Infrared absorption study of CO chemisorption on copper island films

The infrared absorption spectra of chemisorbed CO at 90 K on copper island films deposited by electron-beam evaporation onto surface-oxidized silicon substrates have been measured at normal incidence of radiation. The integrated absorption intensity of the C-O stretch mode at saturation of CO adsorption depends on both the mass thickness of copper and the substrate temperature during copper deposition. The observed CO absorption features are quite different from those observed on bulk single crystals and thick polycrystalline films. The FE-SEM images reveal that at a given mass thickness the islands are larger when deposited at 296 K than when deposited at 473 K. This difference is reflected in the mass thickness dependence of the absorption intensity; the maximum absorption intensity is obtained at a thickness of 9 nm when deposited at 296 K, whereas the corresponding thickness is 16 nm when deposited at 473 K. A feature common to both films is partial aggregation of the islands. The integrated absorption intensity of the C-O stretch mode observed on the 16 nm film is 5 times larger than that observed on the bulk Cu(1 0 0) surface. This absorption enhancement is attributed to the transverse collective electron resonance of the copper islands.