Femtosecond Studies of the Influence of Solvent on Chemical Reaction Dynamics

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In this project funded by the Chemical Structure Dynamics and Mechanism (CDSM-A) program of the Chemistry Division, Professor Stephen Bradforth of the University of Southern California is using photo-electron spectroscopy (PES) and other laser-based techniques to study the structure of liquid ammonia. PES is a technique that uses light to remove electrons from an atom or molecule. By measuring the energy of these electrons, information can be obtained about the structure of the molecule as well as its interactions with other nearby molecules. The Bradforth group is also studying what happens to the interactions between ammonia molecules when extra electrons are dissolved in the liquid. Liquid ammonia was chosen because it can dissolve higher concentrations of electrons, is similar to water in its structure, and because ammonia is relevant to a number of important industrial processes. Since the PES technique involves removal of electrons from molecules caused by light, the research may also provide insights into similar processes such as the damage to living cells caused by light (for example, during cancer radiation therapy). Students and postdocs involved in the project receive training in building instrumentation for new instruments and benefit from training in international labs. As part of the broader impact of this project, Professor Bradforth is continuing an outreach project that provides hands-on summer internships for students from Cerritos College, a minority-serving community college.This research project explores the electronic structure of the hydrogen-bonding liquid ammonia, and the relationship of this structure to that of water. A central focus of the project is to establish how the electronic density of states can be manipulated by solute dopants. The specific objectives of the project are: (i) to determine the liquid valence band orbital delocalization from experimental photoelectron spectroscopy (PES) and photoelectron angular distribution (PAD) measurements; (ii) to identify isolated solvated electrons, spin-paired dielectrons, and the metal-insulator transition at high excess electron concentration using pump-probe PES measurements of liquid ammonia containing solvated electrons (supplied by alkali metals), and (iii) to characterize low-lying conduction band states and their coupling to localized molecular doorway states using continuous two-photon absorption spectroscopy. The transient absorption and photoelectron spectroscopy work is being done in Professor Bradforth's laboratory at USC. The angle-resolved tunable X-ray photoelectron spectroscopy (PAD) studies are being conducted at the Hamburg and Berlin synchrotron light sources.