Carbon monoxide adsorption on 4 monolayer thick fcc-Fe/Cu(1 0 0): infrared reflection absorption and low energy electron diffraction studies

Adsorption behavior of carbon monoxide (CO) on 4 monolayer (ML) thick fcc-Fe films grown on Cu(1 0 0) has been investigated with IR reflection absorption spectroscopy (IRRAS) and low energy electron diffraction (LEED). The as-grown film gives a 5×1 LEED pattern which remains unchanged during CO exposure at 90 K. Simultaneous IRRAS measurements show that the band due to CO adsorbed on the on-top site dominates the spectra at saturation coverage. In contrast CO exposure at room temperature leads to change in the LEED pattern from 5×1 to diffuse c(2×2) accompanied by preferential adsorption of CO on the bridge site of the film surface. These results suggest that room temperature CO exposure gives rise to a change in the reconstructed surface of the as-grown film and c(2×2) overlayer of CO adsorbed on the bridge site, while CO adsorption takes place on the on-top site of the 5×1 reconstructed surface with no long-range ordered overlayer at 90 K.