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Thesis/Dissertation
Date
2025-05
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Chemistry
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https://doi.org/10.34944/wys2-p289
Abstract
While interfaces properties are different from the bulk, probing them is challenging. Iexploit the sensitivity to noncentrosymmetry, intrinsically present at interfaces, of second-order nonlinear optical techniques to investigate oxide interfaces. I have used existing techniques mid-infrared vibrational sum frequency generation (MIR-vSFG) and near-infrared vSFG (NIR-vSFG) and developed new ones NIR second harmonic generation (NIR-vSHG) to study surface phenomena. We revealed the local electric fields of water-acetonitrile mixtures at interfaces with α-Al2O3(0001) and SiO2 by exploiting MIR-vSFG Stark spectroscopy. The nitrile group of acetonitrile probes charged hydroxyl groups (−3.2 MV/cm) at the SiO2 surface at pH 6–11, while at the Al2O3 interface, the local electric field shifts from 4.3 MV/cm at pH 4 to -16 MV/cm at pH 10 due to the influence of multiple charged hydroxyl sites at elevated pH.
We have made progress in understanding electrical permittivity at buried interfaces. We derived an equation for the interfacial dielectric constant ( "!="#"$("$− "# + 6)/2 (2"$ + "#)). Then we validated this expression using MIR-vSFG and Fresnel factor calculations for interfaces of alumina with water (H2O and D2O) and acetonitrile.
We highlighted the impact of NQEs on hydrogen bonding dynamics by comparing the vibrational lifetimes of H2O and D2O at alumina interfaces using ultrafast MIR-pump/MIR-vSFG probe. We propose that the increased anharmonicity of Hâ‚‚O, driven by its higher zero-point energy compared to pure Dâ‚‚O, enhances mode coupling, accelerates spectral diffusion, and results in shorter vibrational lifetimes, as confirmed by HOD experiments. We attribute the differences in the vibrational relaxation lifetimes of the OD and the OH to variations in their relaxation pathways, which depend on the charge state of the interfaces. These nuclear quantum effects (NQEs) collectively account for the observed differences in the vibrational relaxation dynamics between water and its isotopomers.
Using NIR-vSHG, a new and user friendly vibrational nonlinear spectroscopy technique, we investigated the overtone of the free OH group on muscovite mica surfaces in air, as well as the CH stretching vibrations in chloroform and acetonitrile at alumina interfaces. We confirmed the vibrational origin of the NIR-vSHG signal by comparing it with the NIR-vSFG response. Additionally, our results provide the determination of the anharmonicity and dissociation energy of these CH and OH bonds at interfaces.
These results offer new insights into hydrogen bonding networks at interfaces, highlighting how they differ from those in the bulk. We expect that our findings will be of interest to diverse audience of experimental and theoretical scientists working in condensed matter and materials, molecular and optical physics, physics of fluids, atmospheric chemistry, and earth and planetary science.
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