Evaluation of surface vs bulk contributions in sum-frequency vibrational spectroscopy using reflection and transmission geometries

An experiment was designed to measure sum-frequency generation (SFG) spectr a in both reflection and transmission. The result from an octadecyltrichlor osilane monolayer on a substrate reveals that while the resonant spectra in the two geometries both originate from the surface monolayer, the bulk sub strate has a significant contribution to the nonresonant background in the transmitted spectra. Study of a polyethylene film also shows that the bulk contribution to the SFG spectra is only significant for the transmission ge ometry, exhibiting a resonant mode not observable in the reflected spectra.