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We have measured infrared transmission spectra of CO adsorbed on in situ grown iron films on MgO(001) under ultrahigh vacuum conditions. Even at normal incidence we observed strong absorption lines in the C–O stretch region with number, intensities, positions, and shapes dependent on CO coverage and Fe-film thickness and morphology. This absorption must be due to vibrational dipoles oblique to the substrate surface, e.g., due to molecules at island walls. The distinct absorption lines can be assigned to distinct adsorption sites on crystalline facets of epitaxial Fe islands on MgO(001). For each of the films the strongest CO line shows an asymmetric shape. Additionally, the observed absorption is enhanced by at least two orders of magnitude with respect to adiabatic intensities of, e.g., CO on NaCl. Line shapes and intensity let us suggest nonadiabatic coupling of the adsorbate vibration to electronic transitions.

IR absorption lines of CO on ultrathin epitaxial iron films are both enhanced and asymmetric. In this letter we show new experimental results which demonstrate a correlation of the asymmetry of the CO-stretching line to electronic properties of the underlying metal film. The new finding indicates the important role of metal film morphology for nonadiabatic effects. Such effects are strongest slightly above the percolation threshold as our results show.

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