QLC: EAGER: Toward the visualization of chemical control

Regents Of The University Of Michigan - Ann Arbor, Ann Arbor MI

Investigators

Abstract

In this project funded by the Chemical Structure Dynamics and Mechanism (CSDM-A) program of the Chemistry Division, Professor Roseanne Sension of The University of Michigan is using x-ray techniques to develop visual images and movies of the internal motions of the atoms in molecules which have been exposed to a pulse of ultraviolet (UV) or visible (Vis) light. Pulses of UV/Vis light can cause distortions of the chemical bonds in molecules, and even cause them to break or re-form in what we call a photochemical reaction. These light-induced motions occur on very short time scales, from a few femtoseconds to a few nanoseconds (A femtosecond is one quadrillionth of a second. A nanosecond is one billionth of a second). X-ray diffraction has been used to determine molecular structure for more than a century. However, these have been molecules that are stationary in a crystal lattice. In order to capture the extremely fast motions of molecules after they have been exposed to light, Prof. Sensions is using the intense x-rays produced by free electron lasers (for example, at the LINAC Coherent Light Source at Stanford University). Professor Sension and her students are measuring how molecules absorb x-rays, and developing methods to convert x-ray absorption information into images that can be sequenced as frames in a motion picture. The visualization of photochemical reactions at the molecular level helps us understand chemical reactions in general, and also supports the long-term goal to control chemical reactions with light. The project is also training students in the use of advanced experimental facilities and in advanced computer simulation of UV/Vis and x-ray absorption. In addition, the results are being integrated into the undergraduate curriculum to help students understand the relevance of mathematics and physics to chemistry. This project uses femtosecond x-ray free electron lasers to visualize quantum wave packet motion in molecules. A short laser pulse is used to initiate and control a photochemical reaction. A polarized x-ray pulse probes the coherent structural dynamics on the excited state potential energy surface. Cobalamin photochemistry and photophysics is used as a model system for molecular visualization and quantum control with femtosecond x-ray pulses sources. The visualization of coherent excited state dynamics and the correlation of dynamics with the molecular ligands facilitate the understanding and development of cobalamin based molecular devices. The experimental observations are interpreted using quantum chemical calculations and finite difference near edge spectroscopy ab initio code to simulate x-ray absorption spectra. In addition, general simulation methods are being developed to model ultrafast x-ray spectroscopy and coherent excited state dynamics. This research project has implications for a wide range of applications in energy conversion, medical therapeutics, molecular electronics, switches, sensors, and delivery platforms. Many of these applications exploit intrinsically quantum mechanical dynamics of the molecules involved. 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.