Spectroscopy and Molecular Aggregation in Helium Droplets

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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.