Surface enhanced infrared absorption observed with attenuated total reflection (ATR-SEIRA): modeling the optical response

The gain in sensitivity due to Surface Enhanced Infrared Absorption (SEIRA) when analyzing an adsorbate on a metal island film was calculated for the ATR configuration under full consideration of the anisotropy of the film. Modeling the islands as spheroids coated with the dielectric adsorbate leads to such extraordinarily high refractive indices and low absorption indices as experimentally observed and to an enhancement of the absorption bands of the coating. Simulation of the ATR reflectance indicated further enhancement as a result of the optical properties of the island film. The predicted total level of enhancement was about one order of magnitude superior to what commonly is found in experiments. This was attributed to percolation of the metal islands which almost inevitably occurred in practice: the resulting lateral conductivity was found to have a strongly decreasing effect on the enhancement.