摘要
在表面物理学中,详细地了解电子在表面的动力学性质对于理解电子在表面的散射、电荷在界面间转移、电子器件设计等过程都会起到很大作用。过去的几十年里,由于短脉冲激光技术的发展,已可能在实验室中制备和检测许多体系的局域波包。半经典闭合轨道理论由于具有物理图像清晰、应用范围广泛等特点被普遍用来解释原子或离子在强外场中的光吸收现象,成为实现联结经典理论和量子理论的重要桥梁,并是研究和发展量子混沌概念的一个典型实例。在过去十几年里,人们已对表面附近光剥离电子进行了广泛研究,不仅从各个角度验证了半经典闭合轨道理论的正确性,而且发展了回归谱学、标度律、能级统计学等方法。本文分别以闭合轨道理论和量子力学方法研究了表面附近光剥离电子的时间演化和波包动力学。
在第一章中,主要从总体上介绍了波包动力学的研究,闭合轨道理论的发展,表面光剥离特性研究的历史背景和意义以及我们所选题的原因和主要工作。
在第二章里,我们应用半经典理论,特别是闭合轨道理论对于静电场中的氢负离子在弹性墙表面附近的自关联函数进行了研究。结果显示较短的激光脉冲,自关联函数有明显的回归峰,并发现回归峰与电子的闭合轨道有明显的对应关系。但是随着激光脉冲宽度的加大回归峰逐渐变宽,由于相邻峰间的干涉效应,这种对应关系最终消失。同样随着电场强度的增加,时间回归谱峰值的大小和数目都增加了许多。
在第三章中,通过量子力学的方法,我们研究了氢负离子的光剥离电子在强电场中弹性表面附近波包的时间演化和量子拍谱,并且分析了电场强度和脉冲初始动量对波包演化的影响。我们计算了包含不同簇量子态波包的时间演化,分析了电场对波包的调控。计算结果表明包含更多态的电子波包会有更剧烈的空间分布变化,随着电场强度的增加会使波包的分布更趋近于表面。本体系中波包的时间演化和双光子光电效应信号模拟值有很好的相似性。
在第四章中,我们应用量子力学理论通过研究静电场中金属表面光剥离电子的自关联函数对波包动力学性质进行了研究。我们给出了不同簇波包的时间演化解析表达式,并考虑了寿命对波包的影响。结果发现通过改变激光中心能量和外场强度可以调节波包的量子相干和时间演化,同时可以看出寿命对波包的影响是非常明显的。在此基础上我们还详细讨论了本体系中量子与经典的对应关系。
在第五章中,我们给出了文章的主要结论,同时从闭合轨道理论和量子力学方法对其它体系波包动力学性质的研究进行了展望。
In surface physics, a detailed knowledge of electron dynamics at surfaces is crucial for an understanding of a large variety of processes, ranging from electron scattering at surfaces to charge transport dynamics across interfaces, relevant to design electronic devices. In recent years, short-pulse tunable laser technology has been developed constantly and one of its physical application is to excite electronic wave packets. Semiclassical closed orbit theory has been extensively used to explain the photo-absorption spectra of atoms in external fields due to its clear physical picture and wide availability. It has been proved to be a powerful tool for studying the quantum manifestations of classical chaos and a typical example for understanding the conception of chaos. During the past few decades, the closed-orbit theory has been successfully applied to photodetached electron dynamics at surfaces, the validity of the closed orbit theory has been fully confirmed, and the recurrence spectroscopy, sealing property and energy statistics have also been developed. In this paper, photodetached electronic wave packet dynamics at surfaces is investigated by use of the closed orbit theory and quantum approach.
In chapter 1, we have a brief introduction of wave packet dynamics and we also introduce the establishment and development of the closed orbit theory collectively. The background of photodecached electronic wave packet dynamics and the reasons why we choose the subject and the main work we have done is also mentioned.
In chapter 2, the time-resolved spectroscopy of negative hydrogen ion in electric field near an elastic surface is investigated in the framework of semiclassical theory. We calculate the autocorrelation function of the first several closed orbits at a fixed scaled energy and analyze the influences of the laser pulse and the orbit bifurcation on the autocorrelation function. The results show:as the applied pulse width is very narrow, the reviving peaks in the time-resolved spectrum can be attributed to the closed orbits of electrons. With the increase of pulse width, the adjacent peaks mutual influence and merge. Furthermore, with the increase of field strength, the number and amplitude of peaks in the time resolved spectrum increases obviously.
In chapter 3, the quantum dynamics of the photodetached electron of H- in electric field near a surface are studied in the time domain. The evolution of wave packet for different manifold eigenstates with limited lifetimes is obtained analytically. We also discuss the influence of laser central energy and external field on the quantum coherence and temporal evolution of surface electronic wave packet. We demonstrate that the stronger field makes the electron has a high probability close to the surface. Numerical simulation shows that the temporal evolution of photodetached electronic wave packet on the surface exhibits some similar properties of time-resolved two photon photoemission (TR2PPE) signal of surface electron.
In chapter 4, the quantum dynamics of electron in a surface quantum well is studied in the time domain with autocorrelation of wave packet. The evolution of wave packet for different manifold eigenstates with finite and infinite lifetimes is investigated analytically. It is found that the quantum coherence and evolution of surface electronic wave packet can be controlled by the laser central energy and electric field. The results show that the finite lifetime of excited states expedite the dephasing of the coherent electronic wave packet significantly. The correspondence between classical and quantum mechanics is shown explicitly in the system.
In chapter 5, we give some important conclusions of this thesis and make a brief illustration to some problems which can be handled by semiclassical and quantum approach.
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