Abstract
A detailed analysis of the features of C18-alkylthiol/gold (silver) spectra is presented to interpret the results obtained by infrared spectroscopic ellipsometry (IRSE). The interactions between neighboring vibrational bands, the effect of different incident angles, and the Berreman effect are discussed to clarify assignments, intensities, and shifts of absorption peaks. IRSE spectra of octadecanethiol self-assembled monolayers on gold and silver films were measured at different angles of incidence. The symmetric and asymmetric methylene and methyl stretching vibrations show no frequency variation with increasing angle of incidence of the IR beam. The relative tanΨf∕tanΨ0 spectral patterns are similar to those observed in infrared reflection-absorption spectroscopy. The results suggest that weak bands of organic films on metals can be treated with the Lorentz-oscillator model, taking into account the coincidence of parallel and perpendicular modes and the surface selection rule.
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