Dielectric properties of interfacial water and distribution of ions at the air/water interface

This thesis covers several topics, including (i) the vibrational coupling of the H-O-H bending mode of bulk water, (ii) including the dielectric properties of interfacial water on sub-nanometer length scales, and the influence of interfacial dielectric on the surface water spectrum, and (iii) the distribution of ions at the air/water interface. Here, we employ the heterodyne-detected vibrational sum-frequency generation (HD-SFG) spectroscopy and polarization-resolved pump-probe IR measurement for studying the surface and bulk water, respectively.
For bulk water, compared to O-H stretch vibration, information on energy transfer pathways from the H-O-H bending mode is lacking. Here, we reveal that the bend-to-bend coupling is much weaker than the stretch-to-stretch coupling. We find that the intramolecular bend-to-libration energy transfer (≈ 200 fs) takes place much faster than the intermolecular bend-to-bend coupling (≈ 1 ps).
For water surface, HD-VSFG allows one to study the microscopic structure and dielectric environment of the interface. This so-called Fresnel factor correction can change the line shapes and absolute amplitudes of SFG spectra substantially. By comparing the experimental and simulated SFG spectra, we resolve the interfacial dielectric function with the angstrom-level depth resolution. Furthermore, we find that the impact of the vibrational coupling of water surface is largely suppressed due to the Fresnel factor. Next, we investigate the microscopic structure of electric double layer (EDL) created by the differential distribution of anions and cations at the interface. We find that small ions will not deplete from water surface but located in a subsurface region leading to a surface stratification.