He beam scattering studies of the interaction of CO with iron (111)

Abstract

The interaction of CO with Fe(111) has been studied by thermal energy atom scattering (TEAS) at surface temperatures of 250 and 400 K. The behavior of He scattering from both the clean and CO covered Fe(111) surface is different from that on smooth, close-packed surfaces such as Pt(111). Although the specular He intensity from the CO covered surface is not negligible, as it is on smooth surfaces, CO is seen to be a perfectly diffuse scatterer. A model to describe the scattering intensity as a function of CO coverage was developed which takes into account the He intensity scattered from the clean surface and the intensity scattered from the CO covered surface, with no interference effects. Effective cross sections of 56 Å2 for He scattering from molecular CO and 93 Å2 per molecule for He scattering from dissociated CO were obtained. Comparison of the He/CO/Fe(111) system is made with He/CO/Pt(111) and He/H/Pt(111) systems and clear differences are seen. As the temperature of the CO covered surface is increased there are two increases in scattered He intensity at ∼360 and 750 K, corresponding to the desorption of molecular and recombined CO, respectively. This study suggests that, although the interpretation of the data is more complicated than for smooth surfaces, TEAS is a viable technique for application to studies of adsorption on more corrugated surfaces.