Evidence for a nitrogen-induced reconstruction of Fe{1 1 1}

Nitrogen adlayers on Fe{1 1 1} formed by dosing nitrogen ions give rise to a number of ordered structures. With increasing coverage, (3 × 3), (43×43)-R30° and (5 × 5) patterns are observed. Hydrogen desorption spectra from these layers are found to be strikingly different to those from the clean surface. At N coverages above 0.21 ML, the low-coverage H2 desorption peak is at 450 K, approximately 100 K higher than on the clean surface. By comparison with H2 desorption spectra from Fe{1 1 0}, we propose that the difference is due to an N-induced reconstruction of the {1 1 1} substrate, resulting in an almost close-packed top layer of Fe atoms. At lower coverage of N, the H2 desorption spectrum exhibits peaks due to desorption from both reconstructed and unreconstructed patches of the surface. Models are proposed for the observed LEED patterns based on this reconstruction. yers formed by dosing NH3 at room temperature and heating to desorb H2 only give rise to shifted H2 TPD spectra following a complicated sequence of NH3 dosing and annealing, suggesting a significant barrier to surface reconstruction. A second (3 × 3) Fe–N LEED pattern is identified which is not associated with the formation of the reconstruction described above.