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IR-spectroscopy of CO on iron ultrathin films

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Journal of Molecular Structure

Volumes 482-483
25 May 1999
Pages 237-240

PII: S0022-2860(98)00851-5
Copyright © 1999 Elsevier Science B.V. All rights reserved.

Oliver Krauth, Gerhard Fahsold and Annemarie Pucci-Lehmann ^,

Institut für Angewandte Physik, Ruprecht-Karls-Universität Heidelberg, Albert-Ueberle-Strasse 3-5, D-69120 Heidelberg, Germany

Received 24 August 1998; accepted 12 November 1998. Available online 2 June 2000.

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 IR 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.

Author Keywords: Infrared spectroscopy; Thin films; Surface enhanced molecular absorption

1. Introduction
2. Experimental
3. Results and discussion
4. Summary and conclusions
Acknowledgements
References

(11K)
Fig. 1. Relative transmittance of CO on Fe films as measured at 115 K (noisy spectra) and calculations (smooth through lines): Lorentz-oscillator model (on top, parameters see the text) and fits to the strongest absorption line according to the non-adiabatic model [2 and 11]. Fit parameters: ₀ is 2014 cm^-1 for 0.5 nm, 2023 cm^-1 for 1 nm and 1987 cm^-1 for 4.3 nm, respectively. is 103 cm^-1 for 0.5 nm, 87 cm^-1 for 1 nm and 77 cm^-1 for 4.3 nm, respectively. The asymmetry ₀ is 0.4 for 0.5 nm, 1.14 for 1 nm and 0.61 for 4.3 nm, respectively.

² corresponds to the oscillator strength. It is (in 10^-9 A/V) 220 for 0.5 nm and for 1 nm, but it is 500 for 4.3 nm. The broken line displays the baseline change.

(8K)
Fig. 2. Normalized IR reflectance (measured without a polarizer) at CO saturation coverage for oblique incidence at 75° (noisy spectrum). The smooth line is based on a Lorentzian oscillator model and is used for an estimate of the peak area (indicated in the figure).

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Corresponding author. Tel.: 0049 62 2154 9450; fax: 0049 62 2154 9262; email: bj4@ix.uni-heidelberg.de

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Journal of Molecular Structure
Volumes 482-483
25 May 1999
Pages 237-240

43 of 127

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