Adsorption of oxygen on Pt 3Sn(111) studied by scanning tunneling microscopyand x-ray photoelectron diffraction

The adsorption of oxygen on the (111) surface of the (formula presented) alloy was studied by means of scanning tunneling microscopy (STM) and x-ray photoelectron diffraction (XPD). As Auger electron spectroscopy, low-energy ion scattering, and x-ray photoelectron spectroscopy indicate, upon oxygen exposure (range 10000 L, with the sample at circa 770 K) Sn segregates to the surface and forms a two-dimensional tin-oxygen layer. In the x-ray photoemission spectra no pronounced chemical shift is visible that would indicate a thicker tin oxide layer. After exposure to 10000 L (formula presented) at 770 K low-energy electron diffraction shows a streaky2×2 pattern with additional spots. Scanning tunneling microscopy images show a surface with a 2×2periodicity characterized by an high defect density. The corrugation of this surface is substantially higher than that of the clean surface. After annealing in vacuum at temperatures ranging from 600 to 800 K, a sharp 4×4 low-energy electron diffraction pattern can be observed. STM then reveals a superlattice of depressions, the remaining protrusions are slightly laterally displaced from their 2×2 positions. X-ray photoelectron diffraction intensities of the 4×4 phase show hardly any change for Pt (formula presented) whereas (formula presented) azimuthal curves measured at higher polar angles are substantially modified after oxygen exposure. In order to understand the nature of the features observed in the STM images, the experimental XPD curves were compared with single and multiple scattering cluster calculations performed for various structural models. On the basis of these results we propose a model involving the reconstruction of the substrate surface