Vibrational sum frequency spectroscopy of molecules on metal, semiconductor, and aqueous surfaces

2013-06-28Surface-selective vibrational sum frequency generation spectroscopy (SFG) has been used to investigate the structure, orientation, and dynamics of molecules at metal, semiconductor, and aqueous interfaces. In the first part of the thesis, SFG has been used to elucidate the molecular orientation of methyl groups terminating Si(111). Both the symmetric and asymmetric stretches of the surface-bound methyl group showed a pronounced azimuthal anisotropy of the 3-fold symmetry in registry with the signal from the Si(111) substrate, indicating that the propeller-like rotation of the methyl groups was hindered at room temperature. The difference in the SFG line widths for the CH3 symmetric stretch that was observed for different polarization combinations (SPS and PPP for SFG, visible, and IR) indicated that the rotation proceeded on a 1-2 ps time scale, as compared to the ~100 fs rotational dephasing of a free methyl rotor at room temperature. The second part of this thesis describes a simple but general phenomenon observed in temporally-delayed SFG spectra. Depending on the magnitude of the delay, nearby vibrational resonances can flip their relative phase, i.e., appear either in- or out-of-phase with one another, resulting in either constructive or destructive interference in SFG spectra. This is significant for interpretation of the SFG spectra, in particular because the sign of the resonant amplitude provides the absolute molecular orientation (up versus down) of the vibrational chromophore. The third part of the thesis uses SFG to describe CO adsorption to gold surfaces at room temperature and pressure. In the presence of water, CO adsorption was significantly enhanced compared to if no water was present, indicating cooperativity in the CO adsorption behavior. Two CO adsorption peaks have been observed, indicating two different adsorption sites are present on the gold surface. In the last part of the thesis, heterodyne-detected SFG (HD-SFG) has been used to investigate hydrogen bonding at the air-water interface. Implementation of HD-SFG allowed to extract both the phase and amplitude of the spectroscopic signal, thus allowing to subtract the nonresonant (NR) background signal. In this study, the question of just how thin is the water surface is addressed