09/24/18 - 4:00 PM to 5:00 PM
Special Seminar: Professor Wei Xiong
Ultrafast Nonlinear IR Spectroscopy for Exotic Molecular Materials
In this seminar, I will discuss two developments in ultrafast nonlinear IR spectroscopy for exotic molecular materials: (1) 2D IR spectroscopy for molecular vibrational polaritons and (2) transient electric field induced VSFG spectroscopy for probing interfacial charge transfer. Both show the advantages of ultrafast nonlinear IR spectroscopic technique: to decipher hidden physics of exotic molecular materials.
2D IR of Molecular Polaritons1
Molecular vibrational polaritons, half-light, half-matter hybrid quasiparticles, are studied using ultrafast, coherent 2D IR spectroscopy. Molecular vibrational-polaritons are anticipated to produce new opportunities in the photonic and molecular phenomena. Many of these developments hinge on fundamental understanding of physical properties of molecular vibrational polaritons. Using 2D IR spectroscopy to study vibrational-polaritons, we obtained results that challenge and advance both polariton and spectroscopy fields. These results invoke new developments in theory for the spectroscopy, discover observation of new nonlinear optical effects and unexpected responses from hidden dark states. We expect these results to have significant implications in novel infrared photonic devices, lasing, molecular quantum simulation, as well as new chemistry by tailoring potential energy landscapes.
Transient E-field induced VSFG for Direct Interfacial Charge Transfer2
We describe direct electron-transfer at buried interfaces between an organic polymer semiconductor film and a gold substrate, by observing the transient electric-field-induced vibrational sum frequency generation (VSFG). We observe dynamic responses (<150 fs) where electrons are directly transferred from the Fermi level of gold to the LUMO of organic semiconductor. Transient spectra further reveal that, although the interfaces are prepared without deliberate alignment control, a sub-ensemble of surface molecules can adopt conformations for direct electron transfer, supported by DFT calculations. This result will have implications for implementing novel direct electron transfer in energy materials.
- Xiang, B. et al. Two-dimensional infrared spectroscopy of vibrational polaritons. Proc. Natl. Acad. Sci. 115, 4845–4850 (2018).
- Xiang, B., Li, Y., Pham, C. H., Paesani, F. & Xiong, W. Ultrafast Direct Electron Transfer at Organic Semiconductor and Metal Interfaces. Sci. Adv. 3, e1701508 (2017).
Wei Xiong is an assistant professor in the Department of Chemistry and Biochemistry at the University of California, San Diego, where his group develops novel nonlinear optical spectroscopy to study molecular structure and dynamics at interfaces. Previously, he was a postdoctoral researcher in JILA (formerly known as the Joint Institute for Laboratory Astrophysics), University of Colorado, Boulder (2011-2014). He received his doctorate in chemistry from University of Wisconsin-Madison in 2011, and his Bachelor of Science degree in chemistry from Peking University in 2006.
The goal of Professor Xiong's research is to develop new spectroscopic techniques for observing ultrafast processes of interfaces, enabling breakthrough technologies. To achieve this goal, the group is combining concepts and techniques from chemistry and physics, in particular, they have focused on pushing the frontier of a powerful interfacial-sensitive spectroscopy, named Sum Frequency Generation (SFG) spectroscopy, by adding new dimensions (in time, frequency and space) to this established technique.
Primarily, Professor Xiong's research focuses on several directions fields. First, researchers want to prove the electronic structure and dynamics of solid/solid interfaces by developing novel spectroscopies, including Electronic SFG spectroscopy, and Transient Electric-Field-Induced Vibrational SFG spectroscopy. Second, they have been revealing the relationship between molecular conformation and dynamics of electrochemical liquid/solid interfaces, using Heterodyne 2D Vibrational SFG spectroscopy. Third, researchers recently developed spatial-resolved vibrational SFG microscope to image complex molecular self-assembled structures, which reveal micro-size domains with specific molecular packings. Lastly, a recent endeavor has been focused on studying a new hybrid light-matter materials – molecular polariton, by performing the first 2D IR spectroscopy to learn the dynamics and coupling between various states in molecular polariton.