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

许多化学变化，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)的时间内穿过几十个原子。只有使用以类似速度工作的探头才能影响这一运动。这种探测器是一种持续时间为亚飞秒的极端紫外光脉冲，直到本世纪之交才首次报道。在此后的二十年里，阿秒(1As=0.001飞秒)激光和探测技术已经发展到使我们能够跟踪并部分改变由电离光引发的自然变化过程的水平。然而，超快转变的许多方面仍然未知。在这个项目中，Pi和他的团队将使用新的数值技术来研究有机分子中通过吸收短脉冲电离辐射而产生的局域电荷的统计特性和时间演化，这是对其他团队所采用的方法的补充。特别是，Pi的团队将探索如何使用红外脉冲，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。该项目将通过促进超快科学的发展，通过发展本科生、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以及在当地少数族裔服务的高中举办关于分子结构的动手研讨会的外联计划，为国家利益服务。Pi的小组已经开发了基于波函数的从头算关联方法，用于多电子原子和最近的小分子的多光子电离的时间分辨研究。这个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子的稳定、转换和碎裂控制；分子光离子中局域电子空穴的产生、传播和监测；以及由自电离态介导的自由电子-激光脉冲电离经过激光修饰的氦原子的光电子分布的计算。该项目有多个方法学组成部分：在Siegert态基础上实施非厄米Floquite解算器，以确定自动电离极化子的复杂能量面，并确定拓扑学上稳健转换的例外点；在基于波函数的方法内计算产生光电离事件的分子离子系综，以表征空穴局部化；波函数分割，重建激光修饰的氦原子通过长XUV自由电子激光脉冲电离产生的光电子分布。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的原子和分子中的自电离态之间的特异点，从而将相干控制扩展到电离阈值以上，以包括拓扑上健壮的转换协议；表征在小分子的光致电离中产生的离子中的空穴局部化和状态纯度、它们随后的关联动力学以及能够探测这种动力学的光学和光电子观测；以及解释激光修饰的氦通过自由电子激光脉冲电离的符合测量。这项裁决反映了NSF的法定使命，并通过使用基金会的智力优势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.