Effects of mechanical compression on the vibrational spectrum of a self-assembled monolayer

A molecular monolayer (octadecanethiol on gold) is compressed between a bronze ball and a sapphire prism. Vibrational spectra are obtained by optical sum-frequency generation before, during, and after a pressure cycle to 660 MPa (6.6 kbar). Both reversible and permanent effects are observed on the frequency, intensity, and shape of methyl C–H stretching resonances in the monolayer. Spectral broadening indicates the heterogeneity of chemical environments in the contact, while a pressure-dependent loss of orientational order attenuates the resonant signals. These results from a model system demonstrate that contact-modifying molecular films can be investigated spectroscopically.