External reflection infrared spectroscopy of anisotropic adsorbate layers on dielectric substrates

Monolayers of octadecylsiloxane were formed on native silicon (Si/ SiO2) and glass surfaces by adsorption from dilute solutions of octadecyltrichlorosilane and were investigated by polarization- and angle-dependent external reflection infrared spectroscopy. In contrast to metal substrates, both the parallel and perpendicular vibrational components of the adsorbate can be detected on these dielectric surfaces. The monolayer reflection spectra show significant changes as a function of the light incidence angle and the polarization of the infrared radiation, which contain detailed information on the surface orientation of the film molecules. Spectral simulations based on classical electromagnetic theory yield an average 10 degrees tilt angle of the hydrocarbon chains with respect to the surface normal on both silicon and glass surfaces. Despite this apparent structural identity of the monolayer films on silicon and glass, significant differences are observed in the monolayer reflection spectra resulting from purely optical effects of the substrate.