Non-Minimum Energy Pathways in the Dissociation of Energy-Selected Ions

University Of The Pacific, Stockton CA

Investigators

Abstract

In this project, funded by the Chemical Structure, Dynamics, and Mechanisms-A (CSDM-A) Program of the Division of Chemistry, Professors Bálint Sztáray and Anthony Dutoi at the University of the Pacific are investigating the ways that molecules break apart after an electron is removed by ultraviolet light. Until recently, chemists thought of these processes as being controlled almost entirely by the potential energy surface, akin to looking for the lowest pass through mountains. More recently, it was learned that it is not uncommon for the participant atoms to explore previously unrecognized reaction paths or switch between low-energy paths. Sometimes there is not even a clearly unique path, with the atoms spending significant amounts of time “roaming” before the reaction completes. This project combines experimental and computational methods to answer such fundamental questions as how energy flows in the fragmenting ion, what molecular-level parameters govern outcomes of reactions, and how/when reaction mechanisms defy conventional logic. The research involves undergraduate and graduate students from California’s Central Valley who are often the first in the family to get a university degree. This project provides them with advanced experimental and theoretical skills to position them well on the high-tech job market. Non-minimum-energy-pathway (non-MEP) mechanisms, such as nonstatistical dynamic effects, reaction path bifurcations, and roaming, can lead to unusual branching ratios and unusual distributions of internal and kinetic energy in reaction products. This project uses Photoelectron Photoion Coincidence (PEPICO) spectroscopy to investigate unimolecular dissociations involving non-MEP phenomena in energy-selected gas-phase ions. The instruments, built by Professor Sztáray and collaborators, enable measurement of both stable and unstable, thermal and molecular-beam-cooled species, and allow collection of branching, unimolecular rate, and kinetic energy release data with meV resolution. The experimental studies are complemented by Professor Dutoi’s molecular dynamics trajectory calculations to elucidate their mechanisms, further extended by high-level quantum-chemical calculations at select points. Integrating experimentation with computational modeling and theory from the onset allows extraction of physical insights not previously attainable by either experiment or theory alone. 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.