Unveiling Microscopic Structure at Interface by Sum-Frequency Spectroscopy With Better Surface Resolution

Sum frequency spectroscopy (SFS) has been developed into a versatile analytical tool for surface studies because it is highly surface-specific dictated by symmetry under electric dipole approximation. However, there still exist challenges for the current SF spectroscopy, which desiderates further development of the technique. The first one is the lingering doubt that, beyond the electric dipole approximation, the electric-quadrupole bulk contribution to surface SF generation may not be negligible. The problem is particularly important in dealing with media that do not have a strongly polar-oriented surface layer. We develop a phase-sensitive SFS that allows direct measurement of electric-quadrupole spectrum from the bulk, and shows for the first time that we can deduce the true surface spectra of a nonpolar media from proper spectral analysis of transmitted and reflected SF spectra. Our experimental and theoretical approach also provides guidelines for evaluation of the importance of bulk contribution to SFVS in general. The second challenge is to probe the first few monolayers at charged interface, e.g. charged water interface. Generally, water molecules within a distance of a few monolayers away from the charged surface, forming an interface-specific bonding network (labeled as “the bonded interface layer”), governs the properties and functionality of the interface. Unfortunately, the bonded interface layer is usually buried under thick diffuse layer set by screening ions. Despite extensive studies over the years, still little is known about its microscopic structure. We develop a new SFS scheme that allows us for the first time to obtain the vibrational spectrum, and hence the microscopic structural information, of the bonded interface layer of charged water interfaces. This novel spectroscopic technique provides unique opportunities to validate molecular theories of charged water interfaces and to search for better understanding of electrochemistry and biological aqueous interfaces at a deeper molecular level.Reference:1. Tian and Shen, Surf. Sci. Rep. 69, 105(2014); 2. Sun, Tian* et al, PNAS 112, 5883(2015); 3. Wen, Tian* et al, Phys. Rev. Lett.(Accepted) (2015).