IMPROVEMENTS ON THE ANALYSIS AND INSTRUMENTATION FOR SUM FREQUENCY GENERATION SPECTROSCOPY AND MICROSCOPY

Date

2013-05

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Abstract

Sum frequency generation (SFG) vibrational spectroscopy has been established as one of the most essential tools to investigate chemistry of the surfaces and interfaces for several decades. Orientation, structure, and chemical reaction of molecules at the interfaces have been examined by using temporally and spatially resolved SFG. As a unique method to observe the chemistry at the interfaces, SFG has a great potential for further developments. In this dissertation, the possibility of extending the SFG field is gauged, including the introduction of detailed theory and formulation of SFG. Firstly, the principle of sum frequency vibrational depletion spectroscopy (SVDS) technique is presented. Newly developed SVDS has been established for observing vibrational behaviors of molecules on the surface with high spatial resolution below the diffraction limit. This technique is accomplished by depletion of the ground state molecules around the center of probing focal spot on the surface. High energy density IR generated is employed to deplete the ground state molecules to its first vibrational excited state. Then, SFG technique has been conducted to measure the vibrational dipoles of the surface molecules from the confined probing area. The instrumentation, theoretical simulation, depletion efficiency and the preliminary experimental results of this technique is reported. In the following chapter, SFG from acetonitrile on rutile TiO2 (110) surface has been measured using distinct polarization combinations and compared to the spectra obtained by polarization mapping and null angle methods. By varying the polarization combinations of SFG, the magnitude and shape of the spectra undergo substantial change, originating from the interference between the non-resonant signal inherent from the rutile substrate and the resonant signal from the adsorbed molecules. Theory, simulation, and analytical methods for obtaining quantitative orientation information in the presence of non-resonant signal of the adsorbed molecules on a semiconductor are presented. In the last chapter, a refined analytical procedure for SFG imaging microscope (SFGIM) is discussed. In order to determine the surface properties, the images need to be fitted using an SFG model equation which contains the physical properties relevant to interpret the surface. Experimental results obtained from SFGIM analysis of binary system of self-assembled monolayers on gold surface have been reported.

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Keywords

Sum-frequency generation, Vibrational spectroscopy, Nonlinear spectroscopy, Surface chemistry, Vibrational microscopy, Self-assembled monolayers (SAMs)

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