Collaborative Research: Attosecond Charge Dynamics in Atoms and Molecules

在19世纪末，闪光灯摄影使宏观物体的运动速度减慢到可以提取事件的真实顺序的程度。近一个世纪后，飞秒脉冲(1飞秒是十亿分之一秒)使原子的运动在分子形成和分解过程中得以冻结，从而改善了化学的微观图景。短一百万倍的阿秒脉冲的出现，使得最微小的物质构件--电子--的运动可以在轨道上停止。中佛罗里达大学的Chang教授和马里兰大学的Hill教授与他们的学生一起，使用阿秒激光脉冲在与原子和分子系统中的本征运动相称的时间尺度上研究电子电荷动力学。众所周知，电荷迁移是光合作用、生物分子中的辐射损伤和太阳能电池板中使用的光伏等各种重要过程的核心。这个合作项目提供了对原子、分子和固体中普遍存在的电子的集体和交互运动的洞察。提高对电荷迁移的理解将有广泛的应用，从控制分子内的能量流动到调整材料的性能以满足特定的需求。分子中的光致电荷分离是许多化学过程的第一步，也是我们理解电子关联和电子与核运动之间的能量交换的核心，这些运动负责生物分子中的催化、光合作用、光伏和辐射损伤。对复杂分子的全面数值模拟预测，当一个电子从链分子的一端‘突然’移除时，例如小肽，空穴可以在不到10秒的时间内移动到分子的另一端，然后发生电子-核耦合。研究这种自然发生的电荷迁移需要强而孤立的阿秒脉冲。在中佛罗里达大学(UCF)和马里兰大学(UMD)的一个合作项目中，Chang和Hill教授利用UCF的阿秒源，通过阿秒泵浦-阿秒探测实验研究了多电子原子(He)和多原子分子(SO2)中的电荷迁移。泵浦脉冲启动快速激发(在分子情况下没有核运动的情况下)，而探测脉冲监视随后的电荷迁移。两种探测技术被用来‘观察’电荷迁移：瞬时吸收和光电子角分布。与以前的阿秒研究不同，以前的研究是在强外红外场存在的情况下研究电荷迁移的，而UCF-UMD研究是在没有任何外部扰动的情况下探索激发后的电荷迁移。因此，该项目提供了对电荷分离、能量流动和电子-核电荷耦合的更清楚的了解，如上所述，这些与与化学反应、凝聚态系统中的动力学和生物现象有关的各种过程有关。这个项目的第二个目标是开发通用的实验工具，可以转移到更复杂的系统，如ABCU(1-氮杂双环[3.3.3]十一烷)，对于这些系统，存在理论预测，并且激发光谱接近于本研究的模型系统。

In the late 1800s, flash photography enabled the motion of macroscopic objects to be slowed to the point where the true order of events could be extracted. Nearly a century later, femtosecond pulses (1 femtosecond is one millionth of one billionth of a second) enabled the motions of atoms to be frozen during the formation and breakup of molecules, resulting in an improved microscopic picture of chemistry. The advent of attosecond pulses, a million times shorter, allows the motion of the tiniest building block of matter, the electron, to be stopped in its tracks. Professors Chang, of the University of Central Florida, and Hill, of the University of Maryland, together with their students, use attosecond laser pulses to study electronic charge dynamics on timescales commensurate with its indigenous motion in atomic and molecular systems. Charge migration, as it is known, is central to a variety of important processes such as photosynthesis, radiation damage in biomolecules and photovoltaics used in solar panels. This collaborative project is providing insight into the collective and interactive motion of electrons that are ubiquitous in atoms, molecules and solids. An improved understanding of charge migration will have broad application from controlling the flow of energy within a molecule to tailoring the performance of materials to specific needs.Photoinduced charge separation in molecules is the first step in many chemical processes and central to our understanding of electron correlation and the energy exchange between electronic and nuclear motion responsible for catalysis, photosynthesis, photovoltaics and radiation damage in biomolecules. Comprehensive numerical simulations of complex molecules predict that when an electron is 'suddenly' removed from one end of a chain molecule, such as a small peptide, the hole can move to the other end of the molecule in less than 10 fs, before the electron-nuclear coupling takes place. Intense, isolated attosecond pulses are required to study this naturally-occurring charge migration. Professors Chang and Hill, in a collaborative project between the University of Central Florida (UCF) and the University of Maryland (UMD), exploit the attosecond source at UCF to investigate charge migration in multi-electron atoms (He) and multi-atom molecules (SO2) via attosecond pump - attosecond probe experiments. The pump pulse initiates rapid excitation (in the absence of nuclear motion in the molecular case) while the probe pulse monitors the ensuing charge migration. Two probe techniques are used to 'watch' the charge migration: transient absorption and photoelectron angular distribution. In distinction with previous attosecond studies, where charge migration was investigated in the presence of a strong external infrared field, the UCF-UMD study is probing charge migration subsequent to excitation but in the absence of any external perturbation. As a consequence, this project is providing a clearer understanding of charge separation, energy flow and electron-nuclear charge coupling, which, as stated above, are relevant to a variety of processes associated with chemical reactions, dynamics in condensed matter systems and biological phenomena. A secondary goal of this project is the development of general experimental tools that can be transferred to more complicated systems, such as ABCU (1-azabicyclo [3.3.3] undecane), for which theoretical predictions exist and the excitation spectra fall near those of the model systems of this study.