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，都是由电荷运动驱动的。然而，在分子水平上观察和控制电荷的运动，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极短，因此变得非常复杂。一个电子可以在不到十亿分之一秒（1飞秒，fs）的时间内穿过几十个原子。这种运动只能通过使用以相似速度运行的探针来影响。这种探测器是一种持续时间为亚飞秒的极紫外光脉冲，直到本世纪初才首次被报道出来。在此后的20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，使我们能够跟踪并部分改变电离光引发的自然转变过程。然而，超快转换的许多方面仍然未知。在这个项目中，PI和他的团队将从理论上研究有机分子中由电离辐射短脉冲吸收产生的局部电荷的统计特性和时间演化，使用新的数值技术来补充其他团队所使用的技术。特别是，PI的小组将探索如何使用红外脉冲，例如用于脉冲激光手术的脉冲，来提高所产生的量子态的纯度。该项目将通过推进超快科学，通过培养本科生、研究生和研究生水平的研究人员，通过与美国研究小组的协同合作，以及通过包括高中生研究实习和在当地少数民族高中开展分子结构实践讲习班在内的外展计划，为国家利益服务。PI的团队已经开发了基于波函数的从头算相关方法，用于多电子原子的多光子电离的时间分辨研究，最近还用于小分子。该项目解决了三个开放的挑战：原子和分子系统中自电离极化的稳定、转化和碎片控制；分子光子中局域电子空穴的产生、传播和监测并计算了自由电子-激光脉冲在自电离态介导下激光修饰氦原子电离的光电子分布。该项目有多个方法组成部分：在Siegert状态的基础上实现非厄米Floquet解算器，以确定自电离极化的复杂能量面，并确定拓扑鲁棒转换的特殊点；在基于波函数的方法中，计算从光电离事件中出现的分子离子的集合，以表征空穴定位；和波函数的分割，以重建激光修饰氦原子在长XUV FEL脉冲下电离的光电子分布。在现象学方面，这些进步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特殊点，从而将电离阈值以上的相干控制扩展到包括拓扑鲁棒转换协议；表征小分子光电离过程中产生的离子的空穴定位和态纯度、它们随后的相关动力学以及可探测这种动力学的光学和光电子观测值；并解释了用FEL脉冲对激光修饰的氦进行电离的巧合测量。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.