Strong Field and Ultrafast Atomic and Molecular Processes

本项目侧重于从理论上理解强相干激光辐射与电子、原子和分子的相互作用，并研究使用超快激光或电子脉冲对原子和分子中随时间变化的电子过程进行成像。这个项目支持的基础研究对我们在原子尺度上控制物质的方法的理解作出了广泛的贡献。涉及电子超短脉冲的研究可能导致解决原子和分子中电子在时间和空间上运动的方法。通过“观察”电子的运动和与光的相互作用，科学家们将能够更好地控制电子的运动，从而在提高太阳能转换和光合作用等关键过程的效率，以及将光学和电子过程（例如，新兴的光电子学新领域）结合起来制造更快的开关，从而使更快的计算机成为可能的重要应用。对提高高次谐波强度的方法的分析可能有一天会导致相干x射线的来源，从而为可视化活的生物结构以及纳米级材料结构提供了一种新的手段。参与该项目的研究生和博士后研究人员将获得理论原子物理学方面的广泛教育，在科学传播的各个方面获得第一手经验，并在10大土地赠款大学教授本科生。项目成果不仅发表在领先的物理期刊上，并在国内和国际会议上展示，而且还定期在PI和合作者撰写的评论文章中提取和整合其他人的相关工作。过去所有参与该项目的研究生和博士后研究人员都受到了各种其他雇主的追捧，包括技术公司、医学研究人员和其他领先的AMO理论团体。这个项目中包含的过程很难从理论上处理，因为电子与强激光场以及原子和分子势的相互作用很难准确描述；此外，它们与超短激光或电子脉冲的相互作用必须描述为时间相关的。这个小组已经发展了一些理论方法来克服这些困难。特别地，他们基本上准确地解决了短范围势弱束缚电子与强激光场相互作用的问题，并且对这种特殊系统所得到的结果可以准确地推广到其他原子和分子系统的情况。他们还开发了理论方法来模拟超短电子脉冲与原子和分子的时间依赖状态的相互作用。本项目支持的具体研究包括：(1)利用超短持续时间的电子脉冲作为原子中电子运动的时间和空间探针；(2)研究强激光辅助或激光诱导的电子散射、电子复合和电子轫致过程；(3)建立了长波长近似下长、短激光脉冲分子高次谐波产生的解析公式，研究了原子中双色谐波产生光谱的解析公式，解析研究了长波长激光谐波产生光谱中的齐秒干涉特征；(4)研究了高电荷离子中束缚电子的强激光加速中的有限脉冲效应，并建立了隧穿电离的量子描述与高电荷离子中初始束缚电子的激光加速的经典相对论描述相结合。

This project focuses on understanding theoretically the interaction of intense coherent laser radiation with electrons, atoms, and molecules and on investigating the use of ultrafast laser or electron pulses to image time-dependent electronic processes in atoms and molecules. The basic research supported by this project contributes broadly to our understanding of means to control matter on an atomic scale. Investigations involving ultrashort pulses of electrons may lead to ways of resolving electron motion in atoms and molecules both temporally and spatially. In "seeing" how electrons move and interact with light, scientists will be better able to eventually control the electron motion, thereby leading to important applications in improving the efficiency of such key processes as solar energy conversion and photosynthesis, and in combining optical and electronic processes (e.g., the emerging new field of optoelectronics) to make much faster switches that in turn would enable much faster computers. The analyses of ways to increase the intensities of high order harmonics may one day lead to sources of coherent x-rays, thus providing a new means for visualizing living biological structures as well as nanoscale materials structures. Graduate students and postdoctoral researchers involved with this project are given a broad education in theoretical atomic physics, first-hand experience in all aspects of scientific communication, and in teaching undergraduates at a large Big 10 Land Grant university. Project results are not only published in leading physics journals and presented at national and international meetings, but are also periodically distilled and integrated with related work by others in review articles written by the PI and collaborators. All graduate students and postdoctoral researchers involved with this project in the past have been sought after by a variety of other employers, including technology companies, medical researchers, and other leading AMO theory groups.The processes included in this project are difficult to treat theoretically because the interactions of electrons with both intense laser fields and with atomic and molecular potentials are difficult to describe accurately; also, their interactions with ultrashort laser or electron pulses must be described time-dependently. This group has developed a number of theoretical approaches to overcome these difficulties. In particular, they have solved essentially exactly the problem of a weakly bound electron in a short-range potential interacting with an intense laser field, and the results obtained for this particular system can be accurately generalized to the case of other atomic and molecular systems. They have also developed theoretical approaches for simulating the interaction of ultrashort electron pulses with time-dependent states of atoms and molecules. Specific investigations supported in this project include: (1) using ultrashort duration electron pulses as both temporal and spatial probes of electronic motion in atoms; (2) investigating intense laser-assisted or laser-induced electron scattering, electron recombination, and electron bremsstrahlung processes; (3) developing analytic formulas for high-order harmonic generation for molecules for both long and short laser pulses in the long-wavelength approximation, investigating analytic formulas for two-color harmonic generation spectra in atoms, and investigating analytically zeptosecond interference features in harmonic spectra produced by long-wavelength lasers; and (4) investigating finite pulse effects in intense laser acceleration of electrons bound in highly-charged ions, and developing a combined quantum description of tunneling ionization with a classical relativistic description of laser acceleration of electrons initially bound in highly-charged ions.