Enhanced Sum Frequency Generation from a Monolayer Adsorbed on a Composite Dielectric/Metal Substrate

Abstract

A composite dielectric/metal substrate, consisting of a thin mica sheet backed with a gold film, has been employed to enhance the intensity of sum frequency vibrational spectroscopy (SFS) signals arising from model monolayers of octadecylsiloxane (ODS) adsorbed at the mica/air interface. In addition to enhanced intensities in comparison to SF spectra of ODS on mica sheets without gold backing, resonant line profiles were found to vary as a function of the thickness of the mica component of the composite substrate. This experimental result concurs with a recent theoretical study of the composite substrate which showed that interference between SF signals arising from the ODS monolayer and from the displaced gold surface results in variations in the resonant phase of the SF vibrational mode. The interference phase/thickness effect has been investigated for several composite samples, each containing a series of steps of well-defined mica thicknesses. A periodic relationship between the mica thickness and the measured interference phase was observed for both the symmetric (r+) and antisymmetric (r-) methyl stretching modes of the ODS monolayers. The empirically determined periodicities were 3.3 and 3.1 μm for the r+ and r- resonances, respectively. These values agree within error with one of two theoretically predicted periodicities, namely, 3.01 μm. Scatter in the experimental data however is symptomatic of a more complicated interference phase/thickness behavior involving the second shorter theoretically predicted periodicity (162 nm). These experimental observations have been examined in the light of the theoretical model used to simulate the composite substrate. It is concluded that the shorter periodicity component (162 nm) is more susceptible to variations in experimental parameters such as distributions of incident beam angles, nonplanarity of mica, etc. and is partially “averaged out” to leave the less susceptible long periodicity component (3.01 μm) dominant.