Hydrocarbon on Carbon: Coherent Vibrational Spectroscopy of Toluene on Graphite

The ability to study the interactions of hydrocarbons on carbon surfaces is an integral step toward gaining a molecular level understanding of the chemical reactions and physical properties occurring on them. Here, we apply vibrational sum frequency generation (SFG) to determine the tilt angle of toluene, a common organic solvent, on millimeter-thick highly oriented pyrolytic graphite (HOPG). The combination of a time-delay technique, which results in the successful suppression of the nonresonant SFG response, and a null angle method is shown to overcome the "strong optical absorber" problem posed by macroscopically thick carbon samples and yields a molecular tilt angle of toluene in the range of 37 degrees to 42 degrees from the surface normal. The implications of this approach for determining the orientation of organic species adsorbed on carbon interfaces, which are important for energy-relevant processes, are discussed.