Interface-selective Nonlinear Spectroscopy at Aqueous Interface
Aqueous interfaces are ubiquitous and play vital roles in a variety of processes relevant to heterogeneous catalysis, electrochemistry, atmospheric, environmental chemistry, and biological self-assembly. Nevertheless, molecular-level understanding of aqueous interfaces is very limited compared to that of the corresponding bulk media. A huge challenge in investigating the aqueous interface is to selectively observe few molecular layer thick interface in the presence of surrounding more plentiful bulk. Due to lack of interface-selectivity, most of the spectroscopic techniques are inadequate for probing aqueous interfaces. Moreover, unlike a solid, water has substantial vapor pressure, which makes it difficult to probe the water surface by conventional spectroscopy such as X-ray photoelectron spectroscopy (XPS). Thus, it is essential to utilize spectroscopic techniques that can selectively detect interfacial molecules and directly reveal the structure and properties of the interface at the molecular level.
In this seminar, I will introduce interface-selective nonlinear spectroscopy technique called heterodyne-detected vibrational sum frequency generation (HD-VSFG), which I have been actively developing during my PhD and postdoctoral research. HD-VSFG can selectively probe the aqueous interfaces via the vibration of the interfacial molecules1. In fact, HD-VSFG provides the accurate vibrational spectra of interfacial molecules that are comparable to the IR/Raman spectra for the bulk. Moreover, the sign of HD-VSFG spectrum (i.e., positive or negative amplitude of the signal) reveals the absolute orientation of interfacial molecules. As typical examples, I will discuss the application of HD-VSFG that addresses the hydrogen bonding and orientation of water molecules at the air/water interface2, properties of osmolyte and denaturant at water surface3. Furthermore, I will discuss my recent HD-VSFG study that resolved the controversy on the origin of SFG signal for the bend vibration of interfacial water4. The presented results highlight the importance of interface-selective molecular information obtainable with HD-VSFG and its potential application to explore interfacial phenomena in more complex chemical and biological systems.
1 S. Nihonyanagi, J. A. Mondal, S. Yamaguchi, and T. Tahara, Annu. Rev. Phys. Chem. 64, 579 (2013).
2 M. Ahmed, Y. Nojima, S. Nihonyanagi, S. Yamaguchi, and T. Tahara, J. Chem. Phys. 152, 237101 (2020).
3 M. Ahmed, V. Namboodiri, P. Mathi, A. K. Singh, and J. A. Mondal, J. Phys. Chem. C 120, 10252 (2016).
4 M. Ahmed, S. Nihonyanagi, A. Kundu, S. Yamaguchi, and T. Tahara, J. Phys. Chem. Lett. 11, 9123 (2020).