OP:Diffractive Imaging of Complex Isolated Molecules

University Of Nebraska-Lincoln, Lincoln NE

Investigators

Abstract

An important aspect in assuring the continued progress of science and technology is to understand and control the flow of energy at the molecular level. Energy conversion involves changes in the structure of the molecule, which plays a critical role in biological processes such as photosynthesis, vision, and the damage of cells due to absorption of ultraviolet light. From a technological standpoint, the conversion of light into chemical energy and heat at the molecular scale is important for solar energy conversion and storage. When a molecule absorbs a particle of light, it's structure is often significantly changed, but these changes are difficult to observe because of the very small scale and the rapid pace with which the structure changes. Visualizing the molecular structure and how it changes will improve our understanding of how energy is transformed. In this project, the investigators will develop the tools to capture three-dimensional images of isolated molecules with atomic resolution. These methods can then be applied to observe how structures change in time. In order to capture images, the orientation of the molecules will be controlled using very short laser pulses with a duration of less than 1 trillionth of a second. Similarly short electron pulses will be scattered from the molecules, and the structures will be retrieved by analyzing the scattering patterns. The goal of this project is to capture three dimensional images of isolated molecules with atomic resolution. Experiments will start with small molecules and then continue with more complex molecules such as Trifluorotoluene, Stilbene and Diiodotetrafluoroethane. The molecules will be imaged in a cooled gas beam in vacuum. A femtosecond laser pulse will be used to impulsively align the molecules, and a femtosecond or picosecond electron pulse will be used to capture a diffraction pattern of the molecules while they are aligned. The advantage of impulsively aligning the molecules with femtosecond pulses is that the maximum alignment is reached a few picosecond after the interaction with the laser, thus the molecules can be probed in a field-free environment. First, the effect of the laser pulse on the angular distribution and structure of the molecules will be investigated as a function of laser intensity, to determine the optimal alignment parameters such that the alignment is maximized without distorting the structure. Changes in the structure caused by the laser can be detected as changes in the diffraction pattern. Multiple diffraction patterns will be captured for different orientations of the molecules with respect to the detector. Finally, the structure will be retrieved from the diffraction patterns by combining the information in multiple diffraction patterns and using phase retrieval algorithms. If successful, the project will result in a significant advancement in our capability to capture images of isolated molecules.