Second-order nonlinear susceptibilities of surfaces : A systematic study of the wavelength and coverage dependence of thiol adsorption on polycrystalline gold

Systematic studies were carried out with the goal to further develop optical Second-Harmonic Generation (SHG) as a tool for surface and interface analysis. For this purpose hexadecane thiol, HS(CH2)15CH3, on polycrystalline Au films served as a model system to study the spectral behavior of the susceptibility of the gold surface and its variation as a function of coverage. Experiments have been performed at different coverages, with different polarization combinations of the incident and detected light, different angles of incidence and different wavelengths ranging from z=615 nm to 660 nm. In addition to the intensity of the generated second-harmonic light, its phase was measured. As a result, a complete determination of all independent elements of the susceptibility tensor became possible. The studies with different excitation wavelengths reveal the presence of resonances of the tensor elements and strikingly demonstrate the importance of phase measurements for an unambiguous interpretation of the data. The superposition of resonant contributions originating from different elements of the susceptibility tensor is discussed and gives rise to very distinct and peculiar spectral features.