Vibrational Spectroscopic Analysis of Langmuir-Blodgett Multilayers by HREELS: Sampling Depth and Scattering Mechanisms

HREELS analysis of Langmuir-Blodgett mono- and multilayer systems was conducted at various primary electron energies between 1.5 and 6.5 eV and at a specular angle of 60°. Results from the analysis of barium stearate multilayers on polycrystalline Ag indicate that extreme surface sensitivity is being exhibited, as there are no loss features corresponding to carboxylate stretches seen with increasing layer thickness as would be expected if the sampling depth was greater than 40 A; methyl and methylene features dominate the observed spectra. This correlates well with previous work on unsubstituted stearic acid systems analyzed by HREELS. As the layer thickness increases, the elastic peak intensity also increases, supporting a “carpet effect” where the Langmuir-Blodgett multilayers can shield surface imperfections and create a more specular reflecting surface. Primary energy analysis of a single layer of barium behenate was conducted, and enhancement of several vibrational features was observed at approximately 5.5 eV, This correlates well with resonance with C-H impact scattering modes as evidenced in internal photoemission experiments on hexatricontane, showing that the carbonyl electronic feature is not being excited and further substantiating the limited sampling depth of HREELS in these systems. Vibrational loss features are assigned on the basis of previous work to features seen in IR spectroscopy.