Yingmin Li

Short-Range Catalyst–Surface Interactions Revealed by Heterodyne Two-Dimensional Sum Frequency Generation Spectroscopy

Heterodyne 2D sum frequency generation spectroscopy is used to study a model CO2 reduction catalyst, Re(diCN-bpy) (CO)3Cl, as a monolayer on a gold surface. We show that short-range interactions with the surface can cause substantial line-shape differences between vibrational bands from the same molecules. We explain this interaction as the result of couplings between CO vibrational modes of the catalyst molecules and the image dipoles on gold surface, which are sensitive to the relative distance between the molecule and the surface. Thus, by analysis of HD 2D SFG line-shape differences and polarization dependences of IR spectra, we can unambiguously determine the ensemble-averaged orientation of the molecules on the surface. The high sensitivity of HD 2D SFG spectra to short-range interactions can be applied to many other adsorbate-substrate interactions and therefore can serve as a unique tool to determine adsorbate orientations on surfaces.

Two-dimensional infrared spectroscopy of vibrational polaritons

Significance Quantum states of molecular vibrational polaritons, hybrid half-light, half-matter quasi-particles, are studied using ultrafast coherent two-dimensional infrared spectroscopy. Valuable physical insights such as existence of hidden dark states and ultrafast interactions between dark states and bright polariton states are unambiguously revealed. These results not only advance coherent two-dimensional spectroscopy, but also have significant implications in harvesting the creation of molecular vibrational polaritons for infrared photonic devices, lasing, molecular quantum simulation, as well as chemistry by tailoring potential energy landscapes. We report experimental 2D infrared (2D IR) spectra of coherent light–matter excitations––molecular vibrational polaritons. The application of advanced 2D IR spectroscopy to vibrational polaritons challenges and advances our understanding in both fields. First, the 2D IR spectra of polaritons differ drastically from free uncoupled excitations and a new interpretation is needed. Second, 2D IR uniquely resolves excitation of hybrid light–matter polaritons and unexpected dark states in a state-selective manner, revealing otherwise hidden interactions between them. Moreover, 2D IR signals highlight the impact of molecular anharmonicities which are applicable to virtually all molecular systems. A quantum-mechanical model is developed which incorporates both nuclear and electrical anharmonicities and provides the basis for interpreting this class of 2D IR spectra. This work lays the foundation for investigating phenomena of nonlinear photonics and chemistry of molecular vibrational polaritons which cannot be probed with traditional linear spectroscopy.

Ultrafast direct electron transfer at organic semiconductor and metal interfaces

Conformation-specific direct interfacial electron transfer is observed by the first ultrafast electric field–induced VSFG. The ability to control direct electron transfer can facilitate the development of new molecular electronics, light-harvesting materials, and photocatalysis. However, control of direct electron transfer has been rarely reported, and the molecular conformation–electron dynamics relationships remain unclear. We describe direct electron transfer at buried interfaces between an organic polymer semiconductor film and a gold substrate by observing the first dynamical electric field–induced vibrational sum frequency generation (VSFG). In transient electric field–induced VSFG measurements on this system, we observe dynamical responses (<150 fs) that depend on photon energy and polarization, demonstrating that electrons are directly transferred from the Fermi level of gold to the lowest unoccupied molecular orbital of organic semiconductor. Transient spectra further reveal that, although the interfaces are prepared without deliberate alignment control, a subensemble of surface molecules can adopt conformations for direct electron transfer. Density functional theory calculations support the experimental results and ascribe the observed electron transfer to a flat-lying polymer configuration in which electronic orbitals are found to be delocalized across the interface. The present observation of direct electron transfer at complex interfaces and the insights gained into the relationship between molecular conformations and electron dynamics will have implications for implementing novel direct electron transfer in energy materials.