Photoelectron Spectroscopy of Molecular and Cluster Anions

Johns Hopkins University, Baltimore MD

Investigators

Abstract

In this project, funded by the Chemical Structure Dynamics and Mechanism (CSDM-A) program of the Chemistry Division, Professor Kit Bowen of Johns Hopkins University is using anion photoelectron spectroscopy to study a variety of molecular and cluster anions. Photoelectron spectroscopy (PES) entails the use of high energy light (for example ultraviolet or x-ray light) to eject electrons from atoms, molecules or solids. The energy of the ejected electrons reveals information about how strongly the electrons were bound to a molecule as well as the type and strength of chemical bonds within the molecule. Clusters are aggregates of atoms or molecules held together by the same cohesive forces responsible for the existence of solids and liquids. Anions of molecules and clusters are negatively-charged ions of these entities (i.e., they have more than the needed number of electrons). This project uses PES to characterize the extra electrons on these species as these extra electrons can lead to major changes in their chemical properties. This research is providing new insights into the basic nature of chemical bonds. It may also have implications in other areas of science, for example in radiation biology, oncology, and astrochemistry. Graduate students involved in this research project are receiving training not only in the PES technique but in the theoretical aspects of molecular structure. The Bowen group is working with secondary school science teachers to help bring enthusiasm for science into their classrooms. The group offers lab tours and classroom presentations for elementary and middle school teachers and students. This project contributes to knowledge and understanding in chemistry and related fields in several ways. (1) Halogen bonding plays important roles in supramolecular chemistry, crystal engineering, and molecular recognition. This work aims to explore bimolecular halogen bonding interactions in anionic complexes, i.e., at the molecular level. (2) Electron-induced proton transfer (EIPT) is a fundamental process in both chemistry and biology. This work focuses on inter-molecular EIPT among carboxylic acids and triazoles and within hydrated N-heterocyclic molecules. It investigates the relationship between excited state intra-molecular proton transfer (ESIPT) in electronically-excited neutral molecules and intra-molecular EIPT in ground state molecular anions. It also explores the role of inter-molecular EIPT in hydrogen halide-water cluster anions. (3) There are several different interactions between electrons and molecules/clusters that result in diffuse electron states. Binding may have its roots in electron-polarizability, electron-dipole, or electron-quadrupole interactions, or it may derive from correlation effects alone. Even more subtle interactions result in the formation of double Rydberg anions. Professor Bowen's new apparatus combines Rydberg electron transfer and anion photoelectron spectroscopy as a tool for making and studying these entities. (4) Molecular activation is often a prerequisite for reactivity. This work aims to explore electron-assisted molecular activation processes in a variety of systems.