THIOL ADSORPTION ON GOLD STUDIED BY RESONANT 2ND-HARMONIC GENERATION AND PHASE-SENSITIVE DETECTION

The adsorption of hexadecane thiol on polycrystalline gold surfaces has been investigated for fundamental wavelengths between 615 nm and 660 nm. In situ monitoring of the adsorption process exhibits a strong spectral dependence of the intensity of the time dependent second harmonic signal due to resonance effects. Phase sensitive ex situ measurements reveal a much more complex behavior of the second harmonic signal than is obvious from the intensity measurements. At certain wavelengths the change of the phase of the second harmonic signal even reverses its direction during thiol adsorption. The results demonstrate that an interpretation of a second harmonic signal based only on intensity measurements can be misleading and, therefore, the phase of a second harmonic signal should be considered a parameter which is as important to measure as is its intensity. Furthermore, the system studied is an example where different techniques for phase measurements would yield different results.