Photoinitiated Dynamics of Transient Species: Hydrogen-Bonded Clusters

University Of Southern California, Los Angeles CA

Investigators

Abstract

In this project, supported by the Chemical Structure, Dynamics and Mechanisms - A (CSDM-A) and Macromolecular, Supramolecular and Nanochemistry (MSN) Programs of the Division of Chemistry, Professor Hanna Reisler of the University of Southern California uses lasers to initiate chemical reactions and then observe how the molecules dissociate (fall apart), dissipate the energy they absorbed from the laser light, or behave as acids under light exposure. The Reisler group is particularly interested in hydrogen-bonded complexes of molecules and nanoparticles in water containing a few molecules of hydrochloric acid (HCl) and organic molecules such as benzene or phenol. Hydrogen bonds are formed from the attraction of partially negatively-charged atoms such as oxygen on one molecule and the partially positive charge of a hydrogen atom on a neighboring molecule. Hydrogen-bonded networks influence chemical and biological function and are implicated in the identity of icy bodies in the solar system. Hydrogen bonding is not yet well understood because of difficulties in defining the experimental conditions. Professor Reisler overcomes these challenges by selecting water aggregates that contain a specific number of solute molecules, then uses laser techniques that have high detection sensitivity for the molecules and reactions of interest. The broader impacts of the project include potential societal benefits arising from better understanding the effects of hydrogen bonding on the reactivity of molecules in the atmosphere and other water-rich environments, like the human body. Students and postdoctoral researchers build advanced instrumentation, preparing them high technology careers. Professor Reisler, a leader in mentoring of women in science fields, continues her activities in encouraging students from underrepresented groups to pursue careers in science, technology, engineering and mathematics. The current research focuses on three topics: (1) energy flow pathways in the predissociation of dimers of water with aromatic molecules;(2) measurements of hydrogen-bond dissociation energies in cyclic trimers and tetramers with specific HCl and water subunits; and (3) interactions of nanoscale water samples with protons. By recording images of specific rovibrational levels of one fragment of the dissociated cluster, the rovibrational energy distributions in the cofragments are determined, demonstrating how geometry and hydrogen-bonding type influence energy disposal. This provides stringent tests of theories and proposed mechanisms. In order to study the emergence of acidity in condensed phase aggregates, nanofabricated arrays of gold nanoparticles are covered with solid water and rapid heating is achieved by visible irradiation that excites the plasmon resonance of the gold nanoparticles. A 0.5 mm diameter laser pulse excites over a million nanoparticles. The effect of a single laser pulse on solid water doped with HCl is studied using isotope exchange to probe acidity.