Coherent Attosecond Ionization Dynamics in Laser-Dressed Atomic and Molecular Systems

许多化学转化，如燃烧、光合作用和辐射损伤，都是由电荷的运动驱动的。然而，在分子水平上观察和操纵电荷的运动，以便有效地利用能量、传递信息和控制化学反应，由于电子运动的时间尺度极短而变得复杂。一个电子可以在不到十亿分之一秒（一飞秒，fs）的时间内穿过数十个原子。这种运动只能通过使用以类似速度运行的探针来影响。这种探测器是一种亚飞秒持续时间的极紫外光脉冲，直到本世纪之交才首次报道。在过去的二十年里，阿秒（1 as = 0.001 fs）激光和探测技术已经发展到一个水平，使我们能够跟踪和部分改变电离光引发的自然转变过程。然而，超快转变的许多方面仍然是未知的。在这个项目中，PI和他的团队将从理论上研究通过吸收电离辐射的短脉冲在有机分子中产生的局部电荷的统计特性和时间演化，使用与其他团队所采用的技术互补的新数值技术。特别是，PI的团队将探索如何使用红外脉冲，例如脉冲激光手术中使用的脉冲，来提高所产生的量子态的纯度。该项目将通过超快科学的进步，通过本科生，研究生和研究生水平的研究人员的发展，通过与美国研究小组的协同合作，并通过包括高中生的研究实习以及在当地少数民族服务高中的分子结构动手研讨会的推广计划，为国家利益服务。PI的小组已经开发了基于波函数的从头相关方法，用于多电子原子和最近的小分子多光子电离的时间分辨研究。该项目解决了三个开放的挑战：在原子和分子系统中的自电离极化激元的稳定，转换和碎裂控制;分子光离子中局部电子空穴的产生，传播和监测;以及计算由自由电子激光脉冲电离激光修饰的氦原子的光电子分布，由自电离状态介导。该项目有多个方法组成部分：在Siegert状态的基础上实施非Hermitian Floquet解算器，以确定自电离极化激元的复杂能量表面，并确定拓扑鲁棒转换的例外点;在基于波函数的方法内计算从光电离事件中出现的分子离子系综，以表征空穴定位;以及波函数的分段，重建了长脉冲超紫外自由电子激光作用下激光修饰氦原子电离的光电子分布。在现象学方面，这些进展将使该小组能够识别和表征激光修饰原子和分子中自电离状态之间的异常点，从而将相干控制扩展到电离阈值以上，以包括拓扑鲁棒的转换协议;为了表征在小分子的光电离中产生的离子中的空穴定位和状态纯度，它们随后的相关动力学以及能够探测这种动力学的光学和光电子可观测量;并解释激光电离中的符合测量，该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准。

Many chemical transformations, such as combustion, photosynthesis, and radiational damage, are driven by the motion of electric charge. Observing and steering the motion of charge at the molecular level so to efficiently harness energy, transfer information, and control chemical reactions, however, is complicated by the extremely short timescale at which electrons move. An electron can travel across tens of atoms in less than a millionth of a billionth of a second (one femtosecond, fs). This motion can only be affected by using probes that operate at a similar speed. Such a probe, an extreme ultraviolet light pulse with sub-femtosecond duration, was reported for the first time only at the turn of this century. In the two decades elapsed since, attosecond (1 as = 0.001 fs) laser and detection technology has advanced to a level that allows us to follow and to partially alter the natural course of transformations triggered by ionizing light. Many aspects of ultrafast transformations, however, are still unknown. In this project, the PI and his group will theoretically study the statistical properties and the time evolution of localized charges created in organic molecules by the absorption of short pulses of ionizing radiation using novel numerical techniques complementary to those employed by other groups. In particular, the PI's group will explore how infrared pulses, such as those used in pulsed-laser surgery, can be used to increase the purity of the quantum states produced. The project will serve the national interest through the advancement of ultrafast science, through the development of researchers at the undergraduate, graduate, and post-graduate level, through synergistic collaborations with US research groups, and through outreach programs that include research internships of high-school students as well as hands-on workshops on molecular structure at local minority-serving high schools. The PI's group has developed wave-function-based ab initio correlated methods for the time-resolved study of multiphoton ionization of polyelectronic atoms and, more recently, of small molecules. This project tackles three open challenges: the stabilization, conversion, and fragmentation control of autoionizing polaritons in atomic and molecular systems; the creation, propagation, and monitoring of localized electron holes in molecular photoions; and the calculation of the photoelectron distribution from the ionization of laser-dressed helium atoms by free-electron-laser pulses, mediated by autoionizing states. The project has multiple methodological components: the implementation of a non-Hermitian Floquet solver in a basis of Siegert states, to determine the complex energy surfaces of autoionizing polaritons and to identify exceptional points for topologically robust conversions; the calculation, within a wave-function based approach, of the ensemble of molecular ions emerging from a photoionization event, to characterize hole localization; and the segmentation of the wavefunction, to reconstruct the photoelectron distribution from the ionization of laser-dressed helium atoms by long XUV FEL pulses. On the phenomenological side, these advancements will allow the group to identify and characterize exceptional points between autoionizing states in laser dressed atoms and molecules, thus extending coherent control above the ionization threshold to include topologically robust conversion protocols; to characterize hole localization and state purity in the ions generated in the photoionization of small molecules, their subsequent correlated dynamics and the optical and photoelectron observables able to probe such dynamics; and to explain coincidence measurements in the ionization of laser-dressed helium by FEL pulses.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.