Ultrafast 2DIR Studies of Dynamics in Dense Gas and Supercritical Fluid Solutions

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With support from the Chemical Structure, Dynamics, and Mechanisms-A (CSDM-A) Program in the Division of Chemistry, Professor Lawrence Ziegler and his group at Boston University are investigating how molecules behave in supercritical fluids (SCFs). SCFs are a special state of matter found at high pressure and temperature, where the difference between liquid and gas vanishes. How well other molecules dissolve in a SCF is readily tunable, because the fluid properties are sensitively dependent on temperature and pressure. The tunable properties of SCFs are already being used for a wide range of applications in the energy, food, and pharmaceutical industries. However, SCFs have the potential for even greater impact as an inexpensive, efficient, and environmentally clean alternative to organic solvents that have negative environmental and human health consequences. The Ziegler group is conducting experiments with very short-duration pulses of infrared laser light to determine how molecules dissolve and lose their energy in SCFs. These measurements reveal whether dissolved molecules are found in spatial regions that are more liquid- or gas-like, how long they stay in such regions, and what molecular properties determine these behaviors. The research group also uses computational methods to gain additional insight into the behavior of molecules in a SCF by comparison with the experiments. This research challenges fundamental concepts that students in beginning general chemistry classes are taught about SCFs. Aside from the research activities, the research team mentors undergraduate and high school participants in Boston University’s ongoing NSF Research Experience for Undergraduates (REU) and Research in Science and Engineering (RISE) programs, respectively, in order to expand the impact of these studies and contribute to the development of the next generation of molecular scientists. The goal of this project is to exploit ultrafast two-dimensional infrared spectroscopy (2DIR) to gain a more detailed understanding of the local solvation environments in dense gases and supercritical fluids. While previous ultrafast 2DIR studies have been carried out for vibrating molecules in condensed-phase solutions, this project exploits the special characteristics of 2DIR spectra of dense gases and supercritical fluids to learn about rotational and vibrational relaxation in dense fluids, special solvation effects in the supercritical fluid phase, and the evolution of liquid phase character as a function of fluid density. The echo-like properties of 2DIR allow rotational energy relaxation rates of solutes to be precisely determined in high density and supercritical fluid solutions, where the rotational structure is completely unresolvable in the linear IR spectra. Unlike pump-probe spectroscopy, 2DIR directly reports on solvent fluctuations, and thus offers a window on the role of fluctuations in the local solvation dynamics of molecules in the region of the critical point. Additionally, 2DIR spectra distinguish free rotor and liquid-like character at a given state point, thus providing new molecular-level descriptions of these high-pressure and high-temperature regions of phase space that can be compared with previously established descriptions of inhomogeneities in the supercritical region. The different rates of vibrational and rotational energy relaxation are contrasted by these studies, which highlights the quantum bottlenecks for restoring equilibrium in dense, highly excited molecular systems. Finally, the researchers are working with the Boston University Instructional Production Services Team to develop a series of video lectures to accompany each chapter of a book on the principles of linear and nonlinear spectroscopy that is intended for beginning graduate and upper-division undergraduate students, as well as the greater science, technology, engineering, and mathematics (STEM) community.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.