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 thelingering 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,PNAS112,5883(2015); 

3. Wen,Tian*et al, Under Review. 

田传山 教授

男，1978年10月生。复旦大学物理系。分别于2001年和2006年在复旦大学获理学学士和博士学位。2002年9月－12月，在意大利Trieste TASC-INFM同步辐射中心访问研究。2005年6月－11月，在德国Max-Plank微结构物理研究所访问研究。2006年7月－2010年12月，在美国加州大学伯克利分校物理系师从沈元壤教授作博士后研究。2010年12月被复旦大学作为海外优秀人才引进。研究兴趣包括发展新型表面非线性光谱技术，水界面科学，自旋霍尔效应等。