Collaborative Research: Attosecond Electron Dynamics in Polyatomic Molecules Probed by Water Window X-Rays

超短脉冲的激光使x射线相机的快门速度达到最高，可以记录原子、分子和固体中快速移动的带电粒子的慢动作图像。x射线是这方面的理想选择，因为它可以通过自身的吸收特性区分三种重要的元素——碳、氮和氧。在中佛罗里达大学（UCF）和马里兰大学（UMD）的合作研究项目中，张博士和希尔的团队利用超短脉冲的时间分辨能力和x射线的元素分辨能力，观察了有机分子中原子之间电子的跳跃。该计划开发的工具预计将影响其他领域的各种研究。两个例子是：(1)解决与光合作用相关的基本生物学问题；(2)在提高太阳能电池板效率所必需的光伏效应方面取得技术进步。该合作项目为研究生和博士后提供了在原子、分子和光学物理前沿工作的机会，同时培养他们成为超快分子动力学研究领域的领导者和阿秒技术专家。UCF为本科生和研究生提供阿秒光学和物理课程，其中包括在阿秒设施举行的实验室演示（UMD的学生可以通过互联网连接参加）。该项目的一个关键要素是特别努力吸引和吸引来自代表性不足群体的调查人员，并与佛罗里达州和马里兰州历史悠久的黑人学院和大学的学生和调查人员建立联系。UCF-UMD合作已经组装了仪器和人员(1)来研究原型分子中的光致动力学，(2)用最先进的分析工具分析结果。具体来说，超快电荷迁移动力学是通过碳和氧的核心能级态的瞬态吸收实验来探测的，而定量分析是与UCF理论家合作进行的，具有专门的数值代码。阿秒瞬态吸收光谱（ATAS）首次在极紫外（10至120 eV）下进行了演示，使用近红外Ti：蓝宝石激光器驱动的高谐波产生（HHG）光源。在上一个资助期内，利用短波红外驱动的新一代阿秒HHG光源，在氮K-edge （400 eV）下展示了ATAS的光谱范围。目前的调查有两个目的。首先是将阿秒源的光谱范围扩展到覆盖整个水窗，同时在氧k边（530 eV）、氮边和碳k边（280 eV）进行ATAS。这将使电荷迁移在大分子，如丙酸，进行更定量的研究。第二个目标是跟踪红外脉冲在甲烷中去除氢原子（去质子化）时引起的结构变化，并以亚飞秒分辨率观察红外光诱导的乙炔异构化成乙烯。该项目的首要目标是表明，通过测量x射线辐射引起的与元素特定核价跃迁相关的光谱特征，可以用原子空间分辨率和阿秒时间分辨率追踪电荷密度的动态。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。

Ultrashort pulses of laser light enable the highest-speed shutters for X-ray cameras that can record slow-motion pictures of rapidly-moving charged particles in atoms, molecules and solids. X-rays are ideal for this purpose as they can differentiate three important elements -- carbon, nitrogen and oxygen -- by their characteristic absorption features. In a collaborative research program between the University of Central Florida (UCF) and the University of Maryland (UMD), the team of Dr. Chang and Hill observe the hopping of electrons between atoms in organic molecules by taking advantage of the time-resolving power of ultrashort pulses and the element-resolving capabilities of X-rays. The tools developed by this program are anticipated to impact a variety of research in other fields. Two examples are (1) addressing fundamental biological questions associated with photosynthesis and (2) making technological advances in the photovoltaic effect necessary to improve the efficiency of solar panels. The collaborative project provides opportunities for graduate students and postdocs to work at one of the forefronts of atomic, molecular and optical physics, while being trained to become leaders in the research area of ultrafast molecular dynamics and experts in attosecond technologies. Courses on attosecond optics and physics are offered at UCF for undergraduate and graduate students, which include lab demonstrations held in the attosecond facility (students from UMD are able to attend via an internet link). A key element of this program is the special effort made to attract and engage investigators from underrepresented groups and building connections with students and investigators at Historically Black Colleges and Universities in Florida and Maryland.The UCF-UMD collaboration has assembled the instrumentation and personnel (1) to study photoinduced dynamics in prototype molecules and (2) to analyze the results with state-of-the-art analytical tools. Specifically, ultrafast charge-migration dynamics is being probed experimentally with transient absorption of core-level states of carbon and oxygen while quantitative analysis is done in collaboration with UCF theorists with specialized numerical codes. Attosecond transient absorption spectroscopy (ATAS) was first demonstrated in the extreme ultraviolet (10 to 120 eV) using light sources based on high harmonic generation (HHG) driven by near infrared Ti:Sapphire lasers. During the last funding period, the spectrum range of ATAS was demonstrated at the nitrogen K-edge (400 eV) by taking advantage of the new generation attosecond HHG light sources enabled by short-wave infrared drivers. There are two objectives in the current investigation. The first is to extend the spectral range of the attosecond source to cover the full water window, and to conduct ATAS at the oxygen K-edge (530 eV), nitrogen edge, and carbon K-edge (280 eV) simultaneously. This will enable charge migration in large molecules such as propionic acid to be investigated more quantitatively. The second objective is to track structural changes induced by the infrared pulse in methane as hydrogen atoms are removed (deprotonization), and to observe infrared light-induced isomerization of acetylene into vinilydene with sub-femtosecond resolution. An overarching aim of the project is to show that it is possible to trace the dynamics of charge density with atomic spatial resolution and attosecond temporal resolution by measuring spectroscopic features related to the element-specific core-to-valence transitions induced by X-ray radiation.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.