This award, funded by the Experimental Physical Chemistry Program of the Division of Chemistry will support Professor Richard Mabbs of Washington University and his graduate and undergraduate students in their studies of the molecular dynamics of dissociative electron attachment (DEA). The proposed work will extend current molecular level, ultra-fast time-resolved experimental capability to electron collision induced reactions. Techniques based on a novel combination of cluster anion production and ultra-fast laser interrogation will be developed to follow the atomic motions at the instant of the photo-induced electron attachment. Time-resolved photoelectron imaging spectroscopy will follow the evolution of the produced transient anionic species into stable product anions, directly probing the potential energy surface along the reaction coordinate. The initial experimental focus will be on simple methyl halide systems, but later work will extend to larger molecules. The interpretation of experimental results will be supported by ab initio electronic structure calculations and qualitative/semi-quantitative models. The educational component of this CAREER award will incorporate highly visual photoelectron imaging results into the curriculum. The images are intuitively explained using qualitative models, thus providing concrete illustrations of quantum concepts.
DEA is a widespread phenomenon important in such diverse areas as the mechanism of ionizing radiation damage of DNA, plasma remediation of pollutants from air streams, interstellar chemistry and the chemistry of the earth?s ionosphere. Direct study of the fundamental details of DEA will allow testing and refinement of current theoretical treatments and lead to more accurate predictive/interpretive models of mechanisms involving these processes. The active involvement of undergraduate students at all levels is strongly encouraged in this research program. Undergraduates involved in these activities will develop problem solving abilities, analytical skills, critical thinking skills and will develop a long term interest in the physical sciences.
Overview This project developed a novel approach for the molecular level study of electron-molecule interactions. A particular focus was electron capture induced chemical reactions, dissociative electron attachment. Such processes are found in many diverse areas including the mechanism of ionizing radiation damage to DNA, terrestrial and extraterrestrial atmospheric chemistry, plasma based remediation strategies for reduction of volatile organic air pollutants in air streams, electron scavenging processes and the formation of charged soot particles in combustion environments. Products This work has resulted in many peer reviewed scientific publications and presentations at regional, national and international scientific meetings. Copious amounts of data have been generated, usually in the form of photoelectron images. These are available to other researchers on request. Additionally, our research results have produced class room available pedagogical examples which have been reported in the literature and are again available for use by the wider community upon request. Approach Traditionally electron molecule interactions have been studied by impinging free electrons on a sample of target molecules and measuring properties such as deflection angles of the electrons and the nature of any products formed. Under this proposal we have developed a different, photoinitiated approach using the properties of cluster anions coupled with photoelectron imaging detection. We produce cluster anions comprised of an atomic anionic moiety (usually an I− ion) physically bound to a relatively unperturbed neutral molecule (examples include CH3Cl, CH3I, CH3CN, CO2, CH3NO2, C4H5N, CH3CH2Cl). The atomic anion acts as an electron source and the neutral moiety of the cluster represents a molecular target. The electron is liberated from the atomic anion by absorption of a photon and is effectively fired at the target. The kinetic energy is linearly dependent on the photon energy affording excellent energy control in these experiments. The photoelectron imaging detection technique records both the kinetic energy spectrum and angular distribution (in the laboratory frame) of the ejected electrons. The results are compared with those of detachment from the free I− anion. Differences in the energy spectra are good indicators of inelastic interactions (the electron kinetic energy is transferred to excited nuclear motion in the molecule) or dissociative attachment. Such processes become evident by the appearance of new transitions in the photoelectron spectrum. For elastic scattering there are no new transitions in the photoelectron spectrum but the angular distributions undergo distinct changes. The angular distribution is also a sensitive indicator of the presence of competing isoenergetic processes, reflecting the competition between direct and indirect (auto) detachment processes. Summary of Key Results These experiments have demonstrated the electron kinetic energy specificity of electron molecule scattering, autodetachment and dissociative attachment processes. To use an elastic scattering example, comparison of the photoelectron angular distributions of I− and I−·CH3X (X = Cl, I) shows that a detachment from a cluster containing methyl chloride yields an angular distribution which is nearly identical to that of free I− detachment. On the other hand, when methyl iodide is the target molecule the two angular distributions are completely different. These results show the different energies of the low lying virtual states of the target molecule which are essential for electron molecule interaction. Similarly, our experiments have demonstrated strong energy dependence for dissociative attachment and electronic autodetachment. The latter of these shows particularly striking alterations in the photoelectron angular distributions over very narrow energy windows. Other key Outcomes The cluster anion based technique for probing scattering and dissociative attachment developed under this proposal represents a significant enhancement to the arsenal of available techniques to study electron molecule interactions. This development paves the way for experiments using sequential ultrafast laser pulses which will allow the study of dissociative attachment on the timescale of atomic motion. Additionally, with the incorporation of higher resolution detection schemes we will be able to use this technique to study the electronic-nuclear kinetic energy transfer in high detail. Such results will represent valuable tests of existing theoretical models of such processes and lead to a significant enhancement of our understanding of these processes. In addition, the photoelectron imaging technique provides highly visual results that can be used to make interesting pedagogical examples. Photoelectron imaging necessarily involves detection of a quantum object (the electron) and our research results have been used to demonstrate the nature of quantum measurement in freshman chemistry classes. We detailed pedagogical examples resulting from our research in the literature and have had undergraduate and high school student involved in our laboratory pursuing viable research projects.
