Various experimental results on surface enhanced infrared absorption reveal asymmetric line shapes. Whereas the order of magnitude of the enhancement can be understood from electromagnetic field enhancement the unusual line shape remains without satisfactory explanation. An interaction with electron-hole pairs would lead to an asymmetric line but this should be restricted to the first monolayer. However, asymmetry is also observed for vibrations at larger distances from the metal-film surface. Here we show strongly asymmetric lines and their enhancement as a consequence of the interaction of adsorbate vibrations with surface plasmons of metal islands. Both the effects and also the baseline change can be estimated by a proper application of well established effective-media models.

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For ultrathin copper films of various morphology we studied adsorbate-induced changes in broadband infrared transmission at normal incidence of light. Smooth Cu films on Si(111) and mesoscopically rough Cu films on KBr(001) were exposed to CO and to C 2 H 4 . We observed significant broadband changes for each of these gases and for both surfaces. Applying a Drude-type model we calculated the optical spectra in accord with the experiment. We find that the effects related to a change in the electronic relaxation rate are weakly influenced by the mesoscopic roughness of the film, while the effects related to charge transfer are strongly enhanced due to such roughness. This paper shows that the real adsorbate-induced changes can be determined for both the homogeneous films and the inhomogeneous films beyond percolation. The increased surface area owing to mesoscopic roughness is merely one contribution to larger adsorbate-induced effects of inhomogeneous films. The other more interesting contribution is due to depolarization in rough metal films that is responsible for strong enhancement of charge transfer effects.

Vibrational modes of adsorbates on rough metal films can show surface enhanced Raman scattering (SERS) as well as surface enhanced infrared (IR) absorption (SEIRA). Both effects are a consequence of field enhancement and of a so-called chemical effect. Our experiments show that chemical contributions to SERS and to SEIRA from ethylene on Cu may have the same origin. We investigated ethylene adsorbed on ultrathin Cu films at about 100 K and below with IR transmission spectroscopy. At exposures up to 2 L the IR spectra show lines only at the positions of the very intense SERS E-bands. At higher exposures lines at the frequencies of the infrared active vibrations of ethylene molecules develop. All lines are stronger than one expects from molecular IR activities. Co-adsorption experiments performed with ethylene and CO show that E-lines in SEIRA do not occur after pre-coverage of Cu with CO.