MRI: Acquisition of High-Power 100 kHz Laser for Recording Real-Time Movies of Ultrafast Molecular Reactions

预测和控制化学反应结果的能力对于现代物理、化学和生物学的许多领域都是至关重要的。这种能力需要详细了解分子中的单个原子在反应过程中如何移动以打破、形成或重新排列它们之间的化学键。这项研究项目旨在获得这种原子运动的高分辨率电影。由于参与化学反应的原子运动速度极快，通常是在几十飞秒(一飞秒是十亿分之一秒)的时间尺度上，超短的飞秒激光脉冲是唯一能够实时记录此类电影的实验工具。然而，由于原子和分子的量子性质，任何分子反应的结果都不是确定性的：即使一次测量精确地捕捉到了给定时间所有原子的位置，同样的测量如果重复几次，即使在完全相同的条件下，也会有不同的结果。获得每秒发射10万个脉冲(每个脉冲短于10飞秒)的高功率激光系统将能够重复数百万次这样的测量，这是创建量子力学分子电影所必需的。然后，电影的每一帧都反映了原子在反应不同阶段的每一种可能构型的可能性，而不是分子的固定图片。这类电影将增进我们对基本化学过程的理解，并为国家优先领域的应用提供投入，从高效的能量转换和存储到新材料的合成、药物设计和分子电子学。该项目由重大研究仪器计划和已建立的激励竞争性研究计划(EPSCoR)共同资助。在这个项目中，将研究广泛的光化学过程，包括光解离和异构化、电荷转移反应和范德华团簇的形成。对于这些过程中的每一个，主要的科学目标是成像依赖于时间的分子几何结构，并同时表征分子不断演变的电子结构。这将通过使用几种互补的时间分辨技术来实现，包括光电子能谱和离子动量成像、内壳层和激光诱导的光电子衍射，以及用于表征中性碎片的离子束技术和傅里叶变换光谱。对于所有这些技术，促进电子和核自由度同时表征的关键技术方面将是通过获得的激光的高重复率而能够同时检测几个反应产物。同时，其数百瓦的高平均功率将使发射的近红外辐射能够有效地转换为启动和探测待研究反应所需的广泛不同波长(从长波红外到极端紫外线和软X射线)。该奖项反映了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。

The ability to predict and control the outcome of chemical reactions is essential for many areas of modern physics, chemistry and biology. This ability requires detailed knowledge of how individual atoms in molecules move to break, form, or rearrange chemical bonds between them during the reaction. This research project aims at obtaining high-resolution movies of such atomic motion. Since the atoms involved in chemical reactions move extremely fast, typically on a time scale of few tens of femtoseconds (one femtosecond is one millionth of a billionth of a second), ultrashort, femtosecond laser pulses are the only experimental tools capable of recording such movies in real time. However, because of the quantum nature of atoms and molecules, the outcome of any molecular reaction is not deterministic: even if one measurement precisely captures the positions of all atoms at a given time, the same measurement will have different outcomes if repeated several times, even under exactly identical conditions. The acquisition of a high-power laser system delivering a hundred thousand pulses (each shorter than 10 femtoseconds) per second will enable repeating such measurements millions of times, which is needed for the creation of a quantum-mechanical molecular movie. Each frame of a movie then reflects the probability of every possible configuration of atoms at different stages of the reaction, instead of a fixed picture of a molecule. Such movies will advance our understanding of fundamental chemical processes and provide input for applications in areas of national priority, ranging from efficient energy conversion and storage to synthesis of novel materials, drug design and molecular electronics. This project is jointly funded by the Major Research Instrumentation program and the Established Program to Stimulate Competitive Research (EPSCoR). Within this project, a broad range of photochemical processes, including photodissociation and isomerization, charge transfer reactions and formation of van der Waals clusters, will be studied. For each of these processes, the main scientific goal is to image the time-dependent molecular geometry and simultaneously characterize the evolving electronic structure of the molecule. This will be achieved by employing several complementary time-resolved techniques, including photoelectron spectroscopy and ion momentum imaging, inner-shell and laser-induced photoelectron diffraction, as well as ion beam techniques and Fourier-transform spectroscopy for characterization of the neutral fragments. For all these techniques, the key technical aspect facilitating simultaneous characterization of electronic and nuclear degrees of freedom will be the coincident detection of several reaction products, enabled by the high repetition rate of the acquired laser. At the same time, its high average power of several hundred watts will enable the efficient conversion of the emitted near-infrared radiation to a broad range of different wavelengths (from long-wavelength infrared to extreme ultraviolet and soft x-rays) needed for initiating and probing the reactions to be studied.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.