Negative Ion Photoelectron Spectroscopy of Atomic, Molecular and Cluster Anions

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With support from the Chemical Structure, Dynamics, and Mechanisms A (CSDM-A) Program in the Division of Chemistry, Professor Kit Bowen and his group at Johns Hopkins University are studying the negative ions that form when an extra electron is attached to an atom, molecule, or cluster. The addition of electrons to chemical systems can make the unexpected occur, enticing seemingly impossible processes to take place. For example, electrons drive, direct, induce, and initiate an astonishing variety of transformations, including DNA damage, proton transfer, molecular activation, and a vast array of electrochemical reactions. The negative ions that result from electron attachment are also crucial actors in chemical reaction mechanisms, playing key roles in organic chemistry, electrochemistry, and radiation biology. Electron-influenced processes in complex environments are ubiquitous. For these reasons, a microscopic understanding of basic phenomena involving excess electrons is essential. To probe these phenomena, Dr. Bowen and his group use a technique called negative ion photoelectron spectroscopy to measure how tightly electrons are bound to negatively charged atoms, molecules, and clusters. The experiments reveal detailed information about both the ions and their neutral counterparts, while also providing important benchmarks for computational chemistry. Considering the wide-ranging importance of electron-driven processes, the work in Dr. Bowen’s lab has interdisciplinary appeal, with impact ranging across fields as diverse as atmospheric chemistry, catalysis, radiation chemistry and biology, astrochemistry, and mass spectrometry. Dr. Bowen’s work also makes strong contributions to education and human resource development by mentoring graduate students who will become the scientists of tomorrow, by providing undergraduates from Johns Hopkins and other colleges and universities an opportunity to participate in cutting-edge research, and by giving high school, middle school, and elementary school students a glimpse into scientific research. The outreach efforts encourage participation from groups that are underrepresented in science.Dr. Bowen’s research examines electron-induced proton transfer (EIPT) reactions, weakly-bound (diffuse) excess electron states of atoms and molecules, and the properties of transient molecular anions. For example, the research team is exploring the role of EIPT in cluster anions, where they expect the internal interactions to be governed by the excess electron. EIPT is a wide-spread, perhaps even ubiquitous electron-assisted process. The team is also examining the relationship between EIPT in ground-state molecular anions and the process of excited-state intra-molecular proton transfer (ESIPT) in neutral molecules. Dr. Bowen’s research on weakly-bound (diffuse) excess electron states examines some of the most subtle interactions in chemistry; those between electrons and atoms or molecules. In this area, the research team is studying the formation and characterization of polarizability-bound electrons in xenon atomic cluster anions and in ground state anions of buckminsterfullerene; quadrupole-bound electrons in electronically metastable atomic metal anions; and multiple Rydberg electrons in metal-ammonia cluster anions. Although the electrons are weakly bound in each of these cases, they represent steps on a staircase of progressively stronger interactions. Finally, the research team is studying transient molecular anions using a unique approach, developed by Dr. Bowen, that combines Rydberg electron transfer with anion photoelectron spectroscopy (RET-aPES). The research team uses the RET-aPES technique to form and characterize valence anions of nucleobase molecules, the Criegee intermediate, and the hydrogen iodide molecule. They are also using RET-aPES to explore the barely studied, yet fundamentally important process of Penning electron detachment. These studies foster a deeper understanding of the earliest stages of electron damage to DNA, the electrophilic properties of important atmospheric intermediates, the unexpected stability of HI anion, and collisional anion destruction processes in energetic gaseous environments, such as flames and solar atmospheres. The outcomes from this broad research project are expected to have widespread impact across many fields of science. Additionally, the project provides advanced training opportunities for graduate and undergraduate research students, while also engaging K-12 students through Dr. Bowen's outreach activities.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.