Infrared Spectra of Monolayers on Metal Mirrors

Abstract

A double-beam infrared spectrophotometer has been modified so that spectra of films as thin as one or a few monolayers in thickness on metal mirrors can be obtained. It is theoretically predicted and experimentally confirmed that, at the mirror surface, the electric intensity is predominantly perpendicular to the surface. Vibrational modes for which the matrix element of dipole moment is parallel to the surface appear abnormally weak in the spectra. The spectra, therefore, provide information about the orientation as well as the composition of sample films. Spectra of Blodgett films of metal stearates show differences in band positions and band intensities between films one molecular layer in thickness and thicker films. The shifts in band positions are attributed to interactions between carboxylate ions in films thicker than a molecular layer and the absence of such interactions in the first layer. The intensity differences are not explained.