Sulphur segregation on the (111) iron surface studied by LEED, AES, and WFC

With the surface-specific analysis methods of LEED (low energy electron diffraction), AES (Auger electron spectroscopy) and WFC (work function change) the structure of sulphur segregated during heating above 300 °C is investigated. After the first heating of the (111)-oriented iron single crystal to 700 °C an almost monoatomic coverage originates which manifests itself as a p(1 × 1) superstructure. The intensity—energy profiles of the 00 beam and the 10 beam for LEED are analysed for the clean Fe(111) surface and for the p(1 × 1)—S structure according to the kinematic and dynamic scattering theories. Whereas for the clean iron specimen with 0.083 nm the same layer distance as in the bulk is found, the distance between the sulphur layer and the first atomic layer of the substrate is 0.177 nm. From this value the conclusion may be drawn that sulphur as an ion having a double negative charge touches three iron atoms. The positive change of the work function connected with the sulphur segregation leads to an effective dipole length of 0.002 nm, the existence of iron ions in the sulphur layer being explained by this. For the monoatomic Fe—S ion layer on the Fe(111) surface by means of the Haber-Born cycle a binding energy of 2.0 eV is calculated, which explains the extremely great stability of this layer against heat and oxygen.