子散射计算,以及激光与物质相互作用的数值模拟等等。含时量子波包法有许多优点,除了数值计算上的高效外,该方法还为动力学提供了物理意义明确而直观的图像,它即具有经典的直观性,又不缺乏量子力学的准确性。而且,含时量子波包法尤其适用于研究体系随时间演化的问题,因而是研究分子动力学问题的强有力工具。目前,含时量子波包法在研究化学反应、光电离、光解离、气一固表面相互作用、低温物理、半导体物理等诸多领域均有着重要应用。此外,飞秒脉冲激光技术的出现及其在物理学和化学中的广泛应用在客观上推动了含时量子波包法的发展。飞秒脉冲激光场的出现,使人们实现了利用超短脉冲场在分子的指定激发态上制备一个量子波包并实时探测它的演化过程,这类超短脉冲实验需要采用含时量子波包法进行理论模拟。离开含时量子波包法的理论模拟和计算,仅有实验数据并不能说明太多的物理问题,而且我们也无法得到有关分子在中间过渡态的直观的动力学图像。所以说,超短脉冲实验与量子波包动力学理论相辅相成,互相促进,共同推动了分子动力学领域的蓬勃发展。自从含时量子波包法被应用于模拟飞秒激光场和分子的相互作用后,人们逐渐意识到含时方法在处理量子力学问题上有许多的优点,加之快速傅里叶变换(FFT)和离散变量表示(DVR)方案的出现,使得量子基函数的计算更加简洁、精确,含时量子波包方法迅速在分子动力学领域有了广泛的应用。应用现代量子计算方法模拟多原子分子反应散射过程,计算多原子分子本征值问题,模拟分子由超短脉冲场所激发的动力学过程,是含时量子波包法处理分子动力学问题的典范。目前,分子在强场下的光电离和解离动力学行为的理论研究主要集中在一些小分子体系上。自从Zewail等人首次把光电离技术应用于NaI分子体系,光电离技术就迅速成为研究电子激发态包括无荧光辐射态的有效工具。分子的光电子能谱可以提供中间激发态的动力学信息,其最大的优点是适用于任何能够被激光场电离的激发态分子。本论文利用飞秒光电离技术和含时量子波包法成功地研究了Nal分子的非绝热效应、强飞秒激光场下K2分子光电子能谱的Aulter-Townes分裂现象,以及强场调控分子体系的缀饰态选择分布。论文共包括六章,我们的主要工作放在第三章、第四章和第五章。第一章：引言。简要地回顾了近些年来飞秒激光技术的发展和应用,并阐述了强场的范围以及强场引起的动力学现象。随后,我们引出了分子动力学领域的一个重要的概念：波包。对波包的基本性质和波包的制备、探测以及控制的方法做了简单介绍。第二章：含时量子波包法。首先从波恩-奥本海默近似出发,写出外场耦合下的含时薛定谔方程。随后介绍了两种常用的数值方法计算分子的初始波包。在此基础之上,利用几种常用的时间传播方法求得初始波包经过演化后的任意时刻的波包,并给出我们所感兴趣的物理量。第三章：NaI分子的非绝热效应。NaI分子是研究小分子在强场下光电离的热点,但是大多数理论计算都是基于绝热势能面做的动力学计算。由于NaI分子的离子态和共价态之间存在非绝热交叉,分子会出现一些在绝热势能面下无法观测到的动力学现象。根据大量的文献调研,我们发现在绝热势能面下计算的NaI分子动力学信息并不能很好的模拟和解释实验结果。针对上面存在的问题,我们利用精确的含时量子波包法理论研究了NaI分子被飞秒脉冲激光场激发后波包的传播与分裂,并分别计算了NaI分子在绝热和非绝热势能面下电离的光电子能谱。在此基础之上,我们对非绝热效应的来源做了进一步的解释和说明,阐述了一下我们自己的观点。最后,简单地讨论了非绝热效应随脉冲激光场的波长以及演化时间的变化关系。第四章：K2分子光电子能谱的Autler-Townes分裂研究。Autler-Townes分裂现象是分子动力领域的一个热点研究课题,但是其主要研究对象都是针对原子、单分子和量子点系统,然而对分子体系的研究报道主要集中在实验上,并且给出的结论都是基于共振激发的情况。我们利用含时量子波包法理论计算了K2分子系统在强飞秒激光场作用下的光电子能谱,发现了光电子能谱的Autlcr-Towncs分裂现象。在此基础之上,我们研究了K2分子光电子能谱的Autlcr-Towncs分裂随泵浦激光场的强度和波长的变化关系。其中,重点讨论了三种情况下的Autlcr-Towncs分裂：共振激发、近共振激发和远共振激发。在这三种情况下,K2分子光电子能谱表现出来的动力学特性存在很大的差异,本章将给出详细的讨论和解释。第五章：分子系统在飞秒强场下的量子调控：缀饰态选择性分布。本章提出了一种全新的量子调控分子系统的缀饰态能量和分布的方法。K2分子作为理论研究体系,该分子在一束超快强场作用下发生三光子电离,我们利用含时量子波包法计算光电子能谱的triple分裂。根据光电子能谱和缀饰态的对应关系,并通过分析光电子能谱的构型,我们可以得到K2分子缀饰态选择性分布的信息。于此同时,我们通过调节激光场的强度来实现缀饰态能量的调控,通过改变激光场的包络形状和波长来高效地调控缀饰态选择性分布。第六章：总结和展望。对研究工作做了简单的总结,提出了利用含时量子波包法处理分子反应动力学以及量子态调控问题的下一步发展,展望了含时量子波包法在三原子分子体系和光电子成像技术中的应用和发展。更多还原

【Abstract】 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.ChapterⅡ: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.ChapterⅢ: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.更多还原