Spectroscopy and Molecular Aggregation in Helium Droplets

在化学系化学结构动力学和机理（CSDM-A）项目的支持下，南加州大学的Andrey Vilesov教授将使用红外激光研究分子阳离子（去除电子的分子）的结构。阳离子在许多化学反应中都是重要的积极参与者。然而，许多阳离子的分子结构仍然未知，因为它们的高反应性使得制备足够量的研究变得困难。要克服这些实验上的挑战，必须在接近绝对零度（-460华氏度）的温度下，将阳离子隔离在液氦的微小液滴中，在这种温度下，阳离子具有干净的光谱。这一研究方向有望深入了解从外层空间到工业等不同环境下的化学反应的结果是如何受到反应中阳离子中间体结构的影响的。该项目还将研究分子在极低温度下如何旋转和结合。参与本研究项目的学生将获得现代物理化学实验和数据分析的宝贵经验。本项目重点研究了碳正离子在氦液滴中0.4 K超低温下的结构和量子溶剂化。液滴是在低温喷嘴光束膨胀中产生的，掺杂了分子前体，如甲烷、乙烯和其他物质，并通过电子撞击电离。所得到的包封在液滴内的阳离子将由可调谐红外激光器以脉冲方式照射。辐射的共振吸收导致阳离子从液滴中释放出来，随后由四极杆质谱仪检测。所得的C-H延伸范围内的红外光谱与量子化学计算的结果进行了比较，以确定阳离子的结构。通过观察光谱中的旋转结构，研究了阳离子和小分子在超流体4He和正常流体3He液滴中的溶剂化。利用自由电子激光对x射线衍射进行分析，得到分子聚集体的结构。这项工作的更广泛的影响包括探索分子阳离子的结构和溶剂化的潜在社会效益，这些阳离子在凝聚态和气相中作为重要的化学中间体。该项目将通过开发和执行前沿物理化学实验以及准备后续出版物，为科学、技术、工程和数学（STEM）领域的本科生和研究生提供培训机会。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。

With support from the Chemical Structure Dynamics and Mechanisms (CSDM-A) program of the Chemistry Division, Professor Andrey Vilesov of the University of Southern California, will study the structure of molecular cations (molecules with an electron removed) using infrared lasers. Cations are important active participants in a multitude of chemical reactions. However, the molecular structure of many cations remains unknown because their high reactivity makes it difficult to prepare sufficient amounts for study. These experimental challenges are to be overcome by isolating the cations in tiny droplets of liquid helium at temperatures close to absolute zero (-460 F), at which the cations have clean spectra. This line of research is expected to provide insights into how the outcome of a chemical reaction in different settings from outer space to industry is influenced by the structure of the cationic intermediates in the reaction. This project also will examine how molecules rotate and combine at very low temperature. The students engaged in this research project will gain valuable experience in both modern physical chemical experimentation and in data analysis. This project is focused on the study of the structure and quantum solvation of carbocations at an ultra-low temperature of 0.4 K in helium droplets. The droplets are produced in a cryogenic nozzle beam expansion, doped with molecular precursors, such as methane, ethene, and others and ionized by electron impact. The resulting cations encapsulated within the droplets will be irradiated in pulsed fashion from a tunable infrared laser. The resonance absorption of the radiation leads to release of the cations from the droplets followed by detection by the quadrupole mass spectrometer. The resulting infrared spectra in the range of C-H stretches are compared with the results of quantum chemical calculations to determine the structure of the cations. Solvation of the cations and small molecules in superfluid 4He and normal fluid 3He droplets will be studied by observation of the rotational structure in the spectra. The structure of the molecular aggregates will be obtained from the analysis of the x-ray diffraction using a free electron laser. The broader impacts of this work include potential societal benefits from exploring the structure and solvation of molecular cations which act as important chemical intermediates in condensed and gas phases. This project will provide opportunities for training of undergraduate and graduate students in science, technology, engineering, and mathematics (STEM) through the development and execution of cutting-edge physical chemical experiments and the preparation of ensuing publications.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.