Sum Frequency Generation (SFG) is a well-established surface/interface specific non-linear spectroscopy. The laser pulse sequence of SFG Vibrational Spectroscopy (SFG-VS) consists of an on-resonance IR pulse, followed by a non-resonance visible pulse. Since the inherent symmetry of the non-linear response, the SFG signal can only come from non-central symmetric parts of the system. Taking advantage of this property, SFG has been used to investigate many surface/interfacial systems, such as water orientation at the air/water interface.
2D SFG is a perfect match between 2D IR and SFG. In 2D IR, we can learn about the coupling and dynamics of a system. However, the signal of 2D IR is not surface specific, as the signal coming from surface molecular layers can easily be buried in bulk background signal. In order to study this thin molecular layer, we need to incorporate the surface selectivity of SFG into 2D IR. In our previous work, we have demonstrated how the central part of 2D IR, the pulse shaper, can be integrated with SFG. Here, we integrate 2D IR with SFG by adding a non-resonant up-converting visible pulse to the 2D IR pulse sequence, as shown in the figure.
With 2D SFG, we have already studied a variety of systems, including carbon monoxide absorption on a platinum surface, a surface-bound peptide, DNA, and amyloid aggregation on different surfaces. In order to extract the structural information from a 2D SFG spectrum, we also derived the analytical expression of the 2D SFG response functionand and developed a two-exciton Hamiltonian model for spectral simulation. Figure 4 shows how different the 2D SFG spectra can be for different orientations between two coupled vibrational modes.
With a more in-depth understanding of 2D SFG, we can extract both structural and dynamical information of surface/interfacial systems. There are still many more interesting systems to study and we are confident that 2D SFG can provide more insight into surface chemistry.
- Selected Publications
- Ho, J.-J.; Ghosh, A.; Zhang, T. O.; Zanni, M. T. Heterogeneous Amyloid β-Sheet Polymorphs Identified on Hydrogen Bond Promoting Surfaces Using 2D SFG Spectroscopy. J. Phys. Chem. A 2018, 122, 1270–1282.
- Ghosh, A.; Ho, J.-J.; Serrano, A. L.; Skoff, D. R.; Zhang, T.; Zanni, M. T. Two-Dimensional Sum-Frequency Generation (2D SFG) Spectroscopy: Summary of Principles and Its Application to Amyloid Fiber Monolayers. Faraday Discuss. 2015, 177, 493–505.