Carbon monoxide adsorption on ultra-thin Fe film deposited on Cu(100)

This paper is mainly concerned with vibrational properties of CO adsorbed on fcc Fe(100) observed using IR reflection absorption spectroscopy. An epitaxial Fe film of eight monolayers has been grown at 300 K on Cu(100). At 0.1 L exposure only a single C-O stretch band appears at 1920 cm^-1 which shifts to 1977 cm^-1 at 0.55 L. This band further shifts to 1998 cm^-1 and saturates in intensity at 0.88 L. In contrast, at 0.25 L a second C-O stretch band appears at 2020 cm^-1 which increases in intensity and shifts to 2048 cm^-1 at 1.2 L. The initially observed band can be ascribed to the bridging CO and the second band to the on-top CO. Although the bridging CO was stable up to ca. 380 K, the band due to the on-top CO was less stable and disappeared above 313 K. The observed CO adsorption/desorption behavior on the epitaxial fcc Fe(100) surface is compared with that on a vacuum-evaporated bcc Fe polycrystalline film.