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. (C) 2001 American Institute of Physics.