Studies of liquid/solid interfaces and of bulk liquids using second harmonic generation and THz spectroscopy

We have obtained detailed information about the composition and orientation of solute molecules (phenol) adsorbed at the aqueous silica interface. The nonlinear spectroscopic technique of second harmonic generation has been applied to low surface concentrations with the benefit of resonance enhancement. In an aqueous phenol solution, the orientation of phenol molecules at solution/silica interface are found to remain roughtly unchanged with increasing bulk phenol concentration. The ⟨cos3theta⟩/⟨costheta⟩ ratio increases with pH value of the solution increases. This tell us, at higher pH, the molecules tilt in the direction of the surface field. The free energies of adsorption at different pHs were measured. We found the free energy of adsorption has a substantial E field dependence, changing by 30% from pH 4 to pH 8.; In this dissertation, we also demonstrate for the first time that the intensity and angular dependence of the SHG scattered from colloidal particles in suspension can be used to recover the nonlinear susceptibility tensor elements and therefore allow recovery of the coverage and orientation of adsorbed molecules.; SHG signal from molecules adsorbed on colloidal quartz particles in water was detected. The coverage dependence is the same as expected on the basis of planar surface measurements. From measurements of SH angular dependence from colloids at different polarization combinations, we found the orientational information can be recoverable and even for particles as small as 0.15 mum in radius, the surface SH polarizability is well described by P = chiEE. On colloidal silica/water interfaces, as on planar silica/water interfaces, we are able to conclude that adsorbed phenol molecules, like water molecules, align at interfaces when surface charge increases. Also on both interfaces, more water molecules are aligned at interfacial region and hinder the adsorption of phenol while surface field increases.; Finally, with the use of THz-TDS we have measured the real and imaginary part of the dielectric constant of liquid water in the far infrared region from 0.1 to 1.5 THz at five different salt concentration. The data were found to be best fitted by modified damped oscillatory model. Two modes were found. The characteristic frequency of the bending-like intermolecular vibrational mode (nuT2) blue-shifts from 1.5 THz toward 2.3 THz while the damping constant also increase with salt concentration.; The low frequency (nu0 ∼ 0.1 THz) mode in NaCl aqueous solutions measured by THz-TDS may corresponds to a intermolecular process of the dynamical structure of water. The restoring force and damping coefficients are found to be larger and the relaxation times are found to be larger with added salt.