HE BEAM SCATTERING STUDIES OF THE INTERACTION OF CO WITH IRON(111)

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 b ehavior of He scattering from both the clean and CO covered Fe (111) s urface is different from that on smooth, close-packed surfaces such as Pt(111). Although the specular He intensity from the CO covered surfa ce is not negligible, as it is on smooth surfaces, CO is seen to be a perfectly diffuse scatterer. A model to describe the scattering intens ity as a function of CO coverage was developed which takes into accoun t the He intensity scattered from the clean surface and the intensity scattered from the CO covered surface, with no interference effects. E ffective cross sections of 56 angstrom2 for He scattering from molecul ar CO and 93 angstrom2 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 approximately 360 and 750 K , corresponding to the desorption of molecular and recombined CO, resp ectively. This study suggests that, although the interpretation of the data is more complicated than for smooth surfaces, TEAS is a viable t echnique for application to studies of adsorption on more corrugated s urfaces.