2ND-ORDER NONLINEAR SUSCEPTIBILITIES OF SURFACES - A SYSTEMATIC STUDYOF THE WAVELENGTH AND COVERAGE DEPENDENCE OF THIOL ADSORPTION ON POLYCRYSTALLINE GOLD

Systematic studies were carried out with the goal to further develop o ptical Second-Harmonic Generation (SHG) as a tool for surface and inte rface analysis. For this purpose hexadecane thiol, HS(CH2)(15)CH3, on polycrystalline Au films served as a model system to study the spectra l behavior of the susceptibility of the gold surface and its variation as a function of coverage. Experiments have been performed at differe nt coverages, with different polarization combinations of the incident and detected light, different angles of incidence and different wavel engths ranging from lambda = 615 nm to 660 nm. In addition to the inte nsity of the generated second-harmonic light, its phase was measured. As a result, a complete determination of all independent elements of t he susceptibility tenser became possible. The studies with different e xcitation wavelengths reveal the presence of resonances of the tenser elements and strikingly demonstrate the importance of phase measuremen ts for an unambiguous interpretation of the data. The superposition of resonant contributions originating from different elements of the sus ceptibility tenser is discussed and gives rise to very distinct and pe culiar spectral features.