Asymmetric line shapes and surface enhanced infrared absorption of CO adsorbed on thin iron films on MgO(001)

Abstract

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.

Infrared Reflection−Absorption Spectra of C2H4 and C2H6 on Cu: Effect of Surface Roughness

“…In more detail, the experimental setup is described somewhere else. 6 The Cu films used for infrared reflection absorption spectroscopy (IRRAS) were prepared in the following way: Polished polycrystalline Cu plates as substrates were heated in an UHV to degas them. Copper was thermally evaporated on them, the first part at room temperature and, for cold-deposited films, the second part at a 50 K substrate temperature.…”

Surface-Enhanced Infrared Absorption

“…In contrast, the EM calculations predict an enhancement of infrared absorption on transition metals as strong as that on coinage metals [26]. In fact, several recent experiments have demonstrated the SEIRA effect on many other metals such as Pt, Pd, Rh, Ru, Ir, Sn, Fe, In, Pb, and Pt-Fe alloys [16,17,19,32,49,50,70,71,72,73]. Since the dielectric constants of these metals do not differ greatly from those of coinage metals in the infrared region, these results may be a reasonable consequence.…”

“…derivative-like) band shapes are often observed in SEIRA spectra [19,32,49]. In some cases, molecular vibrations are observed as negative absorption peaks, i.e.…”

“…At least two different mechanisms, the electromagnetic (EM) and chemical mechanisms, are supposed to contribute to the total enhancement [10,31,32] as in the case of SERS [1,2,3,4]. The infrared absorption (A) may be written as…”

Line shape of the stretching vibration of CO adsorbed on rough copper surfaces

“…The line shape of the stretching vibration is asymmetric and develops a pronounced shoulder at lower wavenumbers with increasing exposure to CO. In the case of surface enhanced infrared spectra of CO adsorbed at colddeposited Fe films, damping of the adsorbate vibration by electron-hole pair creation [11] was used to explain the asymmetric Fano-like line shape [12]. However in SERS spectra this Fanolike line shape does not explain the experimental observed spectra [7,10].…”