Extended appearance-potential fine-structure measurements of clean and oxidized iron surfaces

We have measured the surface structure of clean and oxidized polycrystalline iron using extended-appearance potential fine structure (EAPFS), and compared the results to bulk extended x-ray-absorption fine-structure measurements on iron. Improvements in experimental techniques permitted the use of much lower incident electron currents and shorter data-acquisition times than previously necessary in EAPFS measurements, while not requiring special equipment. For the clean iron both the nearest-neighbor spacing and the second- and third-neighbor spacings, which have been measured for the first time with EAPFS, are larger than bulk values by an average of 3%. We conclude the surface is relaxed but still highly ordered. In contrast, the oxidized surface is more disordered; only the first iron-oxygen shell and the first iron-iron shell can be clearly resolved. The spacings measured indicate that the oxide structure closely resembles FeO. These results demonstrate the potential of EAPFS for the study of surface structures.