Infrared spectral study of CO adsorption on molecular-beam epitaxially grown, fcc iron films on Cu(100)

Adsorption of CO at 90 K on molecular-beam epitaxially grown iron films on Cu(100) has been investigated using infrared reflection-absorption spectroscopy (IRRAS). The characteristic C-O stretching frequency for linearly bound CO at saturation is correlated well with the surface structures as revealed by reflection high-energy electron diffraction (RHEED). No regular oscillation of RHEED intensity is observed during deposition of less than four monolayers (ML). In this thickness range, a C-O stretch band due to on-top adsorption of CO on the Cu(100) substrate is observed in addition to the band due to adsorption on the iron surface. The additional band decreases in intensity with increasing iron thickness and eventually disappears at a thickness of 4 ML. During deposition of 5 to approximately 10 ML, however, a remarkable regularity of oscillation is observed in the RHEED intensity due to the formation of a continuous face-centered cubic (fcc) Fe(100) surface structure as confirmed by the RHEED patterns. On the films of 5-10 ML the CO band is sharper and its peak is positioned at 2038 cm^-1. Further deposition above 10 ML causes a remarkable decrease in the RHEED intensity as well as in the oscillation amplitude, suggesting a development of body-centered cubic (bcc) structures. As the thickness increases from 10 to 21 ML, the CO band broadens accompanied by a slight red shift. It is clearly shown that the CO adsorption features observed by IRRAS differentiate as-grown iron films at different thicknesses, thus revealing for the first time the vibrational properties of CO adsorbed on the epitaxially grown fcc iron films.