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 Polaritons.¹ 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 Transfer.² 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.

References.

1. Xiang, B. et al. Two-dimensional infrared spectroscopy of vibrational polaritons. Proc. Natl. Acad. Sci. 115, 4845–4850 (2018).
2. 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).