ATR-IR spectroscopy at the metal-liquid interface:: influence of film properties on anomalous band-shape

Attenuated total reflection (ATR) spectra of adsorbates and solvent on thin metal films were investigated with emphasis on the band-shape of absorption bands. Distorted band-shapes are found even far from the critical angle. Strong absorption bands are more distorted. The band-shape strongly depends on the optical constants of the metal film and its thickness. The distortion increases with increasing thickness and increasing refractive index of the thin metal film. For a 10 nm thick Pt film the measured band-shapes for liquid water and ethanol are in good agreement with theoretical predictions using the bulk optical constants for Pt. For CO adsorbed on a 1 nm Pt film a distorted band-shape is observed whereas calculations assuming bulk optical constants predict band-shape distortion only for considerably thicker Pt films. The effective optical constants for very thin metal films deviate considerably from the bulk values, due to the island structure of the film and non-adiabatic effects can lead to distorted band-shapes. Structural changes within a Pt film, induced by hydrogen treatment, leads to a change in the band-shape for adsorbed CO. The results show that band-shape analysis is a valuable tool for in situ ATR IR spectroscopy of metal films.