Photodissociation Dynamics of Transient Species: Hydrogen-Bonded Dimers and Trimers

University Of Southern California, Los Angeles CA

Investigators

Abstract

In this project, supported by the Chemical Structure, Dynamics and Mechanisms - A Program of the Division of Chemistry, Professor Hanna Reisler from the University of Southern California will elucidate mechanisms of predissociation of hydrogen-bonded complexes in the gas phase, in particular dimers and trimers. Despite their weak interaction strength, hydrogen bonds are crucially important in environments ranging from living cells to icy bodies in the solar system. To gain insight into the predissociation mechanisms and determine the hydrogen bond strength, the state resolution and detection sensitivity of photofragment ion imaging will be exploited to obtain pair-correlated distributions of fragments following laser excitation of one subunit of hydrogen-bonded dimers or trimers. Cyclic trimers will serve as prototypes of vibrational energy dissipation in larger hydrogen-bonded networks and for experimental estimates of cooperative interactions that strengthen hydrogen bonding. State-specific energy flow patterns that lead to bond breaking will be inferred from fragment quantum state distributions, and bond dissociation energies will be obtained with spectroscopic accuracy. For example, in HCl-water mixed clusters, the influence of incipient proton transfer from HCl on predissociation dynamics will be revealed. In trimers and larger clusters, the contributions of 2- and 3-body fragmentation processes will be determined. Vibrational predissociation mechanisms will be elucidated in collaboration with the theory group of Professor Joel Bowman at Emory University. This collaboration, which has already resulted in a description of the bond breaking mechanism in water dimers, will be extended to include other dimers and trimers. The ultimate goal is to understand the yet unexplained exquisite state specificity in vibrational energy flow that leads to bond breaking, and to extend to work to larger hydrogen bonding networks. Understanding energy flow through hydrogen-bonded networks has been a major goal of experimental and theoretical studies of molecular behavior in the gas and condensed phases. However, little is currently known about how these weak bonds are formed and broken, what is their flexibility, and how networks, made of many molecules, contribute to the strength of the bonds. Hydrogen bonds are important in environments such as biological systems, molecular solids, hydrated solutions of neutral molecules and ions. The chosen molecular systems are also relevant to atmospheric processes that affect climate change. Students participating in this research observe experimental manifestations of concepts learned in courses on spectroscopy and dynamics, compare their experimental results to theory, and construct dynamical models that emphasize physical insight. Professor Reisler has developed and leads a wide variety of activities aimed at encouraging women in science and engineering to pursue research careers, and incorporates results from this research in her undergraduate and graduate courses.