The enhanced infrared absorption on different types of metal nano-particle films

The surface enhanced infrared absorption (SEIRA) of adsorbates on nano-structured metal films is widely exploited in chemistry and biology, but the basic effects are only partially understood. Our IR transmittance studies of thin metal films under ultra-high vacuum (UHV) conditions showed two types of SEIRA lines. The systems investigated were C₂H₄ and C₂H₆, respectively, on cold-deposited Cu films, on transparent substrates (MgO, KBr) and on Cu films prepared at room temperature on the same substrates. For the room-temperature films only the IR-active vibrations of the adsorbates give SEIRA lines, which can be explained by interaction of the molecules with the electromagnetic field. On cold-deposited films also Raman-active lines of C₂H₄ appear in the IR-transmittance spectra. These lines saturate already below monolayer coverage. For C₂H₆ Raman bands like in SERS of C₂H₆ on cold-deposited silver are not observed. The appearance of the Raman lines of centrosymmetric molecules in the IR spectra is a first-layer effect and should be due to an indirect excitation mechanism involving metal electrons. For this mechanism roughness on an atomic scale and an adsorbate π* state are important.