Adsorption and desorption of oxygen and carbon monoxide on the Si(111) surface studied by ion scattering spectroscopy

The adsorption of O₂ and CO on the Si(111) surface was studied by low-energy helium ion scattering. The adsorption consists of a fast adsorption stage followed by a much slower Sorption process. In the final uptake region CO has a faster rate of increase than O. There is no evidence of He⁺ scattering from C atoms. This fact excludes the CO molecule having its axis parallel to the surface. A comparison of the intensities of the substrate (Si) signals, for the same recorded oxygen content on the surface, shows that carbon monoxide shadows the Si atoms more than oxygen does. An increase in the oxygen signal was observed even after exposures in the range of 10¹⁴–10¹⁵ molecules cm^?2. No substantial diffusion of CO into the bulk can be deduced from these results. Desorption of oxygen by He⁺ ions was observed by following the adsorbate and substrate signals as a function of time. The sputtering cross-section has a maximum for an impact angle of 25° relative to the surface.