Surface-Enhanced Infrared Absorption: Infrared Ellipsometry of Au Evaporated Ultrathin Organic Films

The presented Infrared IR ellipsometric study proves the high potential of this method for quantitative interpretation of the SEIRA Surface Enhanced Infrared Absorption effect in infrared spectra on base of optical layer models incorporating effective medium theory. The IR ellipsometric measurements of gold capped ultrathin nitrobenzene NB films on Si 111 show different line shapes compared to those of the non modified thin NB films. Additionally a strong enhancement of the amplitudes of vibrational bands due to the SEIRA effect is observed. In comparison to vibrational bands of the film without gold on top an enhancement factor of about 5 7 is determined for the stretching vibrations of the NO2 head group of the NB molecules. Experimental results are compared to optical simulations for the NB film with and without gold islands. The observed band shapes can be explained qualitatively by optical simulations for a 3 nm thick NB film with a volume fraction of gold which is close to the percolation border of the film. For simulation of the pure NB film a simple optical layer calculation was used, whereas for Au evaporated films a Bruggeman model for ellipsoids with an effective medium shell was used