RUI: Development of Computational Methods and Applications to Molecules in Microsolvation Environments.

Southeastern Louisiana University, Hammond LA

Investigators

Abstract

Thomas Sommerfeld of Southeastern Louisiana University is supported by an award from the Chemical Theory, Models and Computational Method program in the Chemistry Division and the NSF EPSCoR office to investigate how extra electrons aid in breaking chemical bonds. Chemical bonds themselves are electrons shared between atoms and can be thought of as a glue holding the atoms in a molecule together. The electron-glue analogy, however, only goes so far, because a bond can be weakened by both too little and too much glue, and weak bonds can be cleaved during the permanent motion of the atoms. In other words, adding an electron to a molecule generally weakens its bonds, and the resulting so-called temporary anion may undergo either bond cleavage or, alternatively, reemit the extra electron. Concrete examples where this reaction is applied or happens naturally are: plasma chemistry and plasma etching, chemistry in the ionosphere, damage to living tissue by ionizing radiation, cancer therapy with electron beams, and reduction reactions with solvated electrons, the so-called Birch reduction. This research project supports the development of computer models of the temporary anion, the first intermediate formed in these electron-induced reactions. This research is carried out with collaborators and with undergraduate researchers. In order to introduce undergraduates to this research, Dr. Sommerfeld also designs educational mini-projects, which are only indirectly related to the current main project, but can be addressed with standard quantum chemistry methods, so that his students can be introduced to the research area step-by-step. In this project Dr. Sommerfeld studies electronically metastable states - so called resonance states or, simply, resonances. On the one hand, he develops ab initio methods to characterize resonances, and on the other hand he applies the newly developed methods in selected applications. Computing both the energy and finite lifetime of a resonance is still a notoriously challenging task for quantum chemistry, because it combines an electron-scattering with an electron-correlation problem. To address the continuum aspect, Dr. Sommerfeld either uses complex-absorbing-potentials or the analytic-continuation-in-the-coupling-constant method. To address the correlation aspect, he uses the symmetry-adapted-cluster configuration-interaction method, an electronic structure method closely related to the equation-of-motion coupled-cluster method. One particular point of emphasis is the artificial stabilizing potential added to the Hamiltonian in the analytic-continuation-in-the-coupling-constant method. Dr. Sommerfeld aims to identify a short-range stabilizing potential that improves the subsequent analytic-continuation step. On the application side Dr. Sommerfeld's goals are to examine resonance states, which differ in electronic structure from a closed-shell neutral either by one-hole (Auger-like resonances) or by one excitation (e.g. Penning ionization or resonant photodetachment). For these resonances, treating electron correlation in a balanced way is particularly challenging. Dr. Sommerfeld studies temporary anion resonances embedded in small molecule clusters with the goal of identifying trends in the change of the resonance parameters associated with the interaction strength (permanent dipole, higher order multipoles, dispersion) of the embedding molecules.