Attosecond Electron Dynamics

在这个由化学部化学结构、动力学和机理A （CSDM-A）项目资助的项目中，加州大学伯克利分校的Stephen Leone教授使用复杂的激光技术来研究分子中一些最短时间的过程。现代激光技术使我们能够在化学反应发生时跟踪分子中原子核的运动。然而，构成原子间化学键的电子比原子核的质量小几千倍，因此运动速度要快得多。这个研究项目旨在追踪分子内部在100 - 200阿秒内发生的电子过程（1阿秒是一个小数点后面跟着17个零，然后是一个“1”）。快速测量是通过使用波长在极紫外和x射线光谱区域的激光来完成的。这个项目的目标是描述发生化学反应的分子中电子的运动，并可能在电子云即将选择一种反应路径而不是另一种反应路径时捕捉到它。这项研究对于理解电荷流动（负电子和正原子核）的基本原理以及化学键的断裂和重组对太阳能的利用具有重要意义。激光测量能力的发展推动了化学动力学短时间持续时间的界限，这对社会产生了进一步的好处，使用需要激光和光学平台精确稳定性的工具。这些能力肯定会在许多科学技术领域得到应用。除了直接参与该项目的三名研究生外，Leone研究小组的成员还参与了非正式的教育活动，包括为高中学生和教师参观实验室，以及与劳伦斯科学馆（Lawrence Hall of Science）协调的外展活动。莱昂内实验室开发的极紫外和x射线的超快瞬态吸收技术被用于进行阿秒时间分辨动力学测量，作为探测化学动力学的一种方法，可以将电子动力学时间尺度与核运动分开。电子相关态和叠加态在这些短时间尺度的化学过程中起着核心作用。利用载流子-包络-相位稳定驱动激光脉冲产生高谐波产生阿秒探测脉冲，并利用紫外脉冲通过明确的单光子吸收激发分子。在报告原子中探测的核心能级跃迁是位点和氧化态特异性的，键长敏感的，并且依赖于电子态，提供了一种轨道特异性的方法来测量超快动力学。目的是研究曲线穿越、通过锥形交叉点、电子和振动量子力学叠加以及电荷迁移递归的阿秒和几飞秒时间过程。分子被选择用于研究在多个激发态或分子周围的化学环境中具有非常不同的电子构型的分子，这些分子可以通过光谱手段识别电荷流，或者与化学处理相关。这项工作的更广泛的影响涉及到突破时间限制的工具的开发，这一点很重要，因为设备的尺寸和速度都在下降，分子中的量子原理被认为是计算架构的一部分。参与该项目的学生将学习一系列与高科技行业相关的原理，这些原理可以降低无处不在的电子设备的速度和尺寸。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。

In this project funded by the Chemical Structure, Dynamics, and Mechanisms A (CSDM-A) program of the Chemistry Division, Professor Stephen Leone of the University of California, Berkeley, uses sophisticated laser techniques to study some of the shortest time processes in molecules. Modern laser technology allows us to track the motion of the atomic nuclei in molecules as chemical reactions take place. However, the electrons that make up the bonds between atoms are thousands of times less massive than nuclei and therefore move much faster. This research project seeks to follow electron processes inside molecules that occur within 100 – 200 attoseconds (One attosecond is a decimal point followed by seventeen zeros and then a “1”). The fast measurements are accomplished by using laser light with wavelengths in the extreme ultraviolet and x-ray regions of the spectrum. The goal of this project is to characterize the motions of electrons in molecules undergoing chemical reactions, and perhaps catch the electron cloud in the moment when it is about to choose one reaction path over another. The research is important to understand the fundamental principles of charge flow (negative electrons and positive nuclei) and the breaking and reforming of chemical bonds for utilization of sunlight energy. Further benefits to society arise from the development of laser measurement capabilities that push the boundaries of short time durations in chemical dynamics using tools that require precision stability of lasers and optical platforms. These capabilities are certain find use in many areas of science and technology. In addition to the three graduate students who are directly involved in this project, members of the Leone research group engage in informal educational activities, including laboratory tours for high school students and teachers, and outreach events in coordination with the Lawrence Hall of Science.Ultrafast transient absorption in the extreme ultraviolet and X-ray, techniques developed in the Leone laboratory, are employed to make attosecond time-resolved dynamics measurements as a way to probe chemical dynamics that can separate electron dynamics timescales from nuclear motion. Electron correlation and superposition states play central roles in chemical processes on these short timescales. Attosecond probe pulses are produced via the process of high harmonic generation using carrier-envelope-phase stabilized driver laser pulses, and ultraviolet pulses are used to excite molecules by well-defined one-photon absorption. Core level transitions probed in reporter atoms are site- and oxidation-state specific, bond-length-sensitive, and electronic-state-dependent, providing an orbital-specific way to measure ultrafast dynamics. Objectives are to study attosecond and few-femtosecond time processes of curve crossing, passage through conical intersections, electronic and vibrational quantum mechanical superpositions, and charge migration recurrences. Molecules are chosen for study that have very different electronic configurations in multiple excited states or chemical environments around the molecule that are recognizable by spectroscopic means for charge flow, or for their relevance in chemical processing. The broader impacts of this work involve the development of tools that push the limits of time, important as both the dimensions and speed of devices decrease and as quantum principles in molecules are considered for computing architectures. Students who participate in this project learn an array of principles relevant for high technology professions that enable the speed and dimensions of ubiquitous electronic devices to decrease.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.