IR-spectroscopy of CO on iron ultrathin films

In contrast to the well-known surface enhanced Raman scattering (SERS) the surface enhanced infrared absorption (SEIRA) of molecules adsorbed on metal island films is hardly investigated. The results of our experiments add at least one important fact for a better understanding of the chemical contribution to SEIRA. Under ultrahigh vacuum conditions we measured IR-transmission spectra of in-situ grown iron films on MgO(0 0 1) on which CO was adsorbed. The MgO(0 0 1) surface was prepared by cleavage in UHV. Deposition of iron leads to growth of epitaxial islands. After CO exposure IR-transmission spectra taken at normal incidence show strong absorption lines in the C-O stretch region with number, intensities, positions, and shapes dependent on Fe-film morphology and Fe-film thickness. For each of the films under investigation the strongest C-O line shows an asymmetric (Fano-like) shape which reveals non-adiabatic interaction of vibrations with a continuum of other excitations, i.e. free electron transitions. The dynamic dipole moment is enhanced by about two orders regarding adiabatic values. However. with grazing incidence LR reflection spectroscopy of CO on thick and annealed iron films we observe symmetric lines and intensities without any extra enhancement. Obviously, the Fano-like non-adiabatic interaction is somewhat like the chemical effect in SEIRA. (C) 1999 Elsevier Science B.V. All rights reserved.