Three-dimensional structure of molecules at the water surface

This thesis covers two topics; one is an investigation into the nature of the bending mode of water, while the other is the development of a new probe of the interfacial molecules by extending sum-frequency generation (SFG) spectroscopy to heterodyne-detected and polarization-dependent SFG.
Although the H-O-H bending mode of water is known to be sensitive to the hydrogen bond strength, the information on the vibrational coupling of the bending mode has been poorly understood. Here, we unveil the vibrational coupling and vibrational dynamics of the bending mode by using infrared, Raman, hyper-Raman, SFG, and time-resolved vibrational techniques. We find that the intermolecular coupling of the bending mode is weak compared to the stretching mode, and address the origin of the bending mode SFG signal at the interfaces.
Next, we present the use of polarization-dependent heterodyne-detected SFG (PD-HD-SFG) to study molecular interfacial properties. Surface-specific SFG measurements at different polarization combinations have been used for investigating the molecular orientation at interfaces. We demonstrate that PD-HD-SFG can be used not only to estimate the molecular orientation but also to study the depth profile of molecules, and elucidate details of iodide chemistry at the air-water interface. As such, PD-HD-SFG is a very promising technique for qualitative and quantitative analysis of interfacial molecular properties.