ELECTRON-ENERGY LOSS SPECTROSCOPY OF REAL SURFACES

High-resolution electron energy loss spectroscopy (EELS) was applied to a sputtered carbon (sp-C) film onto which stearic acid had been sublimed, but which had been allowed to be exposed to ordinary atmospheric conditions. This sample is a model ''real'' material as it simulates conditions that might be found in practical materials, such as a lubricated surface. In order to apply EELS it was necessary to understand the signals from the substrate and the adsorbate, so various additional diagnostic experiments were made on Ag and ordered graphite surfaces. Comparison with known organic overlayers shows the presence of sp3 bonded hydrocarbons in the sp-C substrates. Despite the interference from such contaminants, it is possible to detect the spectrum of stearic acid that was sublimed onto sp-C at -150-degrees-C under vacuum. The presence in the EELS spectra of stearic acid of Raman and infrared active modes indicates the importance of impact excitation of the EELS. Spectra of stearic acid sublimed onto highly oriented pyrolytic graphite (HOPG) were used to separate chemical versus geometrical orientation effects in the C = O stretch mode of the acid group. Evidence for undissociated acid groups of stearic acid on graphite is indicated by the presence of the C = O stretch at 212 meV. Comparison of the intensity of the C = O stretch mode of stearic acid sublimed onto HOPG to that of a published spectrum of Langmuir-Blodgett films of vertically oriented stearic acid indicates that the molecules are lying down with the acid groups near the surface in the sublimed overlayers of stearic acid on HOPG. A similar effect is not observed for molecules on a sp-C surface where substrate disorder and roughness average out orientational effects in the spectra.