Polarizability-Induced Vibrational Shifts for Infrared Spectroscopy of Molecular Surfaces: Xe as a Surface Probe of Hexadecanethiol Self-Assembled Monolayers and Hexane Films on Au(111)

Adsorption of atomic and molecular species on several model film surfaces is shown to lead to systematic and reproducible spectral shifts for infrared absorption peaks associated with vibrations at the film/adsorbate interface. These shifts to lower frequencies are demonstrated to be versatile spectral signatures of this exposed interface. While this is found to occur for a wide variety of adsorbates, this work focuses on the adsorption of Xe onto molecular surfaces. By using the pre-Xe film reflectivity as a reference in calculating the spectra, the changes induced by Xe adsorption due to van der Waals forces are isolated from the conventional film spectrum which contains both surface and subsurface spectral contributions. A model-free algorithm containing two user-defined fitting parameters is described to reconstruct the spectrum of the surface from which these shifted features originate. The high surface sensitivity of the technique is exhibited for the case of a hexadecanethiol self-assembled monolayer (SAM), where the reconstruction algorithm successfully reproduces all peaks attributed to the methyl tail groups with minimal spectral contributions from the methylene groups composing the subsurface chain backbone; such methylene spectral features dominate the conventional surface infrared spectra. The algorithm is applied to films of hexane to demonstrate its application to disordered multilayer films as well. The technique and reconstruction algorithm are shown to extract spectroscopic information from the uppermost ∼0.3 nm of the film, representing the fundamental limit of vibrational surface sensitivity. Rudimentary modeling of a CO-Xe3 system using Morse/Lennard-Jones interactions demonstrates the expected distance dependence of the van der Waals interaction which gives rise to the observed vibrational spectral shifts.